TRILL Working Group Radia Perlman
INTERNET-DRAFT Sun Microsystems
Intended status: Proposed Standard Donald Eastlake 3rd
Expires: December 25, 2009 Stellar Switches
Dinesh G. Dutt
Silvano Gai
Cisco Systems
Anoop Ghanwani
Brocade
June 26, 2009
RBridges: Base Protocol Specification
<draft-ietf-trill-rbridge-protocol-13.txt>
Status of This Document
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79. This document may contain material
from IETF Documents or IETF Contributions published or made publicly
available before November 10, 2008. The person(s) controlling the
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R. Perlman, et al [Page 1]
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Abstract
RBridges provide optimal pair-wise forwarding with zero
configuration, safe forwarding even during periods of temporary
loops, and support for multipathing of both unicast and multicast
traffic. They achieve these goals using IS-IS routing and
encapsulation of traffic with a header that includes a hop count.
RBridges are compatible with previous IEEE 802.1 customer bridges as
well as IPv4 and IPv6 routers and end nodes. They are as invisible to
current IP routers as bridges are and, like routers, they terminate
the bridge spanning tree protocol.
The design supports VLANs and optimization of the distribution of
multi-destination frames based on VLAN and IP derived multicast
groups. It also allows forwarding tables to be sized according to
the number of RBridges (rather than the number of end nodes), which
allows internal forwarding tables to be substantially smaller than in
conventional bridges.
Acknowledgements
Many people have contributed to this design, including, in alphabetic
order, Alia Atlas, Ayan Banerjee, Suresh Boddapati, Caitlin Bestler,
Stewart Bryant, James Carlson, Dino Farinacci, Don Fedyk, Bill
Fenner, Eric Gray, Joel Halpern, Andrew Lange, Israel Meilik, David
Melman, Erik Nordmark, Sanjay Sane, Pekka Savola, Matthew Thomas, Joe
Touch, and Mark Townsley. We invite you to join the mailing list at
http://www.postel.org/rbridge.
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Table of Contents
Status of This Document....................................1
Abstract...................................................2
Acknowledgements...........................................2
1. Introduction............................................7
1.1 Algorhyme V2, by Ray Perlner...........................8
1.2 Normative Content and Precedence.......................8
1.3 Terminology and Notation in this document..............8
1.4 Acronyms..............................................10
2. RBridges...............................................12
2.1 End Station Addresses.................................13
2.2 RBridge Encapsulation Architecture....................14
2.2.1 Known-Unicast.......................................16
2.2.2 Multi-destination...................................16
2.3 RBridges and VLANs....................................17
2.3.1 Link VLAN Assumptions...............................17
2.4 RBridges and IEEE 802.1 Bridges.......................18
2.4.1 RBridge and 802.1 Layering..........................18
2.4.2 Incremental Deployment..............................20
3. Details of the TRILL Header............................21
3.1 TRILL Header Format...................................21
3.2 Version (V)...........................................21
3.3 Reserved (R)..........................................22
3.4 Multi-destination (M).................................22
3.5 TRILL Header Options..................................22
3.6 Hop Count.............................................23
3.7 RBridge Nicknames.....................................24
3.7.1 Egress RBridge Nickname.............................24
3.7.2 Ingress RBridge Nickname............................25
3.7.3 RBridge Nickname Selection..........................25
4. Other RBridge Design Details...........................27
4.1 Ethernet Data Encapsulation...........................27
4.1.1 VLAN Tag Information................................29
4.1.2 Inner VLAN Tag......................................30
4.1.3 Outer VLAN Tag......................................30
4.1.4 Frame Check Sequence (FCS)..........................31
4.2 Link State Protocol (IS-IS)...........................31
4.2.1 IS-IS RBridge Identity..............................31
4.2.2 IS-IS Instances.....................................32
4.2.3 TRILL IS-IS Frames..................................32
4.2.4 TRILL Link Hellos, DRBs, and Appointed Forwarders...33
4.2.4.1 P2P Hello Links...................................34
4.2.4.2 Designated RBridge................................34
4.2.4.3 Appointed VLAN-x Forwarder........................35
4.2.4.4 TRILL LSP Information.............................36
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Table of Contents Continued
4.2.5 TRILL ESADI.........................................38
4.2.5.1 TRILL ESADI Participation.........................40
4.2.5.2 TRILL ESADI Information...........................40
4.3 Link MTU Size.........................................40
4.3.1 Determining Campus-Wide MTU Size....................41
4.3.2 Testing MTU Size....................................41
4.4 TRILL-Hello Protocol..................................42
4.4.1 Rationale...........................................42
4.4.2 TRILL-Hello Contents................................43
4.4.2.1 TRILL Neighbor List...............................44
4.4.3 TRILL MTU probe and Hello VLAN Tagging..............45
4.4.4 Multiple Ports on the Same Link.....................47
4.4.5 VLAN Mapping Within a Link..........................47
4.5 Distribution Trees....................................48
4.5.1 Distribution Tree Calculation.......................50
4.5.2 Multi-destination Frame Checks......................50
4.5.3 Pruning the Distribution Tree.......................52
4.5.4 Tree Distribution Optimization......................53
4.5.5 Forwarding Using a Distribution Tree................54
4.6 Frame Processing Behavior.............................55
4.6.1 Receipt of a Native Frame...........................55
4.6.1.1 Native Unicast Case...............................55
4.6.1.2 Native Multicast and Broadcast Frames.............56
4.6.2 Receipt of a TRILL Frame............................57
4.6.2.1 TRILL Control Frames..............................58
4.6.2.2 TRILL ESADI Frames................................58
4.6.2.3 TRILL Data Frames.................................58
4.6.2.4 Known Unicast TRILL Data Frames...................58
4.6.2.5 Multi-Destination TRILL Data Frames...............59
4.6.3 Receipt of a Layer 2 Control Frame..................60
4.7 IGMP, MLD, and MRD Learning...........................60
4.8 End Station Address Details...........................61
4.8.1 Learning End Station Addresses......................61
4.8.2 Forgetting End Station Addresses....................63
4.8.3 Shared VLAN Learning................................64
4.9 RBridge Ports.........................................64
4.9.1 RBridge Port Configuration..........................65
4.9.2 RBridge Port Structure..............................66
4.9.3 BPDU Handling.......................................68
4.9.3.1 Receipt of BPDUs..................................69
4.9.3.2 Root Bridge Changes...............................69
4.9.3.3 Transmission of BPDUs.............................70
4.9.4 Dynamic VLAN Registration...........................70
5. Addresses, Configuration Parameters, and Constants.....71
6. Security Considerations................................73
6.1 VLAN Security Considerations..........................73
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Table of Contents Continued
6.2 BPDU/Hello Denial of Service Considerations...........74
7. Assignment Considerations..............................76
7.1 IANA Considerations...................................76
7.2 IEEE Registration Authority Considerations............76
8. Normative References...................................78
9. Informative References.................................79
Appendix A: Incremental Deployment Considerations.........80
A.1 Link Cost Determination...............................80
A.2 Appointed Forwarders and Bridged LANs.................80
A.3 Wiring Closet Topology................................82
A.3.1 The RBridge Solution................................83
A.3.2 The VLAN Solution...................................83
A.3.3 The Spanning Tree Solution..........................83
A.3.4 Comparison of Solutions.............................84
Appendix B: Trunk and Access Port Configuration...........85
Appendix C: Multipathing..................................86
Appendix D: Determination of VLAN and Priority............88
Appendix E: Support of IEEE 802.1Q-2005 Amendments........89
E.1 Completed Amendments..................................89
E.2 In-process Amendments.................................90
Appendix Z: Revision History..............................91
Changes from -03 to -04...................................91
Changes from -04 to -05...................................92
Changes from -05 to -06...................................93
Changes from -06 to -07...................................93
Changes from -07 to -08...................................95
Changes from -08 to -09...................................96
Changes from -09 to -10...................................97
Changes from -10 to -11...................................98
Changes from -11 to -12...................................98
Changes from -12 to -13...................................99
Authors' Addresses.......................................101
Copyright and IPR Provisions.............................102
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Table of Figures
Figure 2.1: Interconnected RBridges.......................14
Figure 2.2: An Ethernet Encapsulated TRILL Frame..........14
Figure 2.3: A PPP Encapsulated TRILL Frame................15
Figure 2.4: RBridge Port Model............................19
Figure 3.1: TRILL Header..................................21
Figure 3.2: Options Area Initial Flags Octet..............23
Figure 4.1: TRILL Data Encapsulation over Ethernet........28
Figure 4.2: VLAN Tag Information..........................29
Figure 4.3: TRILL IS-IS Frame Format......................33
Figure 4.4: TRILL ESADI Frame Format......................39
Figure 4.5: Detailed RBridge Port Model...................67
Figure A.1: Link Cost of a Bridged Link...................80
Figure A.2: Wiring Closet Topology........................82
Figure C.1: Multi-Destination Multipath...................86
Figure C.2: Known Unicast Multipath.......................87
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1. Introduction
In traditional IPv4 and IPv6 networks, each subnet has a unique
prefix. Therefore, a node in multiple subnets has multiple IP
addresses, typically one per interface. This also means that when an
interface moves from one subnet to another, it changes its IP
address. Administration of IP networks is complicated because IP
routers require significant configuration. Careful IP address
management is required to avoid creating subnets that are sparsely
populated, wasting addresses.
IEEE 802.1 bridges avoid these problems by transparently gluing many
physical links into what appears to IP to be a single LAN [802.1D].
However, 802.1 bridge forwarding using the spanning tree protocol has
some disadvantages:
o The spanning tree protocol blocks ports, limiting the number of
forwarding links, and therefore creates bottlenecks by
concentrating traffic onto selected links.
o Forwarding is not pair-wise shortest path, but is instead whatever
path remains after the spanning tree eliminates redundant paths.
o The Ethernet header does not contain a hop count (or TTL) field.
This is dangerous when there are temporary loops such as when
spanning tree messages are lost or components such as repeaters
are added.
o VLANs can partition when spanning tree reconfigures due to a node
failure or topology change.
This document presents the design for RBridges (Routing Bridges
[RBridges]) which implement the TRILL protocol and are poetically
summarized below. Rbridges combine the advantages of bridges and
routers and in most cases they can incrementally replace IEEE
[802.1Q-2005] or [802.1D] customer bridges. While they can be
applied to a variety of link protocols, this specification focuses on
IEEE [802.3] links.
For further discussion of the problem domain addressed by RBridges
see [RFC5556].
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1.1 Algorhyme V2, by Ray Perlner
I hope that we shall one day see
A graph more lovely than a tree.
A graph to boost efficiency
While still configuration-free.
A network where RBridges can
Route packets to their target LAN.
The paths they find, to our elation,
Are least cost paths to destination!
With packet hop counts we now see,
The network need not be loop-free!
RBridges work transparently,
Without a common spanning tree.
1.2 Normative Content and Precedence
The bulk of the normative material in this specification appears in
Sections 2, 3, and 4 as follows:
Section 2: general RBridge description
Section 3: the TRILL header
Section 4: other TRILL protocol details
In case of conflict, the order of precedence of these section is as
follows, with those appearing earlier in this list having precedence
over those that appear later:
4 > 3 > 2
1.3 Terminology and Notation in this document
"TRILL" is the protocol specified herein while an "RBridge" is a
devices that implement that protocol. The second letter in Rbridge
is case insensitive. Both Rbridge and RBridge are correct.
In this document, the term "link", unless otherwise qualified, means
"bridged LAN", that is to say, the combination of one or more [802.3]
links with zero or more brides, hubs, repeaters, or the like. The
term "simple link" or the like is used indicate a point-to-point or
multi-access link with no included bridges or RBridges.
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This document uses Hexadecimal Notation for MAC addresses. Two
hexadecimal digits represent each octet (that is, 8-bit byte), giving
the value of the octet as an unsigned integer. A hyphen separates
successive octets. This document consistently uses IETF bit ordering
although the physical order of bit transmission within an octet on an
IEEE [802.3] link is from the lowest order bit to the highest order
bit, the reverse.
In this document, Layer 2 frames are divided into five categories:
o layer 2 control frames (such as BPDUs)
o native frames (non-TRILL-encapsulated data frames)
o TRILL data (TRILL-encapsulated data frames)
o TRILL control frames
o TRILL other frames
The way these five types of frames are distinguished is as follows:
o Layer 2 control frames are those with a multicast destination
address in the range 01-80-C2-00-00-00 to 01-80-C2-00-00-0F or
equal to 01-80-C2-00-00-21. RBridges MUST NOT encapsulate and
forward such frames, though they MAY perform the layer 2
function (such as MAC level security or VLAN registration) of
the control frame. Frames with a destination address of
01-80-C2-00-00-00 (BPDU) or 01-80-C2-00-00-21 (VRP) are called
"high level control" frames in this document. All other layer 2
control frames are called "low level control" frames.
o Native frames are those that are not control frames and have an
Ethertype other than "TRILL" or "L2-IS-IS".
o TRILL data frame have the Ethertype "TRILL".
o TRILL control frames have the Ethertype "L2-IS-IS". In
addition, TRILL control frames, and TRILL data frames, if
multicast, each have distinct multicast destination MAC
addresses, one we call "All-RBridges" (for multicast data) and
"All-IS-IS-RBridges" (for multicast control messages). Note
that ESADI frames look on the outside like TRILL data and are
so handled but, when decapsulated, look like TRILL control.
o TRILL other frames are those with any of the 14 multicast
destination addresses reserved for TRILL other than All-
RBridges and All-IS-IS-RBridges. RBridges conformant to this
specification discard TRILL other frames.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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1.4 Acronyms
AllL1ISs - All Level 1 Intermediate Systems
AllL2ISs - All Level 2 Intermediate Systems
BPDU - Bridge PDU
CHbH - Critical Hop-by-Hop
CItE - Critical Ingress-to-Egress
CSNP - Complete Sequence Number PDU
DA - Destination Address
DR - Designated Router
DRB - Designated RBridge
EAP - Extensible Authentication Protocol
ECMP - Equal Cost Multi-Path
EISS - Extended Internal Sublayer Service
ESADI - End Station Address Distribution Information
FCS - Frame Check Sequence
GARP - Generic Attribute Registration Protocol
GVRP - GARP VLAN Registration Protocol
IEEE - Institute of Electrical and Electronics Engineers
IGMP - Internet Group Management Protocol
IP - Internet Protocol
IS-IS - Intermediate System to Intermediate System
ISS - Internal Sublayer Service
LAN - Local Area Network
LSP - Link State PDU
MAC - Media Access Control
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MLD - Multicast Listener Discovery
MRD - Multicast Router Discovery
MTU - Maximum Transmission Unit
MVRP - Multiple VLAN Registration Protocol
NSAP - Network Service Access Point
P2P - Point-to-point
PDU - Protocol Data Unit
PPP - Point-to-Point Protocol
RBridge - Routing Bridge
RPF - Reverse Path Forwarding
SA - Source Address
SNMP - Simple Network Management Protocol
SPF - Shortest Path First
TLV - Type, Length, Value
TRILL - TRansparent Interconnection of Lots of Links
VLAN - Virtual Local Area Network
VRP - VLAN Registration Protocol
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2. RBridges
This section provides a high-level overview of RBridges, which
implement the TRILL protocol, omitting some details. Sections 3 and 4
below provide the main specification.
RBridges run a link state protocol amongst themselves. This gives
them enough information to compute pair-wise optimal paths for
unicast, and calculate distribution trees for delivery of frames
either to unknown MAC destinations or to multicast/broadcast groups.
[RBridges] [RP1999]
To mitigate temporary loop issues, RBridges forward based on a header
with a hop count. RBridges also specify the next hop RBridge as the
frame destination when forwarding unicast frames across a shared-
media link, which avoids spawning additional copies of frames during
a temporary loop. A Reverse Path Forwarding Check and other checks
are performed on multi-destination frames to further control
potentially looping traffic (see Section 4.5.2).
The first RBridge that a unicast frame encounters in a campus, RB1,
encapsulates the received frame with a TRILL header that specifies
the last RBridge, RB2, where the frame is decapsulated. RB1 is known
as the "ingress RBridge" and RB2 is known as the "egress RBridge".
To save room in the TRILL header and simplify forwarding lookups, a
dynamic nickname acquisition protocol is run among the RBridges to
select 2-octet nicknames for RBridges, unique within the campus,
which are an abbreviation for the 6-octet IS-IS system ID of the
RBridge. The 2-octet nicknames are used to specify the ingress and
egress RBridges in the TRILL header.
Multipathing of multi-destination frames through alternative
distribution tree roots and ECMP (Equal Cost MultiPath) of unicast
frames are supported (see Appendix C).
RBridges run a protocol on a link to elect a "Designated RBridge"
(DRB). The TRILL-IS-IS election protocol on a link is a little
different from the IS-IS [ISO10589] election protocol, because in
TRILL it is essential that only one RBridge be elected DRB, whereas
in layer 3 IS-IS it is possible for multiple routers to be elected
Designated Router (Intermediate System). As with an IS-IS router, the
DRB may give a pseudonode name to the link, issue an LSP (Link State
PDU) on behalf of the pseudonode, and issues CSNPs (Complete Sequence
Number PDUs) on the link. Additionally, the DRB has some TRILL-
specific duties, including specifying which VLAN will be the
Designated VLAN used for communication between RBridges on that link.
The DRB either encapsulates/decapsulates all data traffic to/from the
link, or, for load splitting, delegates this responsibility, for one
or more VLANs, to other RBridges on the link. There must at all
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times be at most one RBridge on the link that encapsulates /
decapsulates traffic for a particular VLAN. We will refer to the
RBridge appointed to forward VLAN-x traffic on behalf of the link as
the "appointed VLAN-x forwarder". (Section 2.3 discusses VLANs
further.)
Rbridges SHOULD support SNMPv3 [RFC3411]. The Rbridge MIB will be
specified in a separate document. If IP service is available to an
RBridge, it SHOULD support SNMPv3 over IP; however, management can be
used, within a campus, even by an RBridge that lacks an IP or other
Layer 3 transport stack or which has zero configuration and thus no
Layer 3 address, by transporting SNMP with Ethernet [RFC4789].
2.1 End Station Addresses
An RBridge, RB1, which is the VLAN-x forwarder on any of its links
MUST learn the location of VLAN-x end nodes, both on the links for
which it is VLAN-x forwarder, and on other links in the campus. RB1
learns the port and Layer 2 (MAC) addresses of end nodes on links for
which it is VLAN-x forwarder from the source address of frames
received, as bridges do (for example, see section 8.7 of
[802.1Q-2005]), or through a Layer 2 explicit registration protocol
such as IEEE 802.11 association and authentication. RB1 learns the
Layer 2 address of distant VLAN-x end nodes, and the corresponding
RBridge to which they are attached, by looking at the ingress RBridge
nickname in the TRILL header and the VLAN and source address of the
inner frame of TRILL data frames that it decapsulates.
Additionally, an RBridge that is the appointed VLAN-x forwarder on
one or more links MAY use the End Station Address Distribution
Information (ESADI) protocol to announce some or all of the attached
VLAN-x end nodes on those links. An ESADI could be used to announce
end nodes that have been explicitly enrolled. Such information might
be more authoritative than that learned from data frames being
decapsulated onto the link. Also, it can be more secure because not
only might the enrollment be authenticated (for example by
cryptographically based EAP methods via [802.1X]), but ESADI also
supports cryptographic authentication of its messages [RFC5304].
Even if an ESADI is used to announce attached end nodes, RBridges
MUST still learn from decapsulating data frames unless configured not
to do so.
Advertising end nodes using ESADI is optional, as is learning from
these announcements.
(See Section 4.8 for further end station address details.)
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2.2 RBridge Encapsulation Architecture
The Layer 2 technology used to connect Rbridges may be either IEEE
[802.3] or some other technology such as PPP [RFC1661]. This is
possible since the RBridge relay functionality is layered on top of
the Layer 2 technologies. However, this document specifies only an
IEEE 802.3 encapsulation.
Figure 2.1 shows two RBridges RB1 and RB2 interconnected through an
Ethernet cloud. The Ethernet cloud may include hubs, point-to-point
or shared media, IEEE 802.1D bridges, or 802.1Q bridges.
------------
/ \
+-----+ / Ethernet \ +-----+
| RB1 |----< >---| RB2 |
+-----+ \ Cloud / +-----+
\ /
------------
Figure 2.1: Interconnected RBridges
Figure 2.2 shows the format of a TRILL data or ESADI frame traveling
through the Ethernet cloud between RB1 and RB2.
+--------------------------------+
| Outer Ethernet Header |
+--------------------------------+
| TRILL Header |
+--------------------------------+
| Inner Ethernet Header |
+--------------------------------+
| Ethernet Payload |
+--------------------------------+
| Ethernet FCS |
+--------------------------------+
Figure 2.2: An Ethernet Encapsulated TRILL Frame
In the case of media different from Ethernet, the outer Ethernet
header is replaced by the header specific to that media. For example,
Figure 2.3 shows a TRILL encapsulation over PPP.
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+--------------------------------+
| PPP Header |
+--------------------------------+
| TRILL Header |
+--------------------------------+
| Inner Ethernet Header |
+--------------------------------+
| Ethernet Payload |
+--------------------------------+
| Ethernet FCS |
+--------------------------------+
Figure 2.3: A PPP Encapsulated TRILL Frame
The outer header is link-specific and, although this document
specifies only Ethernet links, other links are allowed.
In both cases the Inner Ethernet Header and the Ethernet Payload come
from the original frame and are encapsulated with a TRILL Header as
they travel between RBridges. Use of a TRILL header offers the
following benefits:
1. loop mitigation through use of a hop count field;
2. elimination of the need for original source and destination MAC
address learning in transit RBridges;
3. direction of frames towards the egress RBridge (this enables
forwarding tables of RBridges to be sized with the number of
RBridges rather than the total number of end nodes); and,
4. provision of a separate VLAN tag for forwarding traffic between
RBridges, independent of the VLAN of the native frame.
When forwarding unicast frames between RBridges across a shared-
media, the outer header has the MAC destination address of the next
hop Rbridge, to avoid frame duplication. Having the outer header
specify the transmitting RBridge as source address ensures that any
bridges inside the Ethernet cloud will not get confused, as they
might be if multipathing is in use and they were to see the original
source or ingress RBridge in the outer header.
From a forwarding standpoint, transit frames may be classified into
two main categories: known-unicast and multi-destination.
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2.2.1 Known-Unicast
These frames have a unicast inner MAC destination address
(Inner.MacDA) and are those for which ingress RBridge knows the
egress RBridge for that destination MAC address.
Such frames are forwarded Rbridge hop by Rbridge hop to their egress
Rbridge.
2.2.2 Multi-destination
These are frames that must be delivered to multiple destinations.
Multi-destination frames include the following:
1. unicast frames for which the destination is unknown: the
Inner.MacDA is unicast, but the ingress RBridge does not know its
location;
2. multicast frames for which the Layer 2 destination address is
derived from an IP multicast address: the Inner.MacDA is
multicast, from the set of Layer 2 multicast addresses derived
from IPv4 [RFC1112] or IPv6 [RFC2464] multicast addresses; these
frames are handled somewhat differently in different subcases:
2.1 IGMP [RFC3376] and MLD [RFC2710] multicast group membership
reports;
2.2 IGMP [RFC3376] and MLD [RFC2710] queries and MRD [RFC4286]
announcement messages;
2.3 other IP derived Layer 2 multicast frames;
3. multicast frames for which the Layer 2 destination address is not
derived from an IP multicast address: the Inner.MacDA is
multicast, and not from the set of Layer 2 multicast addresses
derived from IPv4 or IPv6 multicast addresses;
4. broadcast frames: the Inner.MacDA is broadcast (FF-FF-FF-FF-FF-
FF).
RBridges build distribution trees (see Section 4.5) and use these
trees for forwarding multi-destination frames. These distribution
trees are pruned in different ways for different cases to avoid
unnecessary propagation of the frame.
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2.3 RBridges and VLANs
A VLAN is a way to partition end nodes in a campus into different
Layer 2 communities [802.1Q-2005]. Use of VLANs requires
configuration. By default, the port on which it is initially
received determines the VLAN of a frame sent by an end station. End
stations can also explicitly insert this information in a frame.
IEEE 802.1Q bridges can be configured to support multiple customer
VLANs over a single simple link by inserting/removing a VLAN tag in
the frame. VLAN tags used by TRILL have the same format as VLAN tags
defined in IEEE [802.1Q-2005]. As shown in Figure 2.2 there are two
places where such tags may be present in a TRILL-encapsulated frame
sent over an IEEE [802.3] link: one in the outer header (Outer.VLAN)
and one in the inner header (Inner.VLAN). Inner and outer VLANs are
further discussed in Section 4.1.
RBridges enforce delivery of a native frame originating in a
particular VLAN only to other links in the same VLAN; however, there
are a few differences in the handling of VLANs between an RBridge
campus and an 802.1 bridged LAN as described below.
(See Section 4.2.4 for further discussion of TRILL IS-IS operation on
a link.)
2.3.1 Link VLAN Assumptions
Certain configurations of bridges may cause partitions of a VLAN on a
link. In that case, a frame sent by one RBridge to a neighbor on that
link, might not arrive, if tagged with a VLAN that is partitioned due
to bridge configuration.
TRILL requires at least one VLAN that gives full connectivity to all
the RBridges on each link in the campus. The default VLAN is 1,
though RBridges may be configured to use a different VLAN. The DRB
dictates to the other RBridges which VLAN to use.
Since there will be only one appointed forwarder for any VLAN, say
VLAN-x, on a link, if bridges are configured to cause VLAN-x to be
partitioned on a link, some VLAN-x end nodes on that link may be
orphaned (unable to communicate with the rest of the campus).
It is possible for bridge and port configuration to cause VLAN
mapping on a link (where a VLAN-x frame turns into a VLAN-y frame).
TRILL detects this by inserting a copy of the outer VLAN into TRILL-
Hello messages and checking it on receipt. If detected, it takes
steps to ensure that there is at most a single appointed forwarder on
the link, to avoid possible frame duplication or loops (see Section
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4.2.4.4).
TRILL behaves as conservatively as possible, avoiding loops rather
than avoiding partial connectivity. As a result, lack of connectivity
may result from bridge or port misconfiguration.
2.4 RBridges and IEEE 802.1 Bridges
As described below, RBridge ports are, for the most part, layered on
top of IEEE [802.1Q-2005] port facilities and RBridges can be
incrementally deployed into an existing bridged LAN.
2.4.1 RBridge and 802.1 Layering
RBridges ports make use of [802.1Q-2005] port VLAN and priority
processing. In addition, they MAY implement other lower level 802.1
protocols as well as the protocols for the link in use, such as port
based access control [802.1X] or link aggregation (Clause 43 of
[802.3]). There may in the future be lower level 802.1 protocols
whose support requires modified handling in an RBridge. (See Appendix
E.)
However, RBridges do not use spanning tree and do not block ports as
spanning tree does. Figure 2.4 shows a high-level diagram of an
RBridge port connected to an IEEE 802.3 link. Single lines represent
the flow of control information, double lines the flow of both frames
and control information.
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+-----------------------------------------
| RBridge
|
| Forwarding Engine, IS-IS, Etc.
| Processing of native and TRILL frames
|
+----+---+--------++----------------------
| | || other ports...
+-------------+ | ||
| | ||
+------------+-------------+ | ||
| RBridge | | +----++-------+ <- EISS
| | | | |
| High-level Control Frame | | | 802.1Q-2005 |
| Processing (BPDU, VRP) | | | Port VLAN |
| | | | & Priority |
+-----------++-------------+ | | Processing |
|| | | |
+---------++-----------------+---+-------------+ <-- ISS
| |
| 802.1/802.3 Low Level Control Frame |
| Processing, Port/Link Control Logic |
| |
+-----------++---------------------------------+
||
|| +------------+
|| | 802.3 PHY |
|+--------+ (Physical +--------- 802.3
+---------+ Interface) +--------- Link
| |
+------------+
Figure 2.4: RBridge Port Model
The upper interface to the lower level port/link control logic
corresponds to the Internal Sublayer Service (ISS) in [802.1Q-2005].
In RBridges, high-level control frames are processed above the ISS
interface.
The upper interface to the port VLAN and priority processing
corresponds to the Extended Internal Sublayer Service (EISS) in
[802.1Q-2005]. In RBridges, native and TRILL frames are processed
above the EISS interface and are subject to port VLAN and priority
processing.
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2.4.2 Incremental Deployment
Because RBridges are generally compatible with IEEE [802.1Q-2005]
customer bridges, a bridged LAN can be upgraded by incrementally
replacing such bridges with RBridges. Bridges that have not yet been
replaced are transparent to RBridge traffic. The physical links
directly interconnected by such bridges, together with the bridges
themselves, constitute bridged LANs. These bridged LANs appear to
RBridges to be multi-access links. If the bridges replaced by
RBridges were zero configuration bridges, then their RBridge
replacements will not require configuration.
The RBridge campus will work best if all IEEE 802.1D and 802.1Q-2005
bridges are replaced with RBridges, assuming the RBridges have the
same speed and capacity as the bridges. However, there may be
intermediate states, where only some bridges have been replaced by
RBridges, with inferior performance.
See Appendix A for further discussion of incremental deployment.
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3. Details of the TRILL Header
This section specifies the TRILL header. Section 4 below provides
other RBridge design details.
3.1 TRILL Header Format
The TRILL header is shown in Figure 3.1 and is independent of the
data link layer used. When that layer is IEEE [802.3], it is prefixed
with the 16-bit TRILL Ethertype [RFC5342], making it 64 bit aligned.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| V | R |M|Op-Length| Hop Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Egress RBridge Nickname | Ingress RBridge Nickname |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3.1: TRILL Header
The header contains the following fields that are described in the
sections referenced:
o V (Version): 2-bit unsigned integer. See Section 3.2.
o R (Reserved): 2 bits. See Section 3.3.
o M (Multi-destination): 1 bit. See Section 3.4.
o Op-Length (Options Length): 5-bit unsigned integer. See Section
3.5.
o Hop Count: 6-bit unsigned integer. See Section 3.6.
o Egress RBridge Nickname: 16-bit identifier. See Section 3.7.1.
o Ingress RBridge Nickname: 16-bit identifier. See Section 3.7.2.
3.2 Version (V)
Version (V) is a two-bit field. Version zero of TRILL is specified in
this document. An RBridge RB1 MUST check the V field in a received
TRILL-encapsulated frame. If the V field has a value not recognized
by RB1, then RB1 MUST silently discard the frame.
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3.3 Reserved (R)
The two R bits are reserved for future use in extensions to this
version zero of the TRILL protocol. They MUST be initially set to
zero, transparently copied by transit RBridges, and ignored on
receipt.
3.4 Multi-destination (M)
The Multi-destination bit (see Section 2.2.2) indicates that the
frame is to be delivered to a class of destination end stations via a
distribution tree and that the egress RBridge nickname field
specifies the root for this tree. In particular:
o M = 0 (FALSE) - The egress RBridge nickname contains a nickname of
the egress Rbridge for a known unicast TRILL data frame;
o M = 1 (TRUE) - The egress RBridge nickname field contains a
nickname of the RBridge that is the root of a distribution tree.
This nickname is selected by the ingress RBridge for a TRILL data
frame or by the source RBridge for a TRILL ESADI frame.
3.5 TRILL Header Options
There are provisions to express in the TRILL Header that a frame is
using an optional capability and to encode information into the
header in connection with that capability.
The Op-Length header field gives the length of the expressed options
in units of 4 octets, which allows up to 124 octets of options area.
If Op-Length is zero there are no options expressed. If options are
expressed, they follow immediately after the Ingress Rbridge Nickname
field.
All Rbridges MUST be able to skip the number of 4-octet chunks
indicated by the Op-Length field in order to find the inner frame,
since RBridges must be able to find the destination MAC address and
VLAN tag in the inner frame. (Transit RBridges need such information
to filter VLANs, IP multicast, and the like. Egress Rbridges need to
find the inner header to correctly decapsulate and handle the inner
frame.)
To ensure backward compatible safe operation, when Op-Length is non-
zero indicating that options are present, the top two bits of the
first octet of the options area are specified as follows:
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+------+------+----+----+----+----+----+----+
| CHbH | CItE | Reserved |
+------+------+----+----+----+----+----+----+
Figure 3.2: Options Area Initial Flags Octet
If the CHbH (Critical Hop by Hop) bit is one, one or more critical
hop-by-hop options are present so transit RBridges that support no
options MUST drop the frame. If the CHbH bit is zero, the frame is
safe, from the point of view of options processing, for a transit
RBridge to forward, even if the forwarding RBridge doesn't understand
any options. A transit RBridge that supports no options and forwards
a frame MUST transparently forward the options area.
If the CItE (Critical Ingress to Egress) bit is a one, one or more
critical ingress-to-egress options are present. If it is zero, no
such options are present. If either CHbH or CItE is non-zero, egress
RBridges that support no options MUST drop the frame. If both CHbH
and CItE are zero, the frame is safe, from the point of view of
options, for any egress RBridge to process, even if it doesn't
understand any options.
Options will be further specified in other documents and are expected
to include provisions for hop-by-hop and ingress-to-egress options as
well as critical and non-critical options.
Note: Most RBridge implementations are expected to be optimized for
the simplest and most common cases of frame forwarding and
processing. The inclusion of any options may, and the inclusion of
complex or lengthy options very likely will, cause frame
processing using a "slow path" with markedly inferior performance
to "fast path" processing. Limited slow path throughput may cause
such frames to be lost.
3.6 Hop Count
The Hop Count field is a 6-bit unsigned integer. An Rbridge drops
frames received with a hop count of zero, otherwise it decrements the
hop count. (This behavior is different from IPv4 and IPv6 in order
to support the later addition of a traceroute-like facility that
would be able to get a hop count exceeded from an egress RBridge.)
For known unicast frames, the ingress RBridge SHOULD set the Hop
Count in excess of the number of RBridge hops it expects to the
egress RBridge to allow for alternate routing later in the path.
For multi-destination frames, the Hop Count SHOULD be set by the
ingress RBridge (or source RBridge for a TRILL ESADI frame) to at
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least the expected number of hops to the most distant RBridge. To
accomplish this, RBridge RBn calculates, for each branch from RBn of
the specified distribution tree rooted at RBi, the maximum number of
hops in that branch. When forwarding a multi-destination frame onto a
branch, transit RBridge RBm MAY decrease the hop count by more than 1
unless decreasing the hop count by more than 1 would result in a Hop
Count insufficient to reach all destinations in that branch of the
tree rooted at RBi. Using a Hop Count close or equal to the minimum
needed on multi-destination frames reduces potential problems with
temporary loops when forwarding.
Although the RBridge MAY decrease the hop count by more than 1, under
the circumstances described above, the RBridge forwarding a frame
MUST decrease the hop count by at least 1, and discards the frame if
it cannot do so because the hop count is 0.
3.7 RBridge Nicknames
Nicknames are 16-bit dynamically assigned quantities that act as
abbreviations for RBridge's 48-bit IS-IS System ID to achieve a more
compact encoding and can be used to specify potentially different
trees with the same root. This assignment allows specifying up to
2**16 RBridges; however, the value 0x0000 is reserved to indicate
that a nickname is not specified, the values 0xFFC0 through 0xFFFE
are reserved for future specification, and the value 0xFFFF is
permanently reserved. RBridges piggyback a nickname acquisition
protocol on the link state protocol (see Section 3.7.3) to acquire
one or more nicknames unique within the campus.
3.7.1 Egress RBridge Nickname
There are two cases for the contents of the egress RBridge nickname
field, depending on the M-bit (see Section 3.4). It is filled in by
the ingress RBridge for TRILL data frames and by the source RBridge
for TRILL ESADI frames.
o For known unicast TRILL data frames, M == 0 and the egress RBridge
nickname field specifies the egress RBridge i.e. it specifies the
RBridge that needs to remove the TRILL encapsulation and forward
the native frame. Once the egress nickname field is set, it MUST
NOT be changed by any subsequent transit RBridge.
o For multi-destination TRILL data frames and for TRILL ESADI
frames, M == 1. The egress RBridge nickname field contains a
nickname of the root RBridge of the distribution tree selected to
be used to forward the frame. This root nickname MUST NOT be
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changed by transit RBridges.
3.7.2 Ingress RBridge Nickname
The ingress RBridge nickname is set to a nickname of the ingress
RBridge for TRILL data frames and to a nickname of the source RBridge
for TRILL ESADI frames.
Once the ingress nickname field is set, it MUST NOT be changed by any
subsequent transit RBridge.
3.7.3 RBridge Nickname Selection
The nickname selection protocol is piggybacked on TRILL IS-IS as
follows:
o The nickname or nicknames being used by an RBridge are carried in
an IS-IS TLV (type-length-value data element) along with a
priority of use value. Each RBridge chooses its own nickname or
nicknames.
o Nickname values MAY be configured. An RBridge that has been
configured with one or more nickname values will have priority for
those nickname values over all Rbridges with non-configured
nicknames.
o The nickname values 0x0000 and 0xFFC0 through 0xFFFF are reserved
and MUST NOT be selected by or configured for an RBridge. The
value 0x0000 is used to indicate that a nickname is not known.
o The priority of use field reported with a nickname is an unsigned
8-bit value, where the most significant bit (0x80) indicates that
the nickname value was configured. The bottom 7 bits have the
default value 0x40, but MAY be configured to be some other value.
Additionally, an RBridge MAY increase its priority after holding a
nickname for some amount of time. However, the most significant
bit of the priority MUST NOT be set unless the nickname value was
configured.
o Once an RBridge has successfully acquired a nickname it SHOULD
attempt to reuse it in the case of a reboot.
o Each RBridge is responsible for ensuring that its nickname or each
of its nicknames is unique. If RB1 chooses nickname x, and RB1
discovers, through receipt of RB2's LSP, that RB2 has also chosen
x, then the RBridge with the numerically higher priority keeps the
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nickname, or if there is a tie in priority, the RBridge with the
numerically higher IS-IS System ID keeps the nickname, and the
other RBridge MUST select a new nickname. This can require an
RBridge with a configured nickname to select a replacement
nickname.
o To minimize the probability of nickname collisions, when an
RBridge selects a new nickname, it does so by randomly hashing
some of its parameters, e.g., interface MAC addresses, time and
date, and other entropy sources such as those given in [RFC4086].
There is no reason for all Rbridges to use the same algorithm for
selecting nicknames.
o If two RBridge campuses merge, then transient nickname collisions
are possible. As soon as each RBridge receives the LSPs from the
other RBridges, the RBridges that need to change nicknames select
new nicknames that do not, to the best of their knowledge, collide
with any existing nicknames. Some RBridges may need to change
nicknames more than once before the situation is resolved.
o To minimize the probability of a new RBridge usurping a nickname
already in use, an RBridge SHOULD wait to acquire the link state
database from a neighbor before it announces any nicknames that
were not configured.
An RBridge MAY request multiple nicknames so that it can be the root
of multiple trees for multipathing of multi-destination frames. These
trees would all be shortest path trees from the RBridge but, since
the tree number is used in tie breaking when there are multiple equal
cost paths (see Section 4.5.1), the different trees will likely
utilize different links.
If it is desired for a pseudonode to be a tree root, the DRB MAY
request one or more nicknames in the pseudonode LSP.
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4. Other RBridge Design Details
Section 3 above specifies the TRILL header, while this Section
specifies other RBridge design details.
4.1 Ethernet Data Encapsulation
TRILL data and ESADI frames in transit on Ethernet links are
encapsulated with an outer Ethernet header (see Figure 2.2). This
outer header looks, to a bridge on the path between two RBridges,
like the header of a regular Ethernet frame and therefore bridges
forward the frame as they normally would. To enable RBridges to
distinguish such TRILL frames, a new TRILL Ethertype (see Section
7.2) is used in the outer header.
Figure 4.1 details a TRILL data frame with an outer VLAN tag
traveling on an Ethernet link as shown at the top of the Figure, that
is, between transit RBridges RB3 and RB4. The native frame originated
at end station ESa, was encapsulated by ingress RBridge RB1 and will
ultimately be decapsulated by egress RBridge RB2 and delivered to
destination end station ESb. The encapsulation shown has the
advantage, in the absence of TRILL options, of aligning the original
Ethernet frame at a 64-bit boundary.
When a TRILL data frame is carried over an Ethernet cloud it has
three pairs of addresses:
o Outer Ethernet Header: Outer Destination MAC Address (Outer.MacDA)
and Outer Source MAC Address (Outer.MacSA): These addresses are
used to specify the next hop RBridge and the transmitting RBridge,
respectively.
o TRILL Header: Egress Nickname and Ingress Nickname. These specify
nickname values of the egress and ingress RBridges, respectively,
unless the frame is multi-destination, in which case the Egress
Nickname specifies the root of the distribution tree on which the
frame is being sent.
o Inner Ethernet Header: Inner Destination MAC Address (Inner.MacDA)
and Inner Source MAC Address (Inner.MacSA): These addresses are as
transmitted by the original end station, specifying, respectively,
the destination and source of the inner frame.
A TRILL data frame also potentially has two VLAN tags that can carry
two different VLAN Identifiers and specify priority.
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Flow:
+----+ +-------+ +-------+ +-------+ +-------+ +----+
|ESa +--+ RB1 +---+ RB3 +-------+ RB4 +---+ RB2 +--+ESb |
+----+ |ingress| |transit| ^ |transit| |egress | +----+
+-------+ +-------+ | +-------+ +-------+
|
Outer Ethernet Header: |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Outer Destination MAC Address (RB4) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Outer Destination MAC Address | Outer Source MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Outer Source MAC Address (RB3) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ethertype = C-Tag [802.1Q] | Outer.VLAN Tag Information |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TRILL Header:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ethertype = TRILL | V | R |M|Op-Length| Hop Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Egress (RB2) Nickname | Ingress (RB1) Nickname |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Inner Ethernet Header:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Inner Destination MAC Address (ESb) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Inner Destination MAC Address | Inner Source MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Inner Source MAC Address (ESa) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ethertype = C-Tag [802.1Q] | Inner.VLAN Tag Information |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Payload:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ethertype of Original Payload | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Original Ethernet Payload |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Frame Check Sequence:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| New FCS (Frame Check Sequence) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4.1: TRILL Data Encapsulation over Ethernet
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4.1.1 VLAN Tag Information
A "VLAN Tag" (formerly known as a Q-tag), also known as a "C-tag" for
customer tag, includes a VLAN ID and a priority field as shown in
Figure 4.2. The "VLAN ID" may be zero, indicating the no VLAN is
specified, just a priority, although such frames are called "priority
tagged" rather than "VLAN tagged" [802.1Q-2005].
[802.1Qad] S-tags or service tags are beyond the scope of this
document.
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| Priority | C | VLAN ID |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
Figure 4.2: VLAN Tag Information
As recommended in [802.1Q-2005], Rbridges SHOULD be implemented so as
to allow use of the full range of VLAN IDs from 0x001 through 0xFFE.
Rbridges MAY support a smaller number of simultaneously active VLAN
IDs. VLAN ID zero is the null VLAN identifier and indicates that no
VLAN is specified while VLAN ID 0xFFF is reserved.
The VLAN ID 0xFFF MUST NOT be used. Rbridges MUST discard any frame
they receive with an Outer.VLAN ID of 0xFFF. Rbridges MUST discard
any frame for which they examine the Inner.VLAN ID and find it to be
0xFFF; such examination is required at all egress Rbridges which
decapsulate a frame.
The "C" bit shown in Figure 4.2 is not used in TRILL. It MUST be set
to zero and is ignored by receivers.
As specified in [802.1Q-2005], the priority field contains an
unsigned value from 0 through 7 where 1 indicates the lowest
priority, 7 the highest priority, and the default priority zero is
considered to be higher than priority 1 but lower than priority 2.
The [802.1ad] amendment to [802.1Q-2005] permits mapping some
adjacent pairs of priority levels into a single priority level with
and without drop eligibility. Ongoing work in IEEE 802.1 (802.1az,
Appendix E) suggests the ability to configure "priority groups" that
have a certain guaranteed bandwidth. RBridges ports MAY also
implement such options. RBridges are not required to implement any
particular number of distinct priority levels but may treat one or
more adjacent priority levels in the same fashion.
Frames with the same source address, destination address, VLAN, and
priority that are received on the same port as each other and are
transmitted on the same port MUST be transmitted in the order
received unless the RBridge classifies the frames into more fine
grained flows, in which case this ordering requirement applies to
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each such flow. (Such frames might not be sent out the same port if
multipath is implemented. See Appendix C.)
The C-Tag Ethertype [RFC5342] is 0x8100.
4.1.2 Inner VLAN Tag
The "Inner VLAN Tag Information" (Inner.VLAN) field contains the VLAN
tag information associated with the native frame when it was
ingressed or the VLAN tag information associated with a TRILL ESADI
frame when that frame was created. When a TRILL frame passes through
a transit RBridge, the Inner.VLAN MUST NOT be changed except when
VLAN mapping is being intentionally performed within that RBridge.
When a native frame arrives at an RBridge, the associated VLAN ID and
priority are determined as specified in [802.1Q-2005] (see Appendix D
and [802.1Q-2005] Section 6.7). If the RBridge is an appointed
forwarder for that VLAN and the delivery of the frame requires
transmission to one or more other links, this ingress RBridge forms a
TRILL data frame with the associated VLAN ID and priority placed in
the Inner.VLAN information.
The VLAN ID is required at the ingress Rbridge as one element in
determining the appropriate egress Rbridge for a known unicast frame
and is needed at the ingress and every transit Rbridge for multi-
destination frames to correctly prune the distribution tree.
4.1.3 Outer VLAN Tag
TRILL frames sent by an RBridge, except for some TRILL-Hello frames,
use an Outer.VLAN ID specified by the Designated RBridge (DRB) for
the link onto which they are being sent, referred to as the
Designated VLAN. For TRILL data and ESADI frames, the priority in the
Outer.VLAN tag SHOULD be set to the priority in the Inner.VLAN tag.
TRILL frames forwarded by a transit RBridge use the priority present
in the Inner.VLAN of the frame as received. TRILL data frames are
sent with the priority associated with the corresponding native frame
when received (see Appendix D). TRILL IS-IS frames SHOULD be sent
with priority 7.
Whether an Outer.VLAN tag actually appears on the wire when a TRILL
frame is sent depends on the configuration of the RBridge port
through which it is sent in the same way as the appearance of a VLAN
tag on a frame sent by an [802.1Q-2005] frame depends on the
configuration of the bridge port (see Section 4.9.2).
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4.1.4 Frame Check Sequence (FCS)
Each Ethernet frame has a single Frame Check Sequence (FCS) that is
computed to cover the entire frame, for detecting frame corruption
due to bit errors on a link. Any received frame for which the FCS
check fails SHOULD be discarded (this may not be possible in the case
of cut through forwarding). The FCS normally changes on
encapsulation, decapsulation, and every TRILL hop due to changes in
the outer destination and source addresses, the decrementing of the
hop count, etc.
Although the FCS is normally calculated just before transmission, it
is desirable, when practical, for an FCS to accompany a frame within
an RBridge after receipt. That FCS could then be dynamically updated
to account for changes to the frame during Rbridge processing and
used for transmission or checked against the FCS calculated for frame
transmission. This optional, more continuous use of an FCS would be
helpful in detecting some internal RBridge failures such as memory
errors.
4.2 Link State Protocol (IS-IS)
TRILL uses an extension of IS-IS [ISO10589] as its routing protocol.
IS-IS has the following advantages:
o it runs directly over Layer 2, so therefore it may be run with
zero configuration (no IP addresses need to be assigned);
o it is easy to extend by defining new TLV (type-length-value) data
elements and sub-elements for carrying TRILL information;
This section describes TRILL use of IS-IS, except for the TRILL-Hello
protocol, which is described in Section 4.4, and the MTU-probe and
MTU-ack messages that are described in Section 4.3.
4.2.1 IS-IS RBridge Identity
Each RBridge has a unique unsigned 48-bit (6-octet) IS-IS System ID.
This ID may be derived from any of the RBridge's unique MAC
addresses.
A pseudonode is assigned a 7-octet ID by the DRB that created it, by
taking a 6-octet ID owned by the DRB, and appending another octet.
The 6-octet ID used to form a pseudonode ID SHOULD be the DRB's ID
unless the DRB has to create IDs for pseudonodes for more than 255
links. The only constraint for correct operations is that the 7-octet
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ID be unique within the campus, and that the 7th octet be nonzero. An
RBridge has a 7-octet ID consisting of its 6-octet system ID
concatenated with a zero octet.
In this document we use the term "IS-IS ID" to refer to the 7-octet
quantity that can either be the ID of an RBridge or a pseudonode.
4.2.2 IS-IS Instances
TRILL implements a separate IS-IS instance from any used by Layer 3,
that is, different from the one used by routers. Layer 3 IS-IS frames
must be distinguished from TRILL IS-IS frames even when those Layer 3
IS-IS frames are transiting an RBridge campus.
Layer 3 IS-IS native frames have special multicast destination
addresses specified for that purpose, such as AllL1ISs or AllL2ISs.
When they are TRILL encapsulated, these multicast addresses appear as
the Inner.MacDA and the Outer.MacDA will be the All-RBridges
multicast address.
Within TRILL, there is an IS-IS instance across all Rbridges in the
campus as described in Section 4.2.3. This instance uses TRILL IS-IS
frames that are distinguished by having a different Ethertype "L2-IS-
IS". Additionally, for TRILL IS-IS frames that are multicast, there
is a distinct multicast destination address of All-IS-IS-RBridges.
TRILL IS-IS frames do not have a TRILL Header.
ESADI is a separate protocol from the IS-IS instance implemented by
all the RBridges. There is a separate ESADI instance for each VLAN,
and ESADI frames are encapsulated just like TRILL data frames. After
the TRILL header, the ESADI frame has an inner Ethernet header with
the Inner.MacDA of "All-ESADI-RBridges" and the "L2-IS-IS" Ethertype
followed by the ESADI frame.
4.2.3 TRILL IS-IS Frames
All Rbridges must participate in the TRILL IS-IS instance. TRILL IS-
IS frames are never forwarded by an RBridge but are locally processed
on receipt. (Such processing may cause the RBridge to send additional
TRILL IS-IS frames.)
A TRILL IS-IS frame on an 802.3 link is structured as shown below.
The RBridge port out which such a frame is sent may strip the outer
VLAN tag.
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Outer Ethernet Header:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| All-IS-IS-RBridges Multicast Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| All-IS-IS-RBridges continued | Source RBridge MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source RBridge MAC Address continued |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ethertype = C-Tag [802.1Q] | Outer.VLAN Tag Information |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| L2-IS-IS Ethertype |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IS-IS Payload:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IS-IS Common Header, IS-IS PDU Specific Fields, IS-IS TLVs |
Frame Check Sequence:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FCS (Frame Check Sequence) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4.3: TRILL IS-IS Frame Format
The VLAN specified in the Outer.VLAN information will be the
Designated VLAN for the link on which the frame is sent, except in
the case of some TRILL-Hellos.
4.2.4 TRILL Link Hellos, DRBs, and Appointed Forwarders
RBridges default to using TRILL Hellos unless, on a per port basis,
they are configured to use P2P Hellos. TRILL-Hello frames are
specified in Section 4.4.
RBridges are normally configured to use P2P Hellos only when there
are exactly two of them on a link. However, it can occur that
RBridges are misconfigured as to which type of hello to use. This is
safe but may cause lack of RBridge to RBridge connectivity. An
RBridge configured to use P2P Hellos ignores TRILL Hellos and an
RBridge configured to use TRILL Hellos ignores P2P Hellos.
If any of the RBridges on a link is configured to use TRILL Hellos,
one of such RBridges using TRILL Hellos is elected DRB (Designated
RBridge). This election is based on configured priority (most
significant field), and source MAC address, as communicated by TRILL-
Hello frames. The DRB, as described in Section 4.2.4.2, designates
the VLAN to be used on the link for inter-RBridge communication by
the non-P2P RBridges and appoints itself or other RBridges on the
link as appointed forwarder (see Section 4.2.4.3) for VLANs on the
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link.
4.2.4.1 P2P Hello Links
RBridge ports can be configured to use IS-IS P2P Hellos. This implies
that the port is a point-to-point link to another RBridge. An RBridge
MUST NOT provide any end station (native frame) service on a port
configured to use P2P Hellos.
As with Layer 3 IS-IS, such P2P ports do not participate in a DRB
election. They send all frames VLAN tagged as being in the Desired
Designated VLAN configured for the port. Since all traffic through
the port should be TRILL frames or layer 2 control frames, such a
port cannot be an appointed forwarder. RBridge P2P ports MUST use the
IS-IS three-way handshake so that an extended circuit ID is
associated with the link for tie breaking purposes (see Section
4.5.2).
Even if all simple links in a network are physically point-to-point,
if some of the nodes are bridges, the bridged LANs that include those
bridges appear to be multi-access link to attached RBridges. This
would necessitate using TRILL-Hellos for proper operation in many
cases.
While it is safe to erroneously configure ports as P2P, this may
result in lack of connectivity.
4.2.4.2 Designated RBridge
TRILL IS-IS elects one RBridge for each LAN link to be the Designated
RBridge (DRB), that is, to have special duties. The Designated
RBridge:
o Chooses, for the link, and announces in its Hellos, the Designated
VLAN ID to be used for inter-RBridge communication. This VLAN is
used for all TRILL-encapsulated data and ESADI frames and TRILL
IS-IS frames except some TRILL-Hello frames.
o If the link is represented in the IS-IS topology as a pseudonode,
chooses a pseudonode ID and announces that in its Hellos and
issues an LSP on behalf of the pseudonode.
o Issues CSNPs.
o For each VLAN-x appearing on the link, chooses an RBridge on the
link to be the appointed VLAN-x forwarder (the DRB MAY choose
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itself to be the appointed VLAN-x forwarder for all or some of the
VLANs).
o Before appointing a VLAN-x forwarder (including appointing
itself), wait at least its Holding Time (to ensure it is DRB).
o If configured to send TRILL-Hello frames, continues to send them
on all its enabled VLANs that have been configured in the
Announcing VLANs set of the DRB, which defaults to all enabled
VLANs.
4.2.4.3 Appointed VLAN-x Forwarder
The appointed VLAN-x forwarder for a link is responsible for the
following points. In connection with the loop avoidance points, when
an appointed forwarder for a port is "inhibited", it drops any native
frames it receives and does not transmit but instead drops any native
frames it decapsulates, in the VLAN for which it is appointed
o Loop avoidance:
- Inhibiting itself for a configurable time from zero to 30
seconds, which defaults to 30 second, after it sees a root
bridge change on the link (see Section 4.9.3.2).
- Inhibiting itself for VLAN-x, if it has received a Hello in
which the sender asserts that it is appointed forwarder and
that is either
+ received on VLAN-x (has VLAN-x as its Outer.VLAN) or
+ was originally sent on VLAN-x as indicated inside the body
of the Hello.
- Optionally, not decapsulating a frame from ingress RBridge RBm
unless it has RBm's LSP, and the root bridge on the link it is
about to forward onto is not listed in RBm's list of root
bridges for VLAN-x. This is known as the "decapsulation check"
or "root bridge collision check".
o Unless inhibited (see above), receiving VLAN-x native traffic from
the link and, forwarding it as appropriate.
o Receiving VLAN-x traffic for the link and, if uninhibited,
transmitting it in native form after decapsulating it as
appropriate.
o Learning the MAC address of local VLAN-x nodes by looking at the
source address of VLAN-x frames from the link.
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o Optionally learning the port of local VLAN-x nodes based on any
sort of Layer 2 registration protocols, such as IEEE 802.11
association and authentication.
o Keeping track of the { egress RBridge, VLAN, MAC address } of
distant VLAN-x end nodes, learned by looking at the fields {
ingress RBridge, Inner.VLAN ID, Inner.MacSA } from VLAN-x frames
being received for decapsulation onto the link.
o Optionally observe native IGMP [RFC3376], MLD [RFC2710], and MRD
[RFC4286] frames to learn the presence of local multicast
listeners and multicast routers.
o Optionally listening to TRILL ESADI messages for VLAN-x to learn {
egress RBridge, VLAN-x, MAC address } triplets and the confidence
level of such explicitly advertised end nodes.
o Optionally advertising VLAN-x end nodes, on links for which it is
appointed VLAN-x forwarder, in ESADI messages.
o Send TRILL-Hello frames on VLAN-x.
o Listening to BPDUs on the common spanning tree to learn the root
bridge, if any, for that link and to report in its LSP the
complete set of root bridges seen on any of its links for which it
is appointed forwarder for VLAN-x.
When an appointed forwarder observes that the DRB on a link has
changed, it no longer considers itself appointed for that link until
appointed by the new DRB.
4.2.4.4 TRILL LSP Information
The information in the TRILL IS-IS LSP is listed below. The actual
encoding of this information and the IS-IS Type or sub-Type values
for any new IS-IS TLV or sub-TLV data elements are specified in a
separate document.
1. The IS-IS IDs of neighbors (pseudonodes as well as RBridges) of
RBridge RBn, and the cost of the link to each of those neighbors.
RBridges MUST use the Extended IS Reachability TLV (#22, also
known as "wide metric" [RFC5305]) and MUST NOT use the IS
Reachability TLV (#2, also known as "narrow metric").
2. In connection with the nickname or each of the nicknames of
RBridge RBn:
2.1 The nickname value (2 octets).
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2.2 The unsigned 8-bit priority for RBn to have that nickname (see
Section 3.7.3).
3. The maximum TRILL Header Version supported by RBridge RBn.
4. The following information in connection with distribution tree
determination and announcement. (See Section 4.5 for further
details on how this information is used.)
4.1 The 16-bit unsigned priority of that nickname to becoming a
distribution tree root.
4.2 A second unsigned 16-bit number that is the number of trees
all RBridges in the campus calculate if RBn is highest
priority.
4.3 A third unsigned 16-bit number that is the number of trees RBn
would like to use.
4.4 A forth unsigned 16-bit number that is the maximum number of
distribution trees that RBn is able to calculate.
4.5. A first list of nicknames that are intended roots of
distribution trees all RBridges in the campus must calculate.
4.6 A second list of nicknames that are roots that RBn would like
to use when ingressing multi-destination frames.
5. The list of VLAN IDs of VLANs directly connected to RBn for links
on which RBn is the appointed forwarder for that VLAN. (Note: an
RBridge may advertise that it is connected to additional VLANs in
order to receive additional frames to support certain VLAN based
features beyond the scope of this specification as mentioned in
Section 4.8.3 and in a separate document concerning VLAN mapping
inside RBridges.) In addition, the LSP contains the following
information on a per-VLAN basis:
5.1 Per VLAN Multicast Router attached flags: This is two bits of
information that indicate whether there is an IPv4 and/or IPv6
multicast router attached to the Rbridge on that VLAN. An
RBridge that does not do IP multicast control snooping MUST
set both of these bits (see Section 4.5.4). This information
is used because IGMP [RFC3376] and MLD [RFC2710] Membership
Reports MUST be transmitted to all links with IP multicast
routers, and SHOULD NOT be transmitted to links without such
routers. Also, all frames for IP-derived multicast addresses
MUST be transmitted to all links with IP multicast routers
(within a VLAN), in addition to links from which an IP node
has explicitly asked to join the group the frame is for,
except for some IP multicast addresses that MUST be treated as
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broadcast.
5.2 Per VLAN mandatory announcement of the set of IDs of Root
bridges for any of RBn's links on which RBn is forwarder for
that VLAN. Where MSTP (Multiple Spanning Tree Protocol) is
running on a link, this is the root bridge of the CIST (Common
and Internal Spanning Tree). This is to quickly detect cases
where two Layer 2 clouds accidentally get merged, and where
there might otherwise temporarily be two DRBs for the same
VLAN on the same link. (See Section 4.2.4.3.)
5.3 Optionally, per VLAN Layer 2 multicast addresses derived from
IPv4 IGMP and IPv6 MLD notification messages received from
attached end nodes on that VLAN, indicating the location of
listeners for these multicast addresses (see Section 4.5.5).
5.4 Per VLAN ESADI participation flag, priority, and holding time.
If this flag is one, it indicates that the RBridge wishes to
receive such TRILL ESADI frames (see Section 4.2.5.1).
5.5 Per VLAN appointed forwarder status lost counter (see Section
4.8.2).
6. Optionally, a list of VLAN groups where address learning is shared
across that VLAN group (see Section 4.8.3). Each VLAN group is a
list of VLAN IDs, where the first VLAN ID listed in a group, if
present, is the "primary" and the others are "secondary". This is
to detect misconfiguration of features outside the scope of this
document. RBridges that do not support features such as "shared
VLAN learning" ignore this field.
4.2.5 TRILL ESADI
RBridges that are the appointed VLAN-x forwarder for a link MAY
participate in the TRILL end station address distribution information
(ESADI) protocol for that VLAN. But all transit RBridges MUST
properly forward TRILL ESADI frames as if they were multicast TRILL
data frames. TRILL ESADI frames are structured like IS-IS frames but
are always TRILL encapsulated on the wire as if they were TRILL data
frames.
Because of this forwarding, it appears to an ESADI at an RBridge that
it is directly connected by a shared virtual link to all other
RBridges in the campus running ESADI for that VLAN. RBridges that do
not implement that ESADI or are not appointed forwarder for that VLAN
do not decapsulate or locally process any TRILL ESADI frames they
receive for that VLAN. In other words, these frames are transparently
tunneled through transit RBridges. Such transit RBridges treat them
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exactly as multicast TRILL data frames and no special processing is
invoked due to such forwarding.
TRILL ESADI frames sent on an IEEE 802.3 link are structured as shown
below. The outer VLAN tag will not be present if it was stripped by
the port out which the frame was sent.
Outer Ethernet Header:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Hop Destination Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Hop Destination Address | Sending RBridge MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sending RBridge Port MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ethertype = C-Tag [802.1Q] | Outer.VLAN Tag Information |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TRILL Header:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ethertype = TRILL | V | R |M|Op-Length| Hop Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Egress (Dist. Tree) Nickname | Ingress (Origin) Nickname |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Inner Ethernet Header:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| All-ESADI-RBridges Multicast Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| All-ESADI-RBridges continued | Origin RBridge MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Origin RBridge MAC Address continued |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ethertype = C-Tag [802.1Q] | Inner.VLAN Tag Information |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ethertype = L2-IS-IS |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
ESADI Payload (formatted as IS-IS):
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IS-IS Common Header, IS-IS PDU Specific Fields, IS-IS TLVs |
Frame Check Sequence:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FCS (Frame Check Sequence) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4.4: TRILL ESADI Frame Format
The Next Hop Destination Address or Outer.MacDA is the All-RBridges
multicast address. The VLAN specified in the Outer.VLAN information
will always be the Designated VLAN for the link on which the frame is
sent. The V and R fields will be zero while the M field will be one.
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The VLAN specified in the Inner.VLAN information will be the VLAN to
which the ESADI applies. The Origin RBridge MAC Address or
Inner.MacSA MUST be a globally unique MAC address owned by the
RBridge originating the ESADI frame, for example any of its port MAC
addresses, and each RBridge MUST use the same Inner.MacSA for all of
the ESADI frames that RBridge originates.
4.2.5.1 TRILL ESADI Participation
An RBridge does not send any Hellos because of participation in an
ESADI. The information available in the TRILL IS-IS link state
database is sufficient to determine the ESADI DRB on the virtual link
for each VLAN's ESADI. In particular, the link state database
information for each RBridge includes the VLANs, if any, for which
that RBridge is participating in an ESADI, its priority for being
selected as DRB for each of those ESADIs, its holding time, and its
IS-IS system ID for breaking ties in priority.
The DRB sends TRILL-ESADI-CSNP frames on the ESADI virtual link. For
robustness, a participating RBridge that determines that some other
RBridge should be ESADI DRB on such a virtual link and has not
received or sent a TRILL-ESADI-CSNP in at least the DRB holding time
MAY also send a TRILL-ESADI-CSNP on the virtual link. A participating
RBridge that determines that no other RBridges are participating in
an ESADI for a particular VLAN SHOULD NOT send ESADI information or
TRILL-ESADI-CSNPs on the virtual link.
4.2.5.2 TRILL ESADI Information
The information in ESADI is the list of local end station MAC
addresses known to the originating RBridge and, for each such
address, a one octet unsigned "confidence" rating in the range 0-254
(see Section 4.8). In order to make it practical to optionally
provide for VLAN ID translation, as specified in a separate document,
TRILL ESADI frames MUST NOT contain the VLAN ID in the body of the
frame after the Inner.VLAN tag.
4.3 Link MTU Size
There are two reasons why it is important to know what size of packet
each link in the campus can support:
1. RBridge RB1 must know what size of link state information messages
it can generate, that will be guaranteed to be forwardable across
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all links in the campus.
2. If traffic engineering tools know which links support larger than
minimally acceptable data packet sizes, paths can be computed that
can support large data packets.
4.3.1 Determining Campus-Wide MTU Size
There must be an agreement among all RBridges on the value of "Sz",
the minimum acceptable link size for the campus. Once Sz is known,
all RBridges MUST format their link state information messages to be
in chunks of size at most Sz. Also, every RBridge RB1 SHOULD test
each of its adjacencies, say to RB2, to ensure that the RB1-RB2 link
can forward packets of at least size Sz.
Sz is determined by having each RBridge (optionally) advertise, in
its LSP, its assumption of the value of the campus-wide Sz. This LSP
element is known in IS-IS as the originatingLSPBufferSize, TLV #14.
The default and minimum value for Sz, and the implicitly advertised
value of Sz if the TLV is absent, is 1470 bytes.
The campus-wide value of Sz is the smallest value of Sz advertised by
any RBridge.
4.3.2 Testing MTU Size
There are two new TRILL IS-IS message types for use between pairs of
RBridge neighbors to test the bidirectional packet size capacity of
their connection. These messages are:
-- MTU-probe
-- MTU-ack
Both the MTU-probe and the MTU-ack are padded to the size being
tested.
Sending of MTU-probes is optional; however, an RBridge RB2 that
receives an MTU-probe from RB1 MUST respond with an MTU-ack padded to
the same size as the MTU-probe. The MTU-probe MAY be multicast to
All-RBridges, or unicast to a specific RBridge. The MTU-ack is
normally unicast to the source of the MTU-probe to which it responds
but MAY be multicast to All-RBridges.
If RB1 fails to receive an MTU-ack to a probe of size X from RB2
after k tries (where k is a configurable parameter whose default is
3), then RB1 assumes the RB1-RB2 link cannot support size X. If X is
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not greater than Sz, then RB1 sets the "failed minimum MTU test" flag
for RB2 in RB1's Hello. If size X succeeds, and X > Sz, then RB1
advertises the largest tested X in RB1's LAN Hello, and RB1 MAY
advertise X as an attribute of the link to RB2 in RB1's LSP.
4.4 TRILL-Hello Protocol
The TRILL-Hello protocol is a little different from the layer 3 IS-IS
LAN Hello protocol and uses a new type of TRILL IS-IS message known
as a TRILL-Hello.
4.4.1 Rationale
The reason for defining this new type of link in TRILL is that in
layer 3 IS-IS, the LAN Hello protocol may elect multiple Designated
Routers (DRs) since, when choosing a DR, routers ignore other routers
with whom they do not have 2-way connectivity. Also, layer 3 IS-IS
LAN Hellos are padded, to avoid forming adjacencies between neighbors
that can't speak the maximum sized packet to each other. This means,
in layer 3 IS-IS, that neighbors that have connectivity to each
other, but with an MTU on that connection less than what they
perceive as maximum sized packets, will not see each other's Hellos.
The result is that routers might form cliques, resulting in the link
turning into multiple pseudonodes.
This behavior is fine for layer 3, but not for layer 2, where loops
may form if there are multiple DRBs. Therefore, the TRILL-Hello
protocol is a little different from layer 3 IS-IS's LAN Hello
protocol.
One other issue with TRILL-Hellos is to ensure that subsets of the
information can appear in any single message, and be processable, in
the spirit of IS-IS LSPs and CSNPs. TRILL-Hello frames, completely
independently of whether they are padded or not, can become very
large. An example where this might be the case is when some sort of
backbone technology interconnects hundreds of TRILL sites over what
would appear to TRILL to be a giant Ethernet, where the RBridges
connected to that cloud will perceive that backbone to be a single
link with hundreds of neighbors.
In TRILL (unlike in layer 3 IS-IS), the DRB is selected based solely
on priority and MAC address. In other words, if RB2 receives a TRILL-
Hello from RB1 with higher (priority, MAC), RB2 defers to RB1 as DRB,
regardless of whether RB1 lists RB2 in RB1's TRILL-Hello.
Although the neighbor list in a TRILL-Hello does not influence the
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DRB election, it does determine what is announced in LSPs. RB1 only
reports links to RBridges that it has two-way connectivity with. If
RB1 is DRB on a link, and for whatever reason (MTU mismatch, or one-
way connectivity) RB1 and RB2 do not have two-way connectivity, then
RB2 does not report a link to RB1 (or the pseudonode), and RB1 (or
RB1 on behalf of the pseudonode) does not report a link to RB2.
4.4.2 TRILL-Hello Contents
The TRILL-Hello has a new IS-IS message type. It starts with the same
fixed header as an IS-IS LAN Hello.
The following information MUST appear in every TRILL-Hello.
References to "TLV" may actually be a "sub-TLV" as specified in a
separate document.
1. The VLAN ID of the Designated VLAN for the link.
2. A copy of the Outer.VLAN ID with which the Hello was tagged on
transmission
3. Two flags as follows:
3.a A flag which, if set, indicates that the sender has detected
VLAN mapping on the link, within the past 2 of its Holding
Times.
3.b A flag which, if set, indicates that the sender believes it is
appointed forwarder for the VLAN and port on which the TRILL-
Hello was sent
The following information MAY appear
1. The set of VLANs for which end station service is enabled on the
port.
2. Several flags as follows:
2.a A flag which, if set, indicates that the sender's port was
configured as an access port.
2.b A flag which, if set, indicates that the sender's port was
configured as a trunk port.
2.c A bypass pseudonode flag, as described below in this section.
3. If the sender is DRB, the Rbridges (excluding itself) that it
appoints as forwarders for that link and the VLANs for which it
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appoints them. As described below, this TLV is designed so that
not all the appointment information need be included in each
Hello. Its absence means that appointed forwarders should
continue as previously assigned.
4. The TRILL neighbor list. This is a new TLV, not the same as the
IS-IS Neighbor TLV, in order to accommodate fragmentation and
reporting MTU on the link (see Section 4.4.2.1).
The appointed forwarders TLV specifies a range of VLANs and, within
that range, specifies which Rbridge, if any, other than the DRB, is
appointed forwarder for the VLANs in that range. Such TLVs sent by
the DRB must eventually cover every possible VLAN. Appointing an
RBridge as forwarder on a port for a VLAN which is not enabled on
that port has no effect.
It is anticipated that many links between RBridges will be point-to-
point, in which case using a pseudonode merely adds to the
complexity. If the DRB specifies the bypass pseudonode bit in its
TRILL-Hellos, the RBridges on the link just report their adjacencies
as point-to-point. This has no effect on how LSPs are flooded on a
link. It only affects what LSPs are generated.
For example, if RB1 and RB2 are the only RBridges on the link and RB1
is DRB, then if RB1 creates a pseudonode that is used, there are 3
LSPs: for, say, RB1.25 (the pseudonode), RB1, and RB2, where RB1.25
reports connectivity to RB1 and RB2, and RB1 and RB2 each just say
they are connected to RB1.25. Whereas if DRB RB1 sets the bypass
pseudonode bit in its Hellos, then there will be only 2 LSPs: RB1 and
RB2 each reporting connectivity to each other.
A DRB SHOULD set the bypass pseudonode bit for its links unless, for
a particular link, it has seen at least two simultaneous adjacencies
on the link at some point since it last re-booted.
4.4.2.1 TRILL Neighbor List
The new TRILL Neighbor TLV includes the following information for
each neighbor it lists:
1. The neighbor's MAC address.
2. MTU size to this neighbor as a two-octet unsigned integer in units
of 4-octet chunks. The value zero indicates that the MTU is
untested.
3. A flag for "failed minimum MTU test".
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To allow partial reporting of neighbors, the neighbor IDs MUST be
sorted by ID. If a set of neighbors { X1, X2, X3, ... Xn } are
reported in RB1's Hello, then X1 < X2 < X3, ... < Xn. If RBridge
RB2's ID is between X1 and Xn, and does not appear in RB1's Hello,
then RB2 knows that RB1 has not heard RB2's Hello.
Additionally there are two overall TRILL Neighbor List TLV flags:
"the smallest ID I reported in this Hello is the smallest ID of any
neighbor", and "the largest ID I reported in this Hello is the
largest ID of any neighbor". If all the neighbors fit in RB1's
Hello, both flags will be set.
If RB1 reports { X1, ... Xn } in its Hello, with the "smallest" flag
set, and RB2's ID is smaller than X1, then RB2 knows that RB1 has not
heard RB2's Hello.
To ensure that any RBridge RB2 can definitively determine whether RB1
can hear RB2, RB1's neighbor list must eventually cover every
possible range of IDs. In other words, if X1 is the smallest reported
in one of RB1's neighbor lists, and the "smallest" flag is not set,
then X1 must appear in a different TRILL-Hello fragment as well, as
the largest ID reported in that fragment. Or, fragments may overlap,
as long as there is no gap, such that some range, say between Xi and
Xj, never appears in any fragment.
4.4.3 TRILL MTU probe and Hello VLAN Tagging
The MTU probe mechanism is designed to determine the MTU for
transmissions between RBridges. MTU probes and probe acknowledgements
are only sent on the Designated VLAN.
An RBridge RBn maintains for each port the same VLAN information as a
customer IEEE [802.1Q-2005] bridge, including the set of VLANs
enabled for output through that port (see Section 4.9.2). In
addition, RBn maintains the following TRILL specific VLAN parameters
per port:
a) Desired Designated VLAN: the VLAN that RBn, if it is DRB, will
specify in its TRILL-Hellos as the VLAN to be used by all
RBridges on the link to communicate all TRILL frames, except
some TRILL-Hellos. This MUST be a VLAN enabled on RBn's port.
It defaults to the numerically lowest enabled VLAN ID, which is
VLAN 1 for a zero configuration RBridge.
b) Designated VLAN: the VLAN being used on the link for all TRILL
frames except some TRILL Hellos. This is RBn's Desired
Designated VLAN if RBn believes it is the DRB or the Designated
VLAN in the DRB's Hellos if RBn is not the DRB.
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c) Announcing VLANs set. This defaults to the enabled VLANs set on
the port but may be configured to be a subset of the enabled
VLANs.
d) Forwarding VLANs set: the set of VLANs for which an RBridge
port is appointed VLAN forwarder on the port. This MUST contain
only enabled VLANs for the port, possibly all enabled VLANs.
On each of its ports that is not configured to use P2P Hellos, an
RBridge sends TRILL-Hellos Outer.VLAN tagged with each VLAN in a set
of VLANs. This set depends on the RBridge's DRB status and the above
VLAN parameters. RBridges send TRILL Hellos Outer.VLAN tagged with
the Designated VLAN, unless that VLAN is not enabled on the port. In
addition, the DRB sends TRILL Hellos Outer.VLAN tagged with each
enabled VLAN in its Announcing VLANs set. All non-DRB RBridges send
TRILL-Hellos Outer.VLAN tagged with all enabled VLANs that are in the
intersection of their Forwarding VLANs set and their Announcing VLANs
set. More symbolically, TRILL-Hello frames, when sent, are sent as
follows:
If sender is DRB
intersection ( Enabled VLANs,
union ( Designated VLAN, Announcing VLANs ) )
If sender is not DRB
intersection ( Enabled VLANs,
union ( Designated VLAN,
intersection ( Forwarding VLANs, Announcing VLANs ) ) )
Configuring the Announcing VLANs set to be null minimizes the number
of TRILL-Hellos. In that case, TRILL-Hellos are only tagged with the
Designated VLAN.
The number of TRILL-Hellos is maximized, within this specification,
by configuring the Announcing VLANs set to be the set of all enabled
VLAN IDs, which is the default. In that case, the DRB will send
TRILL-Hello frames tagged with all its Enabled VLAN tags and any non-
DRB RBridge RBn will send TRILL-Hello frames tagged with the
Designated VLAN, if enabled, and tagged with all VLANs for which RBn
is an appointed forwarder. (It is possible to send even more TRILL-
Hellos. In particular, non-DRB RBridges could send TRILL-Hellos on
enabled VLANs for which they are not an appointed forwarder and which
are not the Designated VLAN. This would cause no harm other than a
further communications and processing burden.)
When an RBridge port comes up, until it has heard a TRILL-Hello from
a higher priority RBridge, it considers itself to be DRB on that port
and sends TRILL-Hellos on that basis. Similarly, even if it has at
some time recognized some other RBridge on the link as DRB, if it
receives no TRILL-Hellos on that port from an RBridge with higher
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priority as DRB for a long enough time, as specified by IS-IS, it
will revert to believing itself DRB.
4.4.4 Multiple Ports on the Same Link
It is possible for an RBridge RB1 to have multiple ports onto the
same link. It is important for RB1 to recognize which of its ports
are on the same link, so, for instance, if RB1 is appointed forwarder
for VLAN A, RB1 knows that only one of its ports acts as appointed
forwarder for VLAN A on that link.
RB1 detects this condition based on receiving TRILL-Hello messages
with the same pseudonode ID on multiple ports. RB1 might have one set
of ports, say { p1, p2, p3 } on one link, and another set of ports {
p4, p5 } on a second link, and yet other ports, say p6, p7, p8, that
are each on distinct links. Let us call a set of ports on the same
link as a "port group".
If RB1 detects that a set of ports, say { p1, p2, p3 } are a port
group on a link, then RB1 MUST ensure that it does not cause loops
when it encapsulates and decapsulates traffic from/to that link. If
RB1 is appointed forwarder for VLAN A on that Ethernet link, RB1 MUST
encapsulate/decapsulate VLAN A on only one of the ports. However, if
RB1 is appointed forwarder for more than one VLAN, RB1 MAY choose to
load split among its ports, using one port for some set of VLANs, and
another port for a disjoint set of VLANs.
If RB1 detects VLAN mapping occurring, (see Section 4.4.5), then RB1
MUST NOT load split as appointed forwarder, and instead MUST act as
appointed VLAN forwarder on that link on only one of its ports in the
port group.
When forwarding TRILL-encapsulated multidestination frames to/from a
link on which RB1 has a port group, RB1 MAY choose to load-split
among its ports, provided that it does not duplicate frames, and
provided that it keeps frames for the same flow on the same port. If
RB1's neighbor on that link, RB2, accepts multidestination frames on
that tree on that link from RB1, RB2 MUST accept the frame from any
of RB2's adjacencies to RB1 on that link.
4.4.5 VLAN Mapping Within a Link
IEEE [802.1Q-2005] does not provide for bridges changing the C-tag
VLAN ID for a tagged frame they receive, that is, mapping VLANs.
Nevertheless, some bridge products provide this capability and, in
any case, bridged LANs can be configured to display this behavior.
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For example, a bridge port can be configured to strip VLAN tags on
output and send the resulting untagged frames onto a link leading to
another bridge's port configured to tag these frames with a different
VLAN. Although each port's configuration is legal under
[802.1Q-2005], in the aggregate they perform manipulations not
permitted to a single customer [802.1Q-2005] bridge. Since RBridge
ports have the same VLAN capabilities as customer [802.1Q-2005]
bridges, this can occur even in the absence of bridges. (VLAN mapping
is referred to in IEEE 802.1 as "VLAN ID translation".)
RBridges include the Outer.VLAN ID inside every TRILL-Hello message.
When a TRILL-Hello is received, RBridges compare this saved copy with
the Outer.VLAN ID information associated with the received frame. If
these differ and the VLAN ID inside the Hello is X and the Outer.VLAN
is Y, it can be assumed that VLAN ID X is being mapped into VLAN ID
Y.
When non-DRB RB2 detects VLAN mapping, based on receiving a TRILL-
Hello where the VLAN tag in the body of the Hello differs from the
one in the outer header, it sets a flag in all of its TRILL-Hellos
for a period of two of its Holding Times since the last time RB2
detected VLAN mapping. When DRB RB1 is informed of VLAN mapping,
either because of receiving a TRILL-Hello that has been VLAN-mapped,
or because of seeing the "VLAN Mapping detected" flag in a neighbor's
TRILL-Hello on the link, RB1 re-assigns VLAN forwarders to ensure
there is only a single forwarder on the link for all VLANs.
4.5 Distribution Trees
RBridges use distribution trees to forward multi-destination frames
(see Section 2.2.2). Distribution Trees are bidirectional. Although
a single tree is logically sufficient for the entire campus, the
computation of additional distribution trees is warranted for the
following reasons: it enables multipathing of multi-destination
frames and enables the choice of a tree root closer to or, in the
limit, identical with the ingress RBridge. Such a closer tree root
reduces out-of-order delivery when a unicast address transitions
between unknown and known and improves the efficiency of the delivery
of multi-destination frames that are being delivered to a subset of
the links in the campus.
An additional level of flexibility is the ability of an RBridge to
acquire multiple nicknames, and therefore have multiple trees rooted
at the same RBridge. Since the tree number is used as a tie-breaker
for equal cost paths, the different trees, even if rooted at the same
RBridge, will likely utilize different equal cost paths.
RBridges will precompute all the trees that might be used, and keep
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state for Reverse Path Forwarding Check filters (see Section 4.5.2).
Also, since the tree number is used as a tie-breaker, it is important
for all RBridges to know:
o how many trees to compute
o which trees to compute
o what the tree number for each tree is
o which trees each ingress RBridge might choose (for building
Reverse Path Forwarding Check filters).
Each RBridge advertises in its LSP a "tree root" priority. This is a
16-bit unsigned integer that defaults, for a zero configuration
RBridge, to 0x8000. Tree roots are ordered with highest numerical
priority being highest priority, then with system ID of the root
(numerically higher = higher priority) as tie breaker, and if that
root RBridge has multiple nicknames, numerically higher nicknames of
the same RBridge having priority.
Each RBridge advertises in its LSP the maximum number of trees that
it can compute and the number of trees that it wants all RBridges in
the campus to compute. The number of trees, k, that are computed for
the campus is the number wanted by the RBridge RB1, which has the
highest "tree root" priority, but nomore than the number of trees
supported by the RBridge in the campus which supports the fewest
trees. If RB1 does not specify the trees, then the k highest priority
trees are the trees that will be computed by all RBridges. Note that
some of these k highest priority trees might be rooted at the same
RBridge, if that RBridge has multiple nicknames.
If an RBridge specifies the number of trees it can compute, or the
number of trees it wants computed for the campus, as 0, it is treated
as specifying them as 1. Thus k defaults to 1.
In addition, the highest root priority RBridge RB1 might explicitly
advertise a set of s trees by listing s nicknames. In that case, the
first k of those s trees will be computed. If s is less than k, or if
any of the s nicknames associated with the trees RB1 is advertising
does not exist within the LSP database, then the RBridges still
compute k trees, but the remaining trees they select are the highest
priority trees, such that k trees are computed.
The k trees are ordered from 1 to k, with up to k of the s trees
advertised by RB1 given tree numbers 1 through s, respectively, and
any remaining trees given numbers in order of priority. For example,
if RB1 does not explicitly advertise any trees and k=2, then the
highest priority tree is number 1 and the 2nd highest priority tree
is number 2.
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4.5.1 Distribution Tree Calculation
RBridges do not use the spanning tree protocol to calculate
distribution trees. Instead, distribution trees are calculated based
on the link state information, selecting a particular RBridge
nickname as the root. Each RBridge RBn independently calculates a
tree rooted at RBi by performing the SPF (Shortest Path First)
calculation with RBi as the root without requiring any additional
exchange of information.
It is important, when building a tree, that all RBridges choose the
same links for that tree. Therefore, when there are equal cost paths
for a particular tree, all RBridges need to use the same tie-
breakers. It is also desirable to allow splitting of traffic on as
many links as possible. For this reason, a simple tie-breaker such as
"always choose the parent with lower ID" would not be desirable.
Instead, TRILL uses the tree number as a parameter in the tie-
breaking algorithm.
When building the tree number j, remember all possible equal cost
parents for node N. After calculating the entire "tree" (actually,
directed graph), for each node N, if N has "p" parents, then order
the parents according to 7-byte ID. For tree j, choose N's parent as
choice j mod p.
Note that there might be multiple equal cost links between N and
potential parent P that have no pseudonodes, either because they are
point-to-point links, or pseudonode-suppressed links. Such links will
be treated as a single link for the purpose of tree building, because
they all have the same parent P, whose IS-IS ID is "P.0".
In other words, the set of potential parents for, N for the tree
rooted at R, are those that give equally minimal cost paths from N to
R, and which have distinct 7-octet IDs, based on what is reported in
LSPs.
4.5.2 Multi-destination Frame Checks
When a multi-destination TRILL encapsulated frame is received by an
RBridge, there are four checks performed, each of which may cause the
frame to be discarded:
1. Tree Adjacency Check: Each RBridge RBn keeps a set of adjacencies
( { port, neighbor} pairs ) for each distribution tree it is
calculating. One of these adjacencies is toward the tree root RBi
and the others are toward the leaves. Once the adjacencies are
chosen, it is irrelevant which ones are towards the root RBi, and
which are away from RBi. RBridges MUST drop a multi-destination
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frame that arrives at a port from an RBridge that is not an
adjacency for the tree on which the frame is being distributed.
Let's suppose that RBn has calculated that adjacencies a, c, and f
are in the RBi tree. A multi-destination frame for the
distribution tree RBi is received only from one of the adjacencies
a, c, or f (otherwise it is discarded) and forwarded to the other
two adjacencies. Should RBn have multiple ports on a link, a
multidestination frame it sends on one of these ports will be
received by the others but will be discarded as an RBridge is not
adjacent to itself.
2. RPF Check: Another technique used by RBridges for avoiding
temporary multicast loops during topology changes is the reverse
path forwarding check. It involves checking that a multi-
destination frame, based on the tree and the ingress RBridge,
arrives from the expected link. RBridges MUST drop multi-
destination frames that fail the RPF check.
To limit the amount of state necessary to perform the RPF check,
each RBridge RB2 MUST announce which trees RB2 may choose when RB2
ingresses a multi-destination packet. When any RBridge, say, RB3,
is computing the tree from nickname X, RB3 computes, for each
RBridge RB2 that might act as ingress for tree X, the link on
which RB3 should receive a packet from ingress RB2 on tree X, and
note for that link that RB2 is a legal ingress RBridge for tree X.
The information to specify which trees RB2 might choose is
included in RB2's LSP. Similarly to how the highest priority
RBridge RB1 specifies the k trees that will be computed by all
RBridges, RB2 specifies a number j "number of ingress trees",
explicitly specify a set of nicknames of ingress trees in the
field "specified ingress tree nicknames", or a combination of
specified trees and trees selected from the highest priority
trees. If RB2 specifies any trees that are not in the k trees as
specified by RB1, they are ignored.
The j potential ingress trees for RB2 are the ones with nicknames
that RB2 has explicitly specified in "specified ingress tree
nicknames" (and that are included in the k campus-wide trees
selected by highest priority RBridge RB1), with the remainder (up
to the maximum of {j,k}) being the highest priority of the k
campus-wide trees.
The default value for j is 1. The value 0 for j is special and
means that RB2 can pick any of the k trees being computed for the
campus.
3. Parallel Links Check: If the tree-building and tie-breaking for a
particular tree selects a non-pseudonode link between R1 and R2,
that "R1-R2" link might consist of multiple links. These parallel
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links would be visible to R1 and R2, but not to the rest of the
campus (because the links are not represented by pseudonodes). If
this bundle of parallel links is included in a tree, it is
important for R1 and R2 to decide which link to use, but is
irrelevant to other RBridges, and therefore, the tie-breaking
algorithm need not be visible to any RBridges other than R1 and
R2. In this case, R1-R2 adjacencies are ordered as follows, with
the one "most preferred" adjacency being the one that R1 transmits
to R2 on, and the one that R2 accepts traffic from R1 on:
a) Most preferred are those established by P2P Hellos with tie-
breaking among those based on preferring the one with the
numerically highest Extended Circuit ID.
b) Next considered are those established through TRILL-Hello
frames, with suppressed pseudonodes. Note that the pseudonode
is suppressed in LSPs, but still appears in the TRILL-Hello,
and therefore is available for this tie-breaking. Among these
links, the one with the numerically largest pseudonode ID is
preferred.
4. Port Group Check: If an RBridge has multiple ports attached to the
same link, a multidestination frame it is receiving will arrive on
all of them. All but one received copy of such a frame MUST be
discarded to avoid duplication. All such frames that are part of
the same flow must be accepted on the same port to avoid re-
ordering.
When a topology change occurs (including apparent changes during
start up), an RBridge MUST adjust its input distribution tree filters
no later than it adjusts its output forwarding.
4.5.3 Pruning the Distribution Tree
Each distribution tree SHOULD be pruned per-VLAN, eliminating
branches that have no potential receivers downstream. Multi-
destination TRILL data frames SHOULD only be forwarded on branches
that are not pruned.
Further pruning SHOULD be done in two cases: (1) IGMP [RFC3376], MLD
[RFC2710], and MRD [RFC4286] messages, where these are to be
delivered only to links with IP Multicast routers; and (2) other
multicast frames derived from an IP multicast address that should be
delivered only to links that have registered listeners, plus links
which have IP Multicast routers, except for IP multicast addresses
which must be broadcast. Each of these cases are scoped per-VLAN.
Let's assume that RBridge RBn knows that adjacencies (a, c, and f)
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are in the RBi-distribution tree. RBn marks pruning information for
each of the adjacencies in the RBi-tree. For each adjacency and for
each tree, RBn marks:
o the set of VLANs reachable downstream,
o for each one of those VLANs, flags indicating whether there are
IPv4 or IPv6 multicast routers downstream, and
o the set of Layer 2 multicast addresses derived from IP multicast
groups for which there are receivers downstream.
4.5.4 Tree Distribution Optimization
RBridges MUST determine the VLAN associated with all native frames
they ingress and properly enforce VLAN rules on the emission of
native frames at egress RBridge ports according to how those ports
are configured and appointed forwarders. They SHOULD also prune the
distribution tree of multi-destination frames according to VLAN.
But, since they are not required to do such pruning, they may receive
TRILL data or ESADI frames that should have been VLAN pruned earlier
in the tree distribution. They silently discard such frames. A campus
may contain some Rbridges that prune on VLAN and some that do not.
The situation is more complex for multicast. RBridges SHOULD analyze
IP derived native multicast frames, and learn and announce listeners
and IP multicast routers for such frames as discussed in Section 4.7
below. And they SHOULD prune the distribution of IP derived multicast
frames based on such learning and announcements. But, they are not
required to prune based on IP multicast listener and router
attachment state. And, unlike VLANs, where VLAN attachment state of
ports MUST be maintained and honored, RBridges are not required to
maintain IP multicast listener and router attachment state.
An RBridge that does not examine native IGMP [RFC3376], MLD
[RFC2710], and MRD [RFC4286] frames that it ingresses MUST advertise
that it has IPv4 and IPv6 IP multicast routers attached for all the
VLANs for which it is an appointed forwarder. It need not advertise
any IP derived multicast listeners. This will cause all IP derived
multicast traffic to be sent to this RBridge for those VLANs. It then
egresses that traffic onto the links for which it is appointed
forwarder where the VLAN of the traffic matches the VLAN for which it
is appointed forwarder on that link. (This may cause the suppression
of certain IGMP membership report messages from end stations but that
is not significant as any multicast traffic such reports would be
requesting will be sent to such end stations under these
circumstances.)
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A campus may contain a mixture of Rbridges with different levels of
IP derived multicast optimization. An RBridge may receive IP derived
multicast frames that should have been pruned earlier in the tree
distribution. It silently discards such frames.
See also "Considerations for Internet Group Management Protocol
(IGMP) and Multicast Listener Discovery (MLD) Snooping Switches"
[RFC4541].
4.5.5 Forwarding Using a Distribution Tree
With full optimization, forwarding a multi-destination data frame is
done as follows:
o The RBridge RBn receives a multi-destination TRILL data frame with
inner VLAN-x and a TRILL header indicating that the selected tree
is the RBi-tree;
o if the adjacency from which the frame was received is not one of
the adjacencies in the RBi-tree for the specified ingress RBridge,
the frame is dropped (see Section 4.5.1);
o else, if the frame is an IGMP or MLD announcement message or an MRD
query message, then the encapsulated frame is forwarded onto
adjacencies in the RBi-tree that indicate there are downstream
VLAN-x IPv4 or IPv6 multicast routers as appropriate;
o else, if the frame is for a Layer 2 multicast address derived from
an IP multicast group, but its IP address is not the range of IP
multicast addresses that must be treated as broadcast, the frame
is forwarded onto adjacencies in the RBi-tree that indicate there
are downstream VLAN-x IP multicast routers of the corresponding
type (IPv4 or IPv6), as well as adjacencies that indicate there
are downstream VLAN-x receivers for that group address;
o else (the inner frame is for a Layer 2 multicast address not
derived from an IP multicast group or an unknown destination or
broadcast or an IP multicast address which is required to be
treated as broadcast) the frame is forwarded onto an adjacency if
and only if that adjacency is in the RBi-tree, and marked as
reaching VLAN-x links.
For each link for which RBn is appointed forwarder, RBn additionally
checks to see if it should decapsulate the frame and send it to the
link in native form, or process the frame locally.
TRILL ESADI frames will be delivered only to RBridges that are
appointed forwarders for their VLAN. Such frames will be multicast
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throughout the campus, like other non-IP-derived multicast data
frames, on the distribution tree chosen by the RBridge which created
the TRILL ESADI frame, and pruned according to the Inner.VLAN ID.
Thus all the RBridges that are appointed forwarders for a link in
that VLAN receive them.
4.6 Frame Processing Behavior
This section describes RBridge behavior for all varieties of received
frames, including how they are forwarded when appropriate. Section
4.6.1 covers native frames, Section 4.6.2 covers TRILL frames, and
Section 4.6.3 covers layer 2 control frames. Processing may be
organized or sequenced in a different way than described here as long
as the result is the same.
Corrupt frames, for example frames that are not a multiple of 8 bits,
are too short or long for the link protocol/hardware in use, or have
a bad FCS are discarded on receipt by an RBridge port as they are
discarded on receipt at an IEEE 802.1 bridge port.
Source address information ( { VLAN, Outer.MacSA, port } ) is learned
from any frame with a unicast sources address (see Section 4.8).
4.6.1 Receipt of a Native Frame
If the port is configured as disabled or if end station service is
disabled on the port by configuring it as a trunk port or configuring
it to use P2P Hellos, the frame is discarded.
The ingress Rbridge RB1 determines the VLAN ID for a native frame
according to the same rules as IEEE [802.1Q-2005] bridges do (see
Appendix D and Section 4.9.2). Once the VLAN is determined, if RB1 is
not the appointed forwarder for that VLAN on the port where the frame
was received or is inhibited, the frame is discarded. If it is
appointed forwarder for that VLAN and is not inhibited (see Section
4.2.4.3), then the native frame is forwarded according to 4.6.1.1 if
it is unicast and according to 4.6.1.2 if it is multicast or
broadcast.
4.6.1.1 Native Unicast Case
If the destination MAC address of the native frame is a unicast
address, the following steps are performed.
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The Layer 2 destination address and VLAN are looked up in the ingress
RBridge's database of learned MAC addresses and VLANs to find the
egress RBridge RBm or the local egress port or to discover that the
destination is the receiving RBridge or is unknown. One of the
following four cases will apply:
1. If the destination is the receiving RBridge, the frame is locally
processed.
2. If the destination is known to be on the same link from which the
native frame was received but is not the receiving RBridge, the
RBridge silently discards the frame, since the destination should
already have received it.
3. If the destination is known to be on a different local link for
which RBm is the appointed forwarder, then RB1 converts the native
frame to a TRILL data frame with an Outer.MacDA of the next hop
RBridge towards RBm, a TRILL header with M = 0, the ingress
nickname for RB1, and the egress nickname for RBm. If RBm is RB1,
processing then proceeds as in 4.6.2.4; otherwise, the Outer.MacSA
is set to the MAC address of the RB1 port on the path to the next
hop RBridge towards RBm and the frame is queued for transmission
out that port.
4. If a unicast destination address is unknown, RB1 handles the frame
as described in Section 4.6.1.2 for a broadcast frame except that
the Inner.MacDA is the original native frame's unicast destination
address.
4.6.1.2 Native Multicast and Broadcast Frames
If the RBridge has multiple ports attached to the same link, all but
one received copy of a native multicast or broadcast frame is
discarded to avoid duplication. All such frames that are part of the
same flow must be accepted on the same port to avoid re-ordering.
If the frame is a native IGMP [RFC3376], MLD [RFC2710], or MRD
[RFC4286] frame, then RB1 SHOULD analyze it, learn any group
membership or IP multicast router presence indicated, and announce
that information for the appropriate VLAN in its LSP (see Section
4.7).
For all multi-destination native frames, RB1 forwards the frame in
native form to its links where it is appointed forwarder for the
frame's VLAN, subject to further pruning and inhibition. In addition,
it converts the native frame to a TRILL data frame with the All-
RBridges multicast address as Outer.MacDA, a TRILL header with the
multi-destination bit M = 1, the ingress nickname for RB1, and the
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egress nickname for the root of the distribution tree it decides to
use. It then forwards the frame on the pruned distribution tree (see
Section 4.5) setting the Outer.MacSA of each copy sent to the MAC
address of the RB1 port on which it is sent.
The default is for RB1 to write into the egress nickname field one of
the nicknames for a distribution tree, from the set of distribution
trees RB1 has announced it might use, whose root is least cost from
RB1. RB1 MAY choose different distribution trees for different frames
if RB1 has been configured to path-split multicast. In that case RB1
MUST select a tree by specifying a nickname that is a distribution
tree root (see Section 4.5). Also, RB1 MUST select a nickname that
RB1 has announced (in RB1's own LSP) to be one of those that RB1
might use.
4.6.2 Receipt of a TRILL Frame
A TRILL frame has either the TRILL or L2-IS-IS Ethertype or has a
multicast Outer.MacDA allocated to TRILL (see Section 7.2). The
following tests are then performed sequentially and the first which
matches controls the handling of the frame:
1. If the Outer.MacDA is All-IS-IS-RBridges and the Ethertype is
L2-IS-IS, the frame is handled as described in Section 4.6.2.1.
2. If the Outer.MacDA is a multicast address allocated to TRILL other
than All-RBridges, the frame is discarded.
3. If the Outer.MacDA is a unicast address other than the address of
the receiving Rbridge, the frame is discarded. (Such discarded
frames are most likely addressed to another RBridge on a multi-
access link and that other Rbridge will handle them.)
4. If the Ethertype is not TRILL, the frame is discarded.
5. If the Version field in the TRILL Header is greater than 0, the
frame is discarded.
6. If the hop count is 0, the frame is discarded.
7. If the Outer.MacDA is multicast and the M bit is zero or if the
Outer.MacDA is unicast and M bit is one, the frame is discarded.
8. The port on which the frame was received is checked and the frame
discarded if there is no TRILL IS-IS adjacency on that port.
9. The Inner.MacDA is then tested. If it is the All-ESADI-RBridges
multicast address and RBn implements the ESADI feature, processing
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proceeds as in Section 4.6.2.2 below. If it is any other address
or RBn does not implement the ESADI feature, processing proceeds
as in Section 4.6.2.3.
4.6.2.1 TRILL Control Frames
The frame is processed by the TRILL IS-IS instance on RBn and is not
forwarded.
4.6.2.2 TRILL ESADI Frames
If M == 0, the frame is silently discarded.
The egress nickname designates the distribution tree. The frame is
forwarded as described in Section 4.6.2.5. In addition, if the
forwarding Rbridge is an appointed forwarder for a link in the
specified VLAN and implements a TRILL ESADI for that VLAN and ESADI
is enabled, the inner frame is decapsulated and provided to that
local ESADI.
4.6.2.3 TRILL Data Frames
The M flag is then checked. If it is zero, processing continues as
described in Section 4.6.2.4, if it is one, processing continues as
described in Section 4.6.2.5.
4.6.2.4 Known Unicast TRILL Data Frames
The port on which the frame was received is checked and the frame
discarded if there is no TRILL IS-IS adjacency on that port.
The egress nickname in the TRILL header is examined and, if it is
unknown or reserved, the frame is discarded.
If the egress RBridge indicated is the RBridge performing the
processing (RBn), the frame being forwarded is decapsulated to native
form. The Inner.MacDA is checked: if it is not unicast, the frame is
silently discarded; if it is unicast, the frame is then either (1)
sent onto the link containing the destination if the RBridge is
appointed forwarder for that link for the frame's VLAN and is not
inhibited (or discarded if it is inhibited), (2) locally processed if
the RBridge itself is the destination, or (3) processed as in the
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following paragraph.
A known unicast TRILL data frame can arrive at the egress Rbridge
only to find that the Inner.MacDA is not actually known by that
RBridge. One way this can happen is that the Inner.MacDA may have
timed out in the egress RBridge MAC address cache. In this case, the
egress RBridge sends the native frame out on all links that are in
the frame's VLAN for which the RBridge is appointed forwarder and has
not been inhibited, except that it MAY refrain from sending the frame
on links where it knows there cannot be an end station with the
destination MAC address, for example the link port is configured as a
trunk (see Section 4.9.1).
If RBn is a transit RBridge the hop count is decremented by one and
the frame forwarded to the next hop RBridge towards the egress
RBridge. The Inner.VLAN and ingress nickname are not examined by a
transit RBridge when it forwards a known unicast TRILL data frame.
4.6.2.5 Multi-Destination TRILL Data Frames
The egress and ingress nicknames in the TRILL header are examined
and, if either is unknown or reserved, the frame is discarded.
The Outer.MacSA checked and the frame discarded if it is not a tree
adjacency for the tree indicated by the egress RBridge nickname on
the port where the frame was received. The reverse path forwarding
check is performed on the ingress and egress nicknames and the frame
discarded if it fails. If there are multiple TRILL-Hello pseudonode
suppressed parallel links to the previous hop RBridge, the frame is
discarded if it has been received on the wrong one. If the RBridge
has multiple ports connected to the link, the frame is discarded
unless it was received on the right one. For more information on the
checks in this paragraph, see Section 4.5.2.
If the RBridge is an appointed forwarder for the VLAN of the frame, a
copy of the frame is decapsulated, sent in native form on those links
in its VLAN for which the RBridge is appointed forwarder subject to
additional pruning and inhibition as described in Section 4.2.4.3,
and/or locally processed as appropriate.
The hop count is decreased (see Section 3.6) and the frame is
forwarded down the tree specified by the egress RBridge nickname
pruned as described in Section 4.5.
In the forwarded frame, the Outer.MacSA is set to that of the port on
which the frame is being transmitted and the Outer.MacDA is the All-
RBridges multicast address.
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4.6.3 Receipt of a Layer 2 Control Frame
Low-level control frames received by an RBridge are handled within
the port where they are received as described in Section 4.9.
There are two types of high-level control frames, distinguished by
their destination address, which are handled as described in the
sections referenced below.
Name Section Destination Address
BPDU 4.9.3 01-80-C2-00-00-00
VRP 4.9.4 01-80-C2-00-00-21
4.7 IGMP, MLD, and MRD Learning
Ingress RBridges SHOULD learn, based on seeing native IGMP [RFC3376],
MLD [RFC2710], and MRD [RFC4286] frames, which IP derived multicast
messages should be forwarded onto which links. Such frames are also,
in general, encapsulated as TRILL data frames and distributed as
described below and in Section 4.5.
An IGMP or MLD membership report received in native form from a link
indicates a multicast group listener for that group on that link. An
IGMP or MLD query or an MRD advertisement received in native form
from a link indicates the presence of an IP multicast router on that
link.
IP multicast group membership reports have to be sent throughout the
campus and delivered to all IP multicast routers, distinguishing IPv4
and IPv6. All IP-derived multicast traffic must also be sent to all
IP multicast routers for the same version of IP.
IP multicast data SHOULD only be sent on links where there is either
an IP multicast router for that IP type (IPv4 or IPv6) or an IP
multicast group listener for that IP multicast derived MAC address,
unless the IP multicast address is in the range required to be
treated as broadcast.
RBridges do not need to announce themselves as listeners to the All-
Snoopers multicast group (the group used for MRD reports [RFC4541]),
because the IP multicast address for that group is in the range where
all frames sent to that IP multicast addresses must be broadcast.
See also "Considerations for Internet Group Management Protocol
(IGMP) and Multicast Listener Discovery (MLD) Snooping Switches"
[RFC4541].
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4.8 End Station Address Details
RBridges have to learn the MAC addresses and VLANs of their locally
attached end stations for link/VLAN pairs for which they are the
appointed forwarder so they can
o forward the native form of incoming known unicast TRILL data
frames onto the correct link and
o decide, for an incoming native unicast frame from a link, where
the RBridge is the appointed forwarder for the frame's VLAN,
whether the frame is
- known to have been destined for another end station on the same
link, so the RBridge need do nothing, or
- has to be converted to a TRILL data frame and forwarded.
RBridges need to learn the MAC addresses, VLANs, and remote RBridges
of remotely attached end stations for VLANs for which they and the
remote RBridge are an appointed forwarder, so they can efficiently
direct native frames they receive which are unicast to those
addresses and VLANs.
4.8.1 Learning End Station Addresses
There are five independent ways an RBridge can learn end station
addresses as follows:
1. From the observation of VLAN-x frames received on ports where it
is appointed VLAN-x forwarder, learning the { source MAC, VLAN,
port } triplet of received frames.
2. The { source MAC, VLAN, ingress RBridge nickname } triplet of any
native frames that it decapsulates.
3. By Layer 2 registration protocols learning the { source MAC, VLAN,
port } of end stations registering at a local port.
4. By running one or more TRILL ESADIs that receive remote address
information and transmit local address information.
5. By management configuration.
RBridges MUST implement capabilities 1 and 2 above. RBridges use
these capabilities unless configured, for one or more particular
VLANs and/or ports, to not learn from either received frames or from
decapsulating native frames to be transmitted or both.
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RBridges MAY implement capabilities 3 and 4 above. If capability 4 is
implemented, such ESADIs are run only when the RBridge is configured
to do so on a per-VLAN basis.
RBridges SHOULD implement capability 5.
Entries in the table of learned MAC addresses and associated
information also have a one octet unsigned confidence level
associated with each entry. Such information learned from the
observation of data has a confidence of 0x20 unless configured to
have a different confidence. This confidence level can be configured
on a per RBridge basis separately for information learned from local
native frames and that learned from remotely originated encapsulated
frames. Such information received via TRILL ESADI is accompanied by
a confidence level in the range 0 to 254. Such information configured
by management defaults to a confidence level of 255 but may be
configured to have another value.
The table of learned MAC addresses includes (1) { confidence, VLAN,
MAC address, local port } for addresses learned from local native
frames and local registration protocols, (2) { confidence, VLAN, MAC
address, egress RBridge nickname } for addresses learned from remote
encapsulated frames and ESADI link state databases, and (3)
additional information to implement timeout of learned addresses,
statically configured addresses, and the like.
When a new learned address and related information are to be entered
into the local database there are three possibilities:
A. If this is a new { address, VLAN } pair, the information is
entered accompanied by the confidence level.
B. If there is already an entry for this { address, VLAN } pair with
the same accompanying delivery information, the confidence level
in the local database is set to the maximum of its existing
confidence level and the confidence level with which it is being
learned. In addition, if the information is being learned with the
same or a higher confidence level than its existing confidence
level, timer information is reset.
C. If there is already an entry for this { address, VLAN } pair with
different information, the learned information replaces the older
information only if it is being learned with higher or equal
confidence than that in the database entry. If it replaces older
information, timer information is also reset.
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4.8.2 Forgetting End Station Addresses
While RBridges need to learn end station addresses as described
above, it is equally important that they be able to forget such
information. Otherwise, frames for end stations that have moved to a
different part of the campus could be indefinitely black holed by
RBridges with stale information as to the link to which the end
station is attached.
For end station address information locally learned from frames
received, the time out from the last time a native frame was received
or decapsulated with the information conforms to the recommendations
of [802.1Q-2005]. It is referred to as the "Aging Time" and is
configurable per RBridge with a range of from 10 seconds to 1,000,000
seconds and a default value of 300 seconds.
The situation is different for end station address information
acquired via TRILL ESADI. It is up to the originating RBridge to
decide when to remove such information from the ESADI LSP (or up to
ESADI timeouts if the originating RBridge becomes inaccessible).
When an RBridge ceases to be appointed forwarder for VLAN-x on a
port, it forgets all end station address information learned from the
observation of VLAN-x native frames received on that port. It also
increments a per VLAN counter of the number of times it lost
appointed forwarder status on one of its ports for that VLAN.
When, for all of its ports, RBridge RBn is no longer appointed
forwarder for VLAN-x, it forgets all end station address information
learned from decapsulating VLAN-x native frames. Also, if RBn is
participating in TRILL ESADI for VLAN-x, it ceases to so participate
after sending a final LSP nulling out the end station address
information for that VLAN which it had been originating. In addition,
all other RBridges that are VLAN-x forwarder on at least one of their
ports notice that the link state data for RBn has changed to show
that it no longer has a link on VLAN-x. In response, they forget all
end station address information they have learned from decapsulating
VLAN-x frames that show RBn as the ingress RBridge.
When the appointed forwarder lost counter for RBridge RBn for VLAN-x
is observed to increase via the TRILL link state database but RBn
continues to be an appointed forwarder for VLAN-x on at least one of
its ports, every other RBridge that is an appointed forwarder for
VLAN-x modifies the aging of all the addresses it has learned by
decapsulating native frames in VLAN-x from ingress RBridge RBn as
follows: The time remaining for each entry is adjusted to be no
larger than a per RBridge configuration parameter called (to
correspond to [802.1Q-2005]) "Forward Delay". This parameter is in
the range of 4 to 30 seconds with a default value of 15 seconds.
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4.8.3 Shared VLAN Learning
RBridges can map VLAN IDs into a smaller number of identifiers for
purposes of address learning, as [802.1Q-2005] bridges can. Then,
when a lookup is done in learned address information, this identifier
is used for matching in place of the VLAN ID. If the ID of the VLAN
on which the address was learned is not retained, then there are the
following consequences:
o The RBridge no longer has the information needed to participate in
TRILL ESADI for the VLANs who's ID is not being retained.
o In cases where 4.8.2 above requires the discarding of learned
address information based on a particular VLAN, when the VLAN ID
is not available for entries under a shared VLAN identifier,
instead the time remaining for each entry under that shared VLAN
identifier is adjusted to be no longer than the RBridge's "Forward
Delay".
Although outside the scope of this specification, there are some
Layer 2 features in which a set of VLANs has shared learning, where
one of the VLANs is the "primary" and the other VLANs in the group
are "secondaries". An example of this is where traffic from different
communities are separated using VLAN tags, and yet some resource
(such as an IP router or DHCP server) is to be shared by all the
communities. A method of implementing this feature is to give a VLAN
tag, say Z, to a link containing the shared resource, and have the
other VLANs, say A, C, and D, be part of the group { primary = Z,
secondaries = A, C, D }. An RBridge, aware of this grouping, attached
to one of the secondary VLANs in the group also claims to be attached
to the primary VLAN. So an RBridge attached to A would claim to also
be attached to Z. An RBridge attached to the primary would claim to
be attached to all the VLANs in the group.
This document does not specify how VLAN groups might be used. Only
RBridges that participate in a VLAN group will be configured to know
about the VLAN group. However, to detect misconfiguration, an RBridge
configured to know about a VLAN group SHOULD report the VLAN group in
its LSP.
4.9 RBridge Ports
Section 4.9.1 below describes the several RBridge port configuration
bits, Section 4.9.2 gives a logical port structure in terms of frame
processing, and Sections 4.9.3 and 4.9.4 describe the handling of
high-level control frames.
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4.9.1 RBridge Port Configuration
There are four per port configuration bits as follows:
o Disable port bit. When this bit is set, all frames received or to
be transmitted are discarded, with the possible exception of some
layer 2 control frames that may be generated and transmitted or
received and processed locally.
o End station service disable (trunk port) bit. When this bit is
set, all native frames received on the port and all native frames
that would have been sent on the port are discarded. (See Appendix
B.) (Note that, for this document, "native frames" does not
include layer 2 control frames.)
If a port with end station service disabled reports, in a TRILL-
Hello frame it sends out that port, which VLANs it provides end
station support for, it reports that there are none.
o TRILL traffic disable (access port) bit. If this bit is set, the
goal is to avoid sending any TRILL frames, except TRILL-Hello
frames, on the port since it is intended only for native end
station traffic. This bit is reported in TRILL-Hello frames. If
RB1 is the DRB and has this bit set in its TRILL-Hello, the DRB
still appoints VLAN forwarders. However, usually no pseudonode is
reported, and none of the inter-RBridge links associated with that
link are reported in LSPs.
If the DRB RB1 does not have this bit set, but neighbor RB2 on the
link does have the bit set, then RB1 does not appoint RB2 as
designated forwarder for any VLAN, and none of the RBridges
(including the pseudonode) report RB2 as a neighbor in LSPs.
In some cases even though the DRB has the "access port" flag set,
the DRB MAY choose to create a pseudonode for the access port. In
this case, the other RBridges report connectivity to the
pseudonode in their LSP, but the DRB sets the "overload" flag in
the pseudonode LSP.
o Use P2P Hellos bit. If this bit is set, Hellos sent on this port
are IS-IS P2P Hellos, not the default TRILL-Hellos. In addition,
the IS-IS P2P three-way handshake MUST be used on P2P RBridge
links.
The dominance relationship of these four configuration bits is as
follows, where configuration bits to the left dominate those to the
right. That is to say, when any pair of bits are asserted,
inconsistencies in behavior they mandate are resolved in favor of the
bit to the left in the this list.
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Disable > P2P > Access > Trunk
4.9.2 RBridge Port Structure
An RBridge port can be modeled as having a lower level structure
similar to that of an [802.1Q-2005] bridge port as shown in Figure
4.7. In this figure, the double lines represent the general flow of
the frames and information while single lines represent information
flow only. The dashed lines in connection with VRP (GVRP/MVRP) are to
show that VRP support is optional. An actual RBridge port
implementation may be structured in any way that provides the correct
behavior.
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+----------------------------------------------
| RBridge
|
| Interport Forwarding, IS-IS. Management, ...
|
+----++----------------------+-------------++--
|| | ||
Trill || Data | ||
|| +--+---------+ ||
+-------------++-----+ | TRILL | ||
| Encapsulation | +------+ IS-IS Hello| ||
| Decapsulation | | | Processing | ||
| Processing | | +-----++-----+ ||
+--------------------+ | || ||
| RBridge Appointed +------+ || ||
+---+ Forwarder and | || ||
| | Inhibition Logic +==============\\ || //====++
| +---------+--------+-+ Native \\ ++ //
| | | Frames \++/
| | | ||
+----+-----+ +- - + - - + | ||
| RBridge | | RBridge | | || All TRILL and
| BPDU | | VRP | | || Native Frames
|Processing| |Processing| | ||
+-----++---+ + - - -+- -+ | +--------++--+ <- EISS
|| | | | 802.1Q |
|| | | | Port VLAN |
|| | | |and priority|
|| | | | Processing |
+---++------------++------+------------+------------+ <-- ISS
| 802.1/802.3 Low Level Control Frame |
| Processing, Port/Link Control Logic |
+------------++-------------------------------------+
||
|| +------------+
|| | 802.3 PHY |
++========+ (Physical +======== 802.3
| Interface) | Link
+------------+
Figure 4.5: Detailed RBridge Port Model
Low-level control frames are handled in the lower level port/link
control logic in the same way as in an [802.1Q-2005] bridge. This
can optionally include a variety of 802.1 or link specific protocols
such as link layer discovery, link aggregation (Clause 43 of
[802.3]), MAC security [802.1AE], or port based access control
[802.1X]. While handled at a low level, these frames may affect
higher level processing. For example, a Layer 2 registration protocol
may affect the confidence in learned addresses. The upper interface
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to this lower level port control logic corresponds to the Internal
Sublayer Service (ISS) in [802.1Q-2005].
High-level control frames (BPDUs and, if supported, VRP frames) are
not VLAN tagged. Although they extend through the ISS interface, they
are not subject to port VLAN processing. Behavior on receipt of a
VLAN tagged BPDU or VLAN tagged VRP frame, is unspecified. If a VRP
is not implemented, then all VRP frames are discarded. Handling of
BPDUs is described in Section 4.9.3. Handling of VRP frames is
described in Section 4.9.4.
Frames other than layer 2 control frames, that is, all TRILL and
native frames, are subject to Port VLAN and priority processing which
is the same as for an [802.1Q-2005] bridge. The upper interface to
the port VLAN and priority processing corresponds to the Extended
Internal Sublayer Service (EISS) in [802.1Q-2005].
In this model, RBridge port processing below the EISS layer is
identical to an [802.1Q-2005] bridge except for (1) the handling of
high-level control frames and (2) that the discard of frames that
have exceeded the Maximum Transit Delay is not mandatory but MAY be
done.
Incoming native frames are only accepted if the RBridge is an
uninhibited appointed forwarder for the frame's VLAN, after which
they are normally encapsulated and forwarded. Outgoing native frames
are usually obtained by decapsulation and are only output if the
RBridge is an uninhibited appointed forwarder for the frame's VLAN.
TRILL-Hellos, MTU-probes, and MTU-acks are handled per port and never
forwarded. They can affect the appointed forwarder and inhibition
logic as well as the RBridge's LSP.
Except TRILL-Hellos, MTU-probes, and MTU-acks, all TRILL control as
well as TRILL data and ESADI frames are passed up to higher level
RBridge processing on receipt and transmitted on creation or
forwarding. Note that these frames are never blocked due to the
appointed forwarder and inhibition logic, which affects only native
frames, but there are additional filters on some of them such as the
Reverse Path Forwarding Check.
4.9.3 BPDU Handling
If RBridge campus topology were static, RBridges would simply be end
stations from a bridging perspective, terminating but not otherwise
interacting with spanning tree. However, there are reasons for
RBridges to listen to and sometimes to transmit BPDUs as described
below. Even when RBridges listen to and transmit BPDUs, this is a
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local RBridge port activity. The ports of a particular RBridge never
interact so as to make the RBridge as a whole a spanning tree node.
4.9.3.1 Receipt of BPDUs
Rbridges MUST listen to spanning tree BPDUs received on a port and
keep track of the root bridge, if any, on that link. If MSTP is
running on the link, this is the CIST root. This information is
reported per VLAN by the RBridge in its LSP. In addition, the receipt
of spanning tree BPDUs is used as an indication that a link is a
bridged LAN, which can affect the RBridge transmission of BPDUs.
An RBridge MUST NOT encapsulate or forward any BPDU frame it
receives.
RBridges discard any topology change BPDUs they receive, but note
Section 4.9.3.3.
4.9.3.2 Root Bridge Changes
A change in the root bridge seen in the BPDUs received at an RBridge
port may indicate a change in bridged LAN topology, including the
possibility of the merger of two bridged LANs or the like, without
any physical level indication at the port. During topology
transients, bridges may go into pre-forwarding states that block
TRILL-Hello frames. For these reasons, when an RBridge sees a root
bridge change on a port for which it is appointed forwarder for one
or more VLANs, it is inhibited (discards all native frames received
from or which it would otherwise have sent to the link) for a period
of time between zero and 30 seconds. This time period is configurable
per RBridge and defaults to 30 seconds.
For example, consider two bridged LANs carrying multiple VLANs, each
with various RBridge appointed forwarders. Should they become merged,
due to a cable being plugged in or the like, those RBridges attached
to the original bridged LAN with the lower priority root will see a
root bridge change while those attached to the other original bridged
LAN will not. Thus all appointed forwarders in the first set will be
inhibited for a time period while things are sorted out by BPDUs
within the merged bridged LAN and TRILL-Hello frames between the
RBridges involved.
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4.9.3.3 Transmission of BPDUs
When an RBridge ceases to be appointed forwarder for one or more
VLANs out a particular port it SHOULD, as long as it continues to
receive spanning tree BPDUs on that port, send topology change BPDUs
until it sees the topology change acknowledged in a spanning tree
BPDU.
RBridges MAY support a capability for sending spanning tree BPDUs for
the purpose of attempting to force a bridged LAN to partition as
discussed in Section A.3.3.
4.9.4 Dynamic VLAN Registration
Dynamic VLAN registration provides a means for bridges (and less
commonly end stations) to request that VLANs be enabled or disabled
on ports leading to the requestor. This is done by VLAN registration
protocol (VRP) frames: GVRP or MVRP. RBridges MAY implement GVRP
and/or MVRP as described below.
VRP frames are never encapsulated as TRILL frames between RBridges or
forwarded in native form by an RBridge. If an RBridge does not
implement a VRP, it discards any VRP frames received and sends none.
RBridge ports may have dynamically enabled VLANs. If an RBridge
supports a VRP, the actual enablement of dynamic VLANs is determined
by GVRP/MVRP frames received at the port as it would be for an
[802.1Q-2005] / [802.1ak] bridge.
An RBridge that supports a VRP sends GVRP/MVRP frames as an
[802.1Q-2005] / [802.1ak] bridge would send on each port that is not
configured as an RBridge trunk port or P2P port. For this purpose, it
sends VRP frames to request traffic in the VLANs for which it is
appointed forwarder and in the Designated VLAN, unless the Designated
VLAN is disabled on the port, and to not request traffic in any other
VLAN.
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5. Addresses, Configuration Parameters, and Constants
IS-IS requires each RBridge to have a unique 48-bit (6-octet) System
ID. This is easily obtainable, for example, as any one of the MAC-48
addresses owned by that RBridge.
Two new Ethertypes must be assigned: one indicate a TRILL
encapsulated frame and one to indicate a TRILL control frame.
Three Layer 2 multicast addresses must be assigned:
o All-RBridges for use as Outer.MacDA in TRILL ESADI and multi-
destination TRILL data frames.
o All-IS-IS-RBridges for use as the Outer.MacDA for TRILL IS-IS
frames.
o All-ESADI-RBridges for use as the Inner.MacDA for TRILL ESADI
frames.
The following per RBridge parameters may be configured:
o One or more nicknames and corresponding nickname selection
priorities.
o Priority to be a distribution tree root, a desired number of
distribution trees for the campus, a desired number of
distribution tree to use, and two lists of RBridge nicknames,
as discussed in Section 4.5.
o The per RBridge parameters Aging Timer, Forward Delay, and
Maximum Transit Delay.
RBridges may be configured to have ESADI (end station address
distribution information) protocol instances and to send and/or learn
end station address information via such instances. Static end
address information and priority of such end station information
statically configured and learned in various ways can also be
configured.
The following RBridge per port parameters:
o The same parameters as for an [802.1Q-2005] port in terms of
VLAN C-tags and frame priority code points.
o Four per-port configuration bits: disable port, disable end
station service (trunk), access port, and use P2P Hellos (see
Section 4.9.1).
o Configuration for the optional send-BPDUs solution to the
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wiring closet topology problem (see Section A.3.3) consists of
System ID of the RBridge with lowest System ID. If RB1 and RB2
are part of a wiring closet topology, both need to be
configured to know about this, and that RB1 is the ID that
should be used in the spanning tree protocol on the specified
port.
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6. Security Considerations
Layer 2 bridging in not inherently secure. It is, for example,
subject to spoofing of source addresses and bridging control
messages. A goal for TRILL is that RBridges do not add new issues
beyond those existing in current bridging technology.
Countermeasures are available such as to configure the TRILL IS-IS
and ESADI instances to use IS-IS security [RFC5304] and ignore
unauthenticated TRILL control and ESADI frames received. Since such
authentication requires configuration, RBridges using it are no
longer zero configuration.
IEEE 802.1 port admission and link security mechanisms, such as
[802.1X] and [802.1AE], can also be used. These are best thought of
as being implemented within a port and are outside the scope of TRILL
(just as they are generally out of scope for bridging standards
[802.1D] and 802.1Q); however, TRILL can make use of secure
registration through the confidence level communicated in optional
TRILL ESADI (see Section 4.8).
TRILL encapsulates native frames inside the RBridge campus while they
are in transit between ingress RBridge and egress RBridge(s). Thus,
TRILL ignorant devices with firewall features that cannot be detected
by RBridges as end stations will generally not be able to inspect the
content of such frames for security checking purposes. This may
render them ineffective. Layer 3 routers and hosts appear to
RBridges to be end stations and native frames will be decapsulated
before being sent to such devices. Thus they will not see the TRILL
Ethertype. Firewall devices that do not appear to an RBridge to be an
end station, for example bridges with co-located firewalls, should be
modified to understand TRILL encapsulation.
RBridges do not prevent nodes from impersonating other nodes, for
instance, by issuing bogus ARP/ND replies. However, RBridges do not
interfere with any schemes that would secure neighbor discovery.
6.1 VLAN Security Considerations
TRILL supports VLANs. These provide logical separation of traffic but
care should be taken in using VLANs for security purposes. To have
reasonable assurance of such separation, all the RBridges and links
(including bridged LANs) in a campus must be secured and configured
so as to prohibit end stations from using dynamic VLAN registration
frames or otherwise gaining access to any VLAN carrying traffic for
which they are not authorized to read and/or inject.
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Furthermore, if VLANs were used to keep some information off links
where it might be observed in a bridged LAN, this will no longer work
in general when bridges are replaced with RBridges; with
encapsulation and a different outer VLAN tag, the data will travel
the least cost transit path regardless of VLAN. Appropriate counter
measures are to use end-to-end encryption or an appropriate TRILL
security option should one be specified.
6.2 BPDU/Hello Denial of Service Considerations
The TRILL protocol requires that an appointed forwarder at an RBridge
port be temporarily inhibited if it sees a TRILL-Hello from another
RBridge claiming to be the appointed forwarder for the same VLAN or
sees a root bridge change out that port. Thus it would seem that
forged BPDUs showing repeated root bridge changes and forged TRILL-
Hello frames with the Appointed Forwarder flag set could represent a
significant denial of service attack. However, the situation is not
as bad as it seems.
The best defense against forged TRILL-Hello frames or other IS-IS
messages is the use of IS-IS security [RFC5304]. Rogue end-stations
would not normally have access to the required IS-IS keying material
needed to forge authenticatible messages.
Authentication similar to IS-IS security is usually unavailable for
BPDUs. However, it is also the case that in typical modern wired
LANs, all the links are point-to-point. If you have an all-RBridged
point-to-point campus, then the worst that an end-station can do by
forging BPDUs or TRILL-Hello frames is to deny itself service. This
could be either through falsely inhibiting the forwarding of native
frames by the RBridge to which it is connected or by falsely
activating the optional decapsulation check (see Section 4.2.4.3).
However, when an RBridge campus contains bridged LANs, those bridged
LANs appear to any connected RBridges to be multi-access links. The
forging of BPDUs by an end-station attached to such a bridged LAN
could affect service to other end-stations attached to the same
bridged LAN. Note that bridges never forward BPDUs but process them,
although this processing may result in the issuance of further BPDUs.
Thus, for an end-station to forge BPDUs to cause continuing changes
in the root bridge as seen by an RBridge through intervening bridges
would typically require it to cause root bridge thrashing throughout
the bridged LAN that would be disruptive even in the absence of
RBridges.
Some bridges can be configured to not send BPDUs and/or to ignore
BPDUs on particular ports and RBridges can be configured not to
inhibit appointed forwarding on a port due to root bridge changes;
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however, such configuration should be used with caution as it can be
unsafe.
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7. Assignment Considerations
This section discuses IANA and IEEE 802 assignment considerations.
See [RFC5226].
7.1 IANA Considerations
A new IANA registry is created for TRILL Versions, Nicknames, Version
0 Header Reserved bits, and multicast addresses.
The initial contents of the TRILL Version Registry is as follows:
Version Status
0 As specified in <RFC-this-document>
1-3 Available for allocation by IETF Standards Action
The initial contents of the Version 0 Header Reserved Bits Registry
is as follows:
Bit Status
0x4 Available for allocation by IETF Standards Action
0x2 Available for allocation by IETF Standards Action
0x1 Multi-destination bit as specified in <RFC-this-document>
The initial contents of the TRILL Nicknames Registry is as follows:
0x0000 Reserved to indicate no nickname specified
0x0001-0xFFBF Dynamically allocated by the RBridges within each
RBridge campus
0xFFC0-0xFFFE Available for allocation by RFC Publication (single
value) or IETF Review (single or multiple values)
0xFFFF Permanently reserved
The initial contents of the TRILL Multicast Address Registry is as
follows:
01-80-C2-XX-XX-X0 Assigned as All-RBridges
01-80-C2-XX-XX-X1 Assigned as All-IS-IS-RBridges
01-80-C2-XX-XX-X2 Assigned as All-ESADI-RBridges
01-80-C2-XX-XX-X3 to 01-80-C2-XX-XX-XF Available for allocation
by IETF Review
7.2 IEEE Registration Authority Considerations
The Ethertype <tbd> is assigned by the IEEE Registration Authority to
the TRILL Protocol.
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The Ethertype <tbd> is assigned by the IEEE Registration Authority
for L2-IS-IS.
The block of 16 multicast MAC addresses from <01-80-C2-XX-XX-X0> to
<01-80-C2-XX-XX-XF> are assigned by the IEEE Registration Authority
for IETF TRILL protocol use.
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8. Normative References
[802.1ak] "IEEE Standard for Local and metropolitan area networks /
Virtual Bridged Local Area Networks / Multiple Registration
Protocol", IEEE Standard 802.1ak-2007, 22 June 2007.
[802.1D] "IEEE Standard for Local and metropolitan area networks /
Media Access Control (MAC) Bridges", 802.1D-2004, 9 June 2004.
[802.1Q-2005] "IEEE Standard for Local and metropolitan area networks
/ Virtual Bridged Local Area Networks", 802.1Q-2005, 19 May 2006.
[802.3] "IEEE Standard for Information technology /
Telecommunications and information exchange between systems /
Local and metropolitan area networks / Specific requirements Part
3: Carrier sense multiple access with collision detection
(CSMA/CD) access method and physical layer specifications",
802.3-2005, 9 December 2005
[ISO10589] ISO/IEC 10589:2002, "Intermediate system to Intermediate
system routeing information exchange protocol for use in
conjunction with the Protocol for providing the Connectionless-
mode Network Service (ISO 8473)," ISO/IEC 10589:2002.
[RFC1112] Deering, S., "Host Extensions for IP Multicasting", STD 5,
RFC 1112, Stanford University, August 1989.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2464] Crawford, M., "Transmission of IPv6 Packets over Ethernet
Networks", RFC 2464, December 1998.
[RFC2710] Deering, S., Fenner, W., and B. Haberman, "Multicast
Listener Discovery (MLD) for IPv6", RFC 2710, October 1999.
[RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
Thyagarajan, "Internet Group Management Protocol, Version 3", RFC
3376, October 2002.
[RFC4286] Haberman, B., Martin, J., "Multicast Router Discovery", RFC
4286, December 2005.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, October 2008.
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9. Informative References
[802.1ad] "IEEE Standard for and metropolitan area networks / Virtual
Bridged Local Area Networks / Provider Bridges", 802.1ad-2005, 26
May 2005.
[802.1AE] "IEEE Standard for Local and metropolitan area networks /
Media Access Control (MAC) Security", 802.1AE-2006, 18 August 2006
[802.1X] "IEEE Standard for Local and metropolitan area networks /
Port Based Network Access Control", 802.1X-2004, 13 December 2004.
[RBridges] Perlman, R., "RBridges: Transparent Routing", Proc.
Infocom 2005, March 2004.
[RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
RFC 1661, July 1994.
[RFC3411] Harrington, D., Presuhn, R., and B. Wijnen, "An
Architecture for Describing Simple Network Management Protocol
(SNMP) Management Frameworks", STD 62, RFC 3411, December 2002.
[RFC4086] Eastlake, D., 3rd, Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086, June
2005.
[RFC4541] Christensen, M., Kimball, K., and F. Solensky,
"Considerations for Internet Group Management Protocol (IGMP) and
Multicast Listener Discovery (MLD) Snooping Switches", RFC 4541,
May 2006.
[RFC4789] Schoenwaelder, J. and T. Jeffree, "Simple Network
Management Protocol (SNMP) over IEEE 802 Networks", RFC 4789,
November 2006.
[RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic
Authentication", RFC 5304, October 2008.
[RFC5342] Eastlake 3rd., D., "IANA Considerations and IETF Protocol
Usage for IEEE 802 Parameters", BCP 141, RFC 5342, September 2008.
[RFC5556] Touch, J. and R. Perlman, "Transparent Interconnection of
Lots of Links (TRILL): Problem and Applicability Statement", RFC
5556, May 2009.
[RP1999] Perlman, R., "Interconnection: Bridges, Routers, Switches,
and Internetworking Protocols, 2nd Edition", Addison Wesley
Longman, Chapter 3, 1999.
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Appendix A: Incremental Deployment Considerations
Some aspects of partial RBridge deployment are described below for
link cost determination (Section A.1) and possible congestion due to
appointed forwarder bottlenecks (Section A.2). A particular example
of a problem related to the TRILL use of a single appointed forwarder
per link per VLAN (the "wiring closet topology") is explored in
detail in Section A.3.
A.1 Link Cost Determination
With an RBridged campus having no bridges or repeaters on the links
between RBridges, the RBridges can accurately determine the number of
physical hops involved in a path and the line speed of each hop,
assuming this is reported by their port logic. With intervening
devices, this is no longer possible. For example, as shown in Figure
A.1, the two bridges B1 and B2 can completely hide a slow link so
that both Rbridges RB1 and RB2 incorrectly believe the link is
faster.
+-----+ +----+ +----+ +-----+
| | Fast | | Slow | | Fast | |
| RB1 +--------+ B1 +--------+ B2 +--------+ RB2 |
| | Link | | Link | | Link | |
+-----+ +----+ +----+ +-----+
Figure A.1: Link Cost of a Bridged Link
Even in the case of a single intervening bridge, two RBridges may
know they are connected but each see the link as a different speed
from how it is seen by the other.
However, this problem is not unique to RBridges. For example, routers
can encounter similar situations due to links hidden by bridges,
repeaters or Rbridges.
A.2 Appointed Forwarders and Bridged LANs
With partial RBridge deployment, the RBridges may partition a bridged
LAN into a relatively small number of relatively large remnant
bridged LANs, or possibly not partition it at all so a single bridged
LAN remains. Such configuration can result in the following problem:
The requirement that native frames enter and leave a link via the
link's appointed forwarder for the VLAN of the frame can cause
congestion or suboptimal routing. (Similar problems can occur within
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a bridged LAN due to the spanning tree algorithm.) The extent to
which such a problem will occur is highly dependent on the network
topology. For example, if a bridged LAN had a star-like structure
with core bridges that connected only to other bridges and peripheral
bridges that connected to end stations and are connected to core
bridges, the replacement of all of the core bridges by RBridges
without replacing the peripheral bridges would generally improve
performance without inducing appointed forwarder congestion.
Solutions to this problem are discussed below and a particular
example explored in Section A.3.
Inserting RBridges so that all the bridged portions of the LAN stay
connected to each other and have multiple RBridge connections is
generally the least efficient arrangement.
There are four techniques that may help if the problem above occurs
and which can, to some extent, be used in combination:
1. Replace more IEEE 802.1 bridges with RBridges so as to minimize
the size of the remnant bridged LANs between RBridges. This
requires no configuration of the RBridges unless the bridges they
replace required configuration.
2. Re-arrange network topology to minimize the problem. If the
bridges and RBridges involved are configured, this may require
changes in their configuration.
3. Configure the RBridges and bridges so that end stations on a
remnant bridged LAN are separated into different VLANs that have
different appointed forwarders. If the end stations were already
assigned to different VLANs, this is straightforward (see Section
4.2.4.2). If the end stations were on the same VLAN and have to be
split into different VLANs, this technique may lead to
connectivity problems between end stations.
4. Configure the RBridges such that their ports which are connected
to the bridged LAN send BPDUs (see Section A.3.3) in such a way as
to force the partition of the bridged LAN. (Note: A spanning tree
is never formed through an RBridge but always terminates at
RBridge ports.) To use this technique, the RBridges must support
this optional feature, and would need to be configured to make use
of it, but the bridges involved would rarely have to be
configured. Warning: This technique makes the bridged LAN
unavailable for TRILL through traffic because the bridged LAN
partitions.
Conversely to item 3 above, there may be bridged LANs which use
VLANs, or use more VLANs than would otherwise be necessary, to
support the Multiple Spanning Tree Protocol or otherwise reduce the
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congestion that can be caused by a single spanning tree. Replacing
the IEEE 802.1 bridges in such LANs with RBridges may enable a
reduction in or elimination of VLANs and configuration complexity.
A.3 Wiring Closet Topology
If 802.1 bridges are present and RBridges are not properly
configured, the bridge spanning tree or the DRB may make
inappropriate decisions. Below is a specific example of the more
general problem that can occur when a bridged LAN is connected to
multiple RBridges.
In cases where there are two (or more) groups of end nodes, each
attached to a bridge (say B1 and B2), and each bridge is attached to
an RBridge (say RB1 and RB2 respectively), with an additional link
connecting B1 and B2 (see Figure A.2), it may be desirable to have
the B1-B2 link only as a backup in case one of RB1 or RB2 or one of
the links B1-RB1 or B2-RB2 fail.
+-------------------------------+
| | | |
| Data +-----+ +-----+ |
| Center -| RB1 |----| RB2 |- |
| +-----+ +-----+ |
| | | |
+-------------------------------+
| |
| |
+-------------------------------+
| | | |
| +----+ +----+ |
| Wiring | B1 |-----| B2 | |
| Closet +----+ +----+ |
| Bridged |
| LAN |
+-------------------------------+
Figure A.2: Wiring Closet Topology
For example, B1 and B2 may be in a wiring closet and it may be easy
to provide a short, high bandwidth, low cost link between them while
RB1 and RB2 are at a distant data center such that the RB1-B1 and
RB2-B2 links are slower and more expensive.
Default behavior might be that one of RB1 or RB2 (say RB1) would
become DRB for the bridged LAN including B1 and B2 and appoint itself
forwarder for the VLANs on that bridged LAN. As a result, RB1 would
forward all traffic to/from the link, so end nodes attached to B2
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would be connected to the campus via the path B2-B1-RB1, rather than
the desired B2-RB2. This wastes the bandwidth of the B2-RB2 path and
cuts available bandwidth between the end stations and the data center
in half. The desired behavior would be to make use of both the RB1-B1
and RB2-B2 links.
Three solutions to this problem are described below.
A.3.1 The RBridge Solution
Of course, if B1 and B2 are replaced with RBridges, the right thing
will happen with zero configuration (other than VLAN support), but
this may not be immediately practical if bridges are being
incrementally replaced by RBridges.
A.3.2 The VLAN Solution
If the end stations attached to B1 and B2 are already divided among a
number of VLANs, RB1 and RB2 could be configured so that which ever
becomes DRB for this link will appoint itself forwarder for some of
these VLANs and appoint the other RBridge for the remaining VLANs.
Should either of the RBridges fail or become disconnected, the other
will have only itself to appoint as forwarder for all the VLANs.
If the end stations are all on a single VLAN, then it would be
necessary to assign them between at least two VLANs to use this
solution. This may lead to connectivity problems that might require
further measures to rectify.
A.3.3 The Spanning Tree Solution
Another solution is to configure RB1 and RB2 to be part of a "wiring
closet group", with a configured System ID RBx (which may be RB1 or
RB2's System ID). Both RB1 and RB2 emit BPDUs on their configured
ports as highest priority root RBx. This causes the spanning tree to
logically partition the bridged LAN as desired by blocking the B1-B2
link at one end or the other (unless one of the bridges is configured
to also have highest priority and has a lower ID, which we consider
to be a misconfiguration). With the B1-B2 link blocked, RB1 and RB2
cannot see each other's TRILL-Hellos via that link and each acts as
Designated RBridge and appointed forwarder for its respective
partition. Of course, with this partition, no TRILL through traffic
can flow over the RB1-B1-B2-RB2 path.
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In the spanning tree BPDU, the Root is "RBx" with highest priority,
cost to Root is 0, Designated Bridge ID is "RB1" when RB1 transmits
and "RB2" when RB2 transmits, and port ID is a value chosen
independently by each of RB1 and RB2 to distinguish each of its own
ports. (If RB1 and RB2 were actually bridges on the same shared
medium with no bridges between them, the result would be that the one
with the larger ID sees "better" BPDUs (because of the tiebreaker on
the third field: the ID of the transmitting RBridge), and would turn
off its port.)
Should either RB1 or the RB1-B1 link or RB2 or the RB2-B2 link fail,
the spanning tree algorithm will stop seeing one of the RBx roots and
will unblock the B1-B2 link maintaining connectivity of all the end
stations with the data center.
If the link RB1-B1-B2-RB2 is on the cut set of the campus and RB2 and
RB1 have been configured to believe they are part of a wiring closet
group, the campus becomes partitioned as the link is blocked.
A.3.4 Comparison of Solutions
Replacing all 802.1 bridges with RBridges is usually the best
solution with the least amount of configuration required, possibly
none.
The VLAN solution works well with a relatively small amount of
configuration if the end stations are already divided among a number
of VLANs. If they are not, it becomes more complex and problematic.
The spanning tree solution does quite well in this particular case.
But it depends on both RB1 and RB2 having implemented the optional
feature of being able to configure a port to emit BPDUs as described
in Section A.3.3 above. It also makes the bridged LAN whose partition
is being forced unavailable for through traffic. Finally, while in
this specific example it neatly breaks the link between the two
bridges B1 and B2, if there were a more complex bridged LAN, instead
of exactly two bridges, there is no guarantee that it would partition
into roughly equal pieces. In such a case, you might end up with a
highly unbalanced load on the RB1-B1 link and the RB2-B2 link.
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Appendix B: Trunk and Access Port Configuration
Many modern bridged LANs are organized into a core and access model,
The core bridges have only point-to-point links to other bridges
while the access bridges connect to end stations, core bridges, and
possibly other access bridges. It seems likely that some RBridge
campuses will be organized in a similar fashion.
An RBridge port can be configured as a trunk port, that is, a link to
another RBridge or RBridges, by configuring it to disable end station
support. There is no reason for such a port to have more than one
VLAN enabled and in its Announcing Set on the port. Of course, the
RBridge (or RBridges) to which it is connected must have the same
VLAN enabled. There is no reason for this VLAN to be other than the
default VLAN 1 unless, perhaps, the link is actually over carrier
Ethernet facilities that only provide some other specific VLAN or the
like. Such configuration minimizes wasted TRILL-Hellos and eliminates
useless decapsulation and transmission of multi-destination traffic
in native form onto the link. (see Sections 4.2.4 and 4.9.1)
An RBridge access port would be expected to lead to a link with end
stations and possibly one or more bridges. Such a link might also
have more than one RBridge connected to it to provide more reliable
service to the end stations. It would be a goal to minimize or
eliminate transit traffic on such as link as it is intended for end
station native traffic. This can be accomplished by turning on the
access port configuration bit for the RBridge port or ports connected
to the link as further detailed in Section 4.9.1.
When designing RBridge configuration user interfaces, consideration
should be given to making it convenient to configure ports as trunk
and access ports.
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Appendix C: Multipathing
Rbridges support multipathing of both known unicast and multi-
destination traffic. Implementation of multipathing is optional.
Multi-destination traffic can be multipathed by using different
distribution tree roots for different frames. For example, assume
that in Figure C.1 end stations attached to RBy are the source of
various multicast streams each of which has multiple listeners
attached to various of RB1 through RB9. Assuming equal bandwidth
links, a distribution tree rooted at RBy will predominantly use the
vertical links among RB1 through RB9 while one rooted at RBz will
predominantly use the horizontal. If RBy chooses its nickname as the
distribution tree root for half of this traffic and an RBz nickname
as the root for the other half, it may be able to substantially
increase the aggregate bandwidth by making use of both the vertical
and horizontal links among RB1 through RB9.
Since the distribution trees an RBridge must calculate are the same
for all RBridges and transit RBridges MUST respect the tree root
specified by the ingress RBridge, a campus will operate correctly
with a mix of RBridges some of which use different roots for
different multi-destination frames they ingress and some of which use
a single root for all such frames.
+---+
|RBy|---------------+
+---+ |
/ | \ |
/ | \ |
/ | \ |
+---+ +---+ +---+ |
|RB1|---|RB2|---|RB3| |
+---+ +---+ +---+\ |
| | | \ |
+---+ +---+ +---+ \+---+
|RB4|---|RB5|---|RB6|-----|RBz|
+---+ +---+ +---+ /+---+
| | | /
+---+ +---+ +---+/
|RB7|---|RB8|---|RB9|
+---+ +---+ +---+
Figure C.1: Multi-Destination Multipath
Known unicast equal cost multipathing (ECMP) can occur if, instead of
using a tie-breaker criterion when building an SPF path between
ingress and egress RBridges, information about equal cost paths is
retained. Different unicast frames can then be sent via different
equal cost paths. For example, in Figure C.2, there are three equal
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cost paths between RB1 and RB2 and two equal cost paths between RB2
and RB5.
A transit RBridge receiving a known unicast frame forwards it towards
the egress RBridge and is not concerned with whether it believes
itself to be on any particular path from the ingress RBridge or a
previous transit RBridge. Thus a campus will operate correctly with
a mix of RBridges some of which implement ECMP and some of which do
not.
As an alternative to multipathing, it might be possible to combine
the three paths between RB1 and RB2 into one logical link through the
"link aggregation" feature of 802.3 (see Clause 43 of [802.3]).
Rbridges MAY implement link aggregation. However, link aggregation
requires multiple single hop equal bandwidth links (no intervening
bridges). Equal cost multipathing is more general in that there can
be multiple hops with intervening bridges and RBridges and links of
different costs as long as the path cost is the same. (Generally,
the default estimate of the cost of a link is proportional to the
reciprocal of its line speed.)
+---+ double line = 10 Gbps
----- ===|RB3|--- single line = 1 Gbps
/ \ // +---+ \
+---+ +---+ +---+
===|RB1|-----|RB2| |RB5|===
+---+ +---+ +---+
\ / \ +---+ //
----- ----|RB4|===
+---+
Figure C.2: Known Unicast Multipath
When multipathing is used, frames that follow different paths will be
subject to different delays and may be re-ordered. While some
traffic may be order/delay insensitive, typically most traffic
consists of flows of frames where re-ordering within a flow is
damaging. How to determine flows or what granularity flows should
have is beyond the scope of this document but, as an example, under
many circumstances it would be safe to consider all the frames
flowing between a particular pair of end station ports to be a flow.
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Appendix D: Determination of VLAN and Priority
A high-level, informative summary of how VLAN ID and priority are
determined for incoming native frames, omitting some details, is
given in the bulleted items below. For more detailed information, see
[802.1Q-2005].
o When an untagged native frame arrives, a zero configuration
RBridge associates the default priority zero and the VLAN ID 1
with it. It actually sets the VLAN for the untagged frame to be
the "port VLAN ID" associated with that port. The port VLAN ID
defaults to VLAN ID 1 but may be configured to be any other VLAN
ID. An Rbridge may also be configured on a per port basis to
discard such frames or to associate a different priority code
point with them. Determination of the VLAN ID associated with an
incoming untagged non-control frame may also be made dependent on
the Ethertype or NSAP (referred to in 802.1 as the Protocol) of
the arriving frame, the source MAC address, or other local rules.
o When a priority tagged native frame arrives, a zero configuration
RBridge associates with it both the port VLAN ID, which defaults
to 1, and the priority code point provided in the priority tag in
the frame. An Rbridge may be configured on a per port basis to
discard such frames or to associate them with a different VLAN ID
as described in the point immediately above. It may also be
configured to map the priority code point provided in the frame by
specifying, for each of the eight possible values that might be in
the frame, what actual priority code point will be associated with
the frame by the RBridge.
o When a C-tagged (formerly called Q-tagged) native frame arrives, a
zero configuration RBridge associates with it the VLAN ID and
priority in the C-tag. An RBridge may be configured on a per port
per VLAN basis to discard such frames. It may also be configured
on a per port basis to map the priority value as specified above
for priority tagged frames.
In 802.1, the process of associating a priority code point with a
frame, including mapping a priority provided in the frame to another
priority, is referred to as priority "regeneration".
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Appendix E: Support of IEEE 802.1Q-2005 Amendments
This informational appendix briefly comments on RBridge support for
completed and in-process amendments to IEEE [802.1Q-2005]. There is
no assurance that existing RBridge protocol specifications or
existing bridges will support not yet specified future [802.1Q-2005]
amendments just as there is no assurance that existing bridge
protocol specifications or existing RBridges will support not yet
specified future TRILL amendments.
E.1 Completed Amendments
802.1ad-2005 Provider Bridges - Sometimes called "Q-in-Q", because
VLAN tags used to be called "Q-tags", 802.1ad specifies
Provider Bridges that tunnel customer bridge traffic within
service VLAN tags (S-tags). If the customer LAN is an RBridge
campus, that traffic will be bridged by Provider Bridges.
Customer bridge features involving Provider Bridge awareness,
such as the ability to configure a customer bridge port to add
an S-tag to a frame before sending it to a Provider Bridge, are
below the EISS layer and can be supported in RBridge ports
without modification to the TRILL protocol.
802.1ag-2007 Connectivity Fault Management (CFM) - This 802.1 feature
is at least in part dependent on the symmetric path and other
characteristics of spanning tree. The informal comments
provided to the IETF TRILL working group by the IEEE 802.1
working group stated that "TRILL weakens the applicability of
CFM.".
802.1ak-2007 Multiple Registration Protocol - Supported to the extent
described in Section 4.9.4.
802.1ah-2008 Provider Backbone Bridges - Sometimes called "MAC-in-
MAC", 802.1ah provides for Provider Backbone Bridges that
tunnel customer bridge traffic within different outer MAC
addresses and using a tag (the "I-tag") to preserve the
original MAC addresses and signal other information. If the
customer LAN is an RBridge campus, that traffic will be bridged
by Provider Backbone Bridges. Customer bridge features
involving Provider Backbone Bridge awareness, such as the
ability to configure a customer bridge port to add an I-tag to
a frame before sending it to a Provider Backbone Bridge, are
below the EISS layer and can be supported in RBridge ports
without modification to the TRILL protocol.
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E.2 In-process Amendments
The following are amendments to IEEE [802.1Q-2005] that are in
process. As such, the brief comments below are based on drafts and
may be incorrect for later versions or any final amendment.
802.1aq Shortest Path Bridging - This amendment provides for improved
routing in bridged LANs.
802.1Qat Stream Reservation Protocol - Modification to 802.1Q to
support the 802.1 Timing and Synchronization protocol. The
effort required to support 802.1Qat in RBridges has not been
examined.
802.1Qau Congestion Notification - It currently appears that
modifications to RBridge behavior above the EISS level would be
needed to support this amendment.
802.1Qav Forwarding and Queuing Enhancements for Time-Sensitive
Streams - Modification to 802.1Q to support the 802.1 Timing
and Synchronization protocol. The effort involved to support
802.1Qav in RBridges has not been examined.
802.1Qaw Management of Data-Driven and Data-Dependent Connectivity
Fault - Amendment building on 802.1ag. See comments on
802.1ag-2007 above.
802.1Qay Provider Backbone Bridge Traffic Engineering - Amendment
building on 802.1ah. See comments on 802.1ah-2008 above.
802.1Qaz Enhanced Transmission Selection - It appears that this
amendment will be below the EISS layer and can be supported in
RBridge ports without modification to the TRILL protocol.
802.1Qbb Priority-based Flow Control - Commonly called "per priority
pause", it appears that this amendment will be below the EISS
layer and can be supported in RBridge ports without
modification to the TRILL protocol.
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Appendix Z: Revision History
RFC Editor: Please delete this Appendix Z before publication. In
addition, please replace the string "<RFC-this-document>" where it
occurs in this document with "RFC xxxx" where xxxx is the RFC number
assigned to this document.
Changes from -03 to -04
1. Divide IANA Considerations section into IANA and IEEE parts. Add
IANA considerations for TRILL Header variations and reserved bit
and normative references to RFCs 2434 and 4020.
2. Add note on the terms Rbridge and TRILL to section 1.2.
3. Remove IS-IS marketing text.
4. Split Section 3 into Sections 3 and 4. Add a new top level
section "5. Pseudo Code", renumbering following sections. Move
pseudo code that was in old Section 3 into Section 5 and make
section 3 more textural. This idea is that Section 3 and 4 have
more readable text descriptions with some corner cases left out
for simplicity while section 5 has more structured and complete
coverage.
5. Revised and extended Security Considerations section.
6. Move multicast router attachment bit and IGMP membership report
information from the per-VLAN IS-IS instance to the core IS-IS
instance so the information can be used by core RBridges to prune
distribution trees.
7. Remove ARP/ND optimization.
8. Change TRILL Header to add option feature. Add option section.
9. Change TRILL Header to expand Version field to the Variation
field. Add TRILL message variations (8 bits) supported to the per
RBridge link state information.
10. Distinguish TRILL data and IS-IS messages by using Variation = 0
and 1.
11. Consistently state that VLAN pruning and IP derived multicast
pruning of distribution trees are SHOULD.
12. Add text and pseudo code to discard TRILL Ethertype data frames
received on a port that does not have an IS-IS adjacency on it.
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13. Add end station address learning section. Specify end station
address learning from decapsulated native frames.
14. Add nickname allocation priority and optional nickname
configuration. Reserve nickname values zero and 0xFFFF.
15. Explain about multiple Designated RBridges because of multiple
VLANS.
16. Add Incremental Deployment Considerations Section incorporating
expanded Wiring Closet Topology Section.
17. Add more detail on VLAN tag information and material on frame
priority.
18. Miscellaneous minor editing and terminology updates.
Changes from -04 to -05
NOTE: Section 5 was NOT updated as indicated below but the remainder
of the draft was so updated.
1. Mention optional VLAN and multicast optimization in Abstract.
2. Change to distinguish TRILL IS-IS from TRILL data frames based on
the Inner.MacDA instead of a TRILL Header bit.
3. Split IP multicast router attached bit in two so you can
separately indicate attachment of IPv4 and IPv6 routers. Provide
that these bits must be set if an RBridge does not actually do
multicast control snooping on ingressed traffic.
4. Add the term "port VLAN ID" (PVID).
5. Drop references to PIM. Improve discussions of IGMP, MLD, and MRD
messages.
6. Move M bit over one and create two-bit pruning field at the
bottom of the "V" combined field.
7. Add pruning control values of V and discussion of same.
8. Permit optional unicast transmission of multi-destination frames
when there is only one received out a port.
9. Miscellaneous minor editing and terminology updates.
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Changes from -05 to -06
1. Revise Section 2 discussion of DRB determination in the presence
of VLANs and move it to Section 2.2. Adjust VLAN handling
description.
2. Change "V" field to be a 2-bit version fields followed by 2
reserved bits. Make corresponding changes to eliminate the
inclusion in the header of frame analysis indicating type of
multi-destination pruning which is proper for frame. Thus all
non-ingress RBridges that wish to perform such pruning are forced
to do full frame analysis. Make further corresponding changes in
IANA Considerations.
3. The Inner.MacDA for TRILL IS-IS frames is changed to a second
multicast address: All-IS-IS-RBridges. IEEE Allocation
Considerations, etc., are correspondingly changed.
4. Note in Section 6 that bridges can hide slow links and generally
make it harder from RBridges to determine the cost of an RBridge
to RBridge hop that is a bridged LAN.
5. Add material noting that replacement of bridges by RBridges can
cause connectivity between previously isolated islands of the
same VLAN.
6. Expand Security Considerations by mentioning RFC 3567 and
indicating that TRILL enveloping may reduce the effectively of
TRILL-ignorant firewall functionality.
7. Extensive updates to pseudo code.
8. Change to one DRB per physical link that dictates the inter-
RBridge VLAN for the link, appoints forwarders per-VLAN, can be
configured to send Hellos on multiple VLANs, etc.
9. Add a minimal management by SNMP statement to Section 2.
10. Delete explicit requirement to process TRILL frames arriving on a
port even if the port implements spanning tree and is in spanning
tree blocked state.
11. Miscellaneous minor editing and terminology updates.
Changes from -06 to -07
[WARNING: Section 5 of draft -07 was not fully updated to incorporate
the changes below.]
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1. Drop recommendation to set "bridge" flags in some 802.1AB frame
fields.
2. Add Section 2.5 giving an informative description of zero
configuration behavior for 802.1D and 802.1Q-2005 bridges and
RBridges.
3. Add Section 4.7 (renumbering the former 4.7 to be 4.9) on the
receipt, handing, and transmission of MVRP and other MRP frames
by RBridges. Add references to 802.1ak.
4. Add Section 4.8 on Multipathing.
5. Partial changes to Section 5 to correspond with changes elsewhere
in the draft.
6. Addition of frame category definitions in Section 1.2.
7. Addition of Section 10, Acronyms.
8. Add note in Section 6.2 that difficult in link cost determination
due to intervening devices is not confined to RBridges.
9. Re-ordered some sections in Section 6.
10. Added a paragraph about taking care if trying to use VLANs for
security to Security Considerations Section and re-ordered
paragraphs in that section.
11. Added mention of being able to configure a port so that native
frames are not send and are dropped on receipt. Probably need to
say more about this.
12. Remove material about pseudo node suppression.
13. Fix a few cases where hop count was off by one.
14. Add option critical bits when option area length non-zero.
15. Replace some remaining references to Q-tag with C-tag.
16. Miscellaneous minor editing and terminology updates. Changed
Figure numbers to be relative to major section. Added Table
captions.
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Changes from -07 to -08
1. Add "low" and "high" level control frame definitions to Section
1.2 and note concerning frames that would qualify as both "TRILL"
and "control" frames. Utilize these defined frame types more
consistently through the document.
2. Move substantial areas of tutorial, motivational, and
informational text to Appendices, or a separate document,
including Sections numbered 2.5, 4.8, 6.3, and 6.4 in version
-07. Remove pseudo-code (Section 5 in version -07).
3. Move link Hellos / VLAN specification and discussion to a new
subsection of Section 4.
4. Replace distribution tree root flag per RBridge with new logic
which orders all RBridges in a campus as to their priority to be
a distribution tree root and provides for the highest priority
distribution tree root to dictate the numbers of trees in the
campus. RBridges use the tree with least cost from themselves to
the tree root for multi-destination frame distribution, or the n
such trees if they multi-path multi-destination traffic.
5. Add "Access" port configuration bit and Appendix on Trunk and
Access Links.
6. Add statement that use of S-tags in TRILL is outside the scope of
this document.
7. Add new section on RBridge port structure (Section 4.7) which
includes discussion of RBridge interactions with BPDUs and
revised interactions with VRP frames. Make provisions for dynamic
VLAN registration a "MAY" implement and agnostic between GVRP and
MVRP. Remove references to 802.1ak. Simplify text related to VRP.
Remove related configuration option.
8. Add requirement to adjust input filters no later than output
forwarding.
9. Add requirement for configurable (default 30 second) inhibition
on RBridge decapsulation out a port if a root bridge change has
just been observed on that port.
10. Add provisions for propagating topology change to attached
bridged LAN when an RBridge is de-appointed forwarder. Also other
end station addressing forgetting details including per VLAN
forwarding status dropped counter.
11. Delete requirement that appointed forwarder wait until it has
received all the LSPs listed in the first CSNP (if any) it has
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received from its neighbors before forwarding frames off a link.
12. Add explicit criterion for when an RBridge port defers to the DRB
indicated in a Hello it receives even if that Hello is not from
the DRB or even from an RBridge in direct communication with the
DRB.
13. Add provisions for pseudonode minimization.
14. Update reference to RFC 2434 to be to RFC 5226.
15. Miscellaneous minor editing and terminology updates. Add Figures
index after Table of Contents.
Changes from -08 to -09
1. Specify SHOULD as the implementation requirement for SNMPv3
management.
2. Change default confidence level to 0x20 for addresses learned
from observing locally received native frames and from
decapsulating TRILL data frames. This provides more space for
lower confidence levels.
3. Add security consideration for observation of traffic no longer
constrained to links in its Inner.VLAN due to TRILL
encapsulation.
4. Updated bridge configuration assumptions in Section 2.3.1.
5. Use "inhibited" to describe the status of an appointed forwarder
when it is temporarily discarding all received native frames and
not sending any native frames.
6. In Section 4.4, there was an implication that the priority to be
a tree root and the number of trees to be computed had not only
default values for a zero configuration RBridge but could also be
individually present or absent in the LSP for the RBridge. This
tends to lead to a variable-length sub-TLV or multiple sub-TLVs
in the LSP that leads to additional code paths to test. So
various "if advertised" conditional clauses have been removed.
7. Reserve nicknames 0xFFC0 through 0xFFFE as well as 0x0000 and
0xFFFF and provide IANA Considerations for their allocation.
8. Improve Figure 4.1, "TRILL Data Encapsulation over Ethernet" by
generalizing it and adding an RBridge diagram.
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9. Add "access port" bit to Hello. Extend and clarify behavior for
access ports and for the occurrence of the IS Neighbor TLV in
TRILL Hellos.
10. Miscellaneous minor editing.
Changes from -09 to -10
1. Split Section 2.4 into two subsections inserting 2.4.1 with a
simplified RBridge port diagram and discussion of how RBridges
mostly use the mechanisms of IEEE 802.1Q-2005 bridges below the
EISS layer.
2. Remove the "SHOULD" requirement that the hop count for multi-
destination frames not be set by the ingress RBridge in excess of
the distance through the distribution tree to the most remote
RBridge.
3. Remove any implication that addresses received by ESADI are
always better than those learned from the data plane.
4. Rephrase language concerning the case where a known unicast
native frame in receive by an RBridge to be output in native form
on another link of that RBridge so that instead of describing
this as logically forwarding the frame in native form it is
described as logically encapsulating and then decapsulating the
frame.
5. Remove language saying that a TRILL Ethertype frame with a
broadcast outer destination address MAY be treated as if its
outer destination address was All-RBridges.
6. Clarify that all TRILL data frames with unknown or reserved
egress nicknames are discarded.
7. Substantially expand Figure 4.5 at the upper port layers and
correspondingly expand the accompanying text that is now Section
4.7.2.
8. Change TRILL IS-IS frames so they are no longer encapsulated but
have the All-IS-IS-RBridges Outer.MacDA. Change the Inner.MacDA
of ESADI frames to be the new All-ESADI-RBridges multicast
address.
9. Update reference to RFC 3567 to be to RFC 5304.
10. Miscellaneous minor editing changes.
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Changes from -10 to -11
1. Add BPDU/Hello denial of service section to Security
Considerations.
2. Remove general prohibition on RBridges sending spanning tree
BPDUs.
3. Change ESADI from "End Station Address Distribution Instance" to
"End Station Address Distribution Information".
4. Delete redundant requirement that TRILL IS-IS Hellos be
distinguished by the port from which they are sent.
5. Add Maximum Transit Delay for RBridges with enforcement a MAY.
6. Confused note re DRB deferral deleted.
7. Update boilerplate and make miscellaneous minor editing changes.
Changes from -11 to -12
1. Changes in the determination of the distribution trees to allow
the highest priority RBridge to explicitly list some or all of
the tree roots. Change the listing of distribution trees an
RBridge can use in encapsulating multi-destination frames to
allow the RBridge to not explicitly list all the roots it can
use.
2. Add figures and a little text illustrating the structure of TRILL
IS-IS and TRILL ESADI frames.
3. Add brief discussion of Hello size limitations.
4. Extend appointed forwarder inhibition to also occur on receiving
a Hello sent on VLAN-x as well as received on VLAN-x in cases of
VLAN translation.
5. Provide for the allocation of a block of 16 multicast addresses
for TRILL use by the IETF Registration Authority. RBridges
conforming to this specification discard frames sent to any of
these addresses other than All-RBridges and All-IS-IS-RBridges.
(All-ESADI-RBridges is only allowed as an Inner.MacDA.)
6. Add text on MTU and add Protection Hellos so there are now two
kinds of Hellos, Adjacency and Protection.
7. Add text mandating the RBridges with the Extended IS Adjacency
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TLV (#22) and do not use the IS Adjacency TLV (#2).
8. Add text requiring and specifying "tie-breaking" to select only
one when sending multi-destination frames between RBridges
connected by multiple parallel links. Mandate three-way handshake
on links configured to use P2P Hellos to provide Extended Circuit
ID.
9. Add section and material on using P2P Hellos.
10. Miscellaneous minor editing changes.
Changes from -12 to -13
1. Eliminate all references to "Hello time", replacing with
appropriate references to Holding Time.
2. Response to IEEE 802.1 comments: Replace all occurrences to
"[802.1Q]" with "[802.1Q-2005]" to make it absolutely, positively
clear that we don't claim to support the "current" 802.1Q as
amended. Add Appendix E to summarize the current state of support
by this draft for the current 802.1Q adopted and in-process
amendments.
3. Improve wording on frame types terminology in Section 1.3.
4. Permit multiple nicknames per Rbridge.
5. Make tie breaker on building distribution trees be the "tree
number" which counts trees rooted at different nicknames at the
same Rbridge as different trees.
6. Renumber 4.3 through 4.7 to be 4.5 through 4.9 and add new
sections 4.3 on MTU-probe and MTU-probe-ack and 4.4 on the TRILL-
Hello protocol that approximately corresponds to the previous
Section 4.2.4.
7. Change TRILL control (IS-IS) messages to be indicated by the
L2-IS-IS Ethertype. Add that Ethertype to Section 5 and Section
7.2.
8. Eliminate references to TRILL IS-IS instance as "core".
9. Eliminate feature whereby encapsulated multi-destination frames
being sent to only one next hop RBridge of interest could be sent
unicast.
10. Update references to Problem and Applicability Statement draft to
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be to RFC 5556.
11. Change how the Hop Count is handled so it is tested on receipt of
an encapsulated frame and discarded if it is zero and then not
tested when decremented on forwarding.
12. Clarify and correct handling of multiple parallel links between
adjacent RBridges providing tie-breaking.
13. Correct DRB election to specify MAC address as tie breaker, not
System ID.
14. Change default number of multi-destination frames to be
calculated for the campus from 2 to 1. Provide for RBridges to
advertise how many trees they can calculate and limit number of
trees to the minimum such number across all RBridges in the
campus.
15. Provide that When an appointed forwarder observes that the DRB on
a link has changed, it no longer considers itself appointed for
that link until appointed by the new DRB.
16. Add new section 4.4.4 and material in 4.5.2 and other spots on
port groups, renumbering the former 4.4.4 as 4.4.5.
17. Miscellaneous editing changes.
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Authors' Addresses
Radia Perlman
Sun Microsystems
16 Network Circle
Menlo Park, CA 94025
Phone: +1-650-960-1300
Email: Radia.Perlman@sun.com
Donald E. Eastlake, 3rd
Stellar Switches
155 Beaver Street
Milford, MA 01757 USA
Phone: +1-508-333-2270
Email: d3e3e3@gmail.com
Dinesh G. Dutt
Cisco Systems
170 Tasman Drive
San Jose, CA 95134-1706 USA
Phone: +1-408-527-0955
Email: ddutt@cisco.com
Silvano Gai
Cisco Systems
2600 San Tomas Expressway
Santa Clara, CA 95051 USA
Phone: +1-408-387-6123
Email: sgai@nuovasystems.com
Anoop Ghanwani
Brocade Communications Systems
1745 Technology Drive
San Jose, CA 95110 USA
Phone: +1-408-333-7149
Email: anoop@brocade.com
R. Perlman, et al [Page 101]
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R. Perlman, et al [Page 102]
