TRILL Working Group Radia Perlman
INTERNET-DRAFT Sun Microsystems
Intended status: Proposed Standard Donald Eastlake 3rd
Expires: May 1, 2009 Eastlake Enterprises
Dinesh G. Dutt
Cisco Systems
Silvano Gai
Nuova Systems
Anoop Ghanwani
Brocade
November 2, 2008
Rbridges: Base Protocol Specification
<draft-ietf-trill-rbridge-protocol-10.txt>
Status of This Document
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Distribution of this document is unlimited. Comments should be sent
to the TRILL working group mailing list <rbridge@postel.org>.
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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, 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...............................................9
2. RBridges...............................................12
2.1 End Station Addresses.................................13
2.2 RBridge Encapsulation Architecture....................13
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 Core TRILL IS-IS....................................32
4.2.3.1 Core IS-IS Link Protocol..........................32
4.2.3.2 Designated RBridge................................36
4.2.3.3 Appointed VLAN-x Forwarder........................37
4.2.3.4 Core TRILL IS-IS LSP Information..................38
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Table of Contents Continued
4.2.4 TRILL ESADI IS-IS...................................40
4.2.4.1 TRILL ESADI Participation.........................40
4.2.4.2 TRILL ESADI Information...........................41
4.3 Distribution Trees....................................41
4.3.1 Distribution Tree Calculation and Checks............42
4.3.2 Pruning the Distribution Tree.......................43
4.3.3 Tree Distribution Optimization......................44
4.3.4 Forwarding Using a Distribution Tree................45
4.4 Frame Processing Behavior.............................46
4.4.1 Receipt of a Native Frame...........................46
4.4.1.1 Native Unicast Case...............................46
4.4.1.2 Native Multicast and Broadcast Frames.............47
4.4.2 Receipt of a TRILL Frame............................48
4.4.2.1 TRILL IS-IS Frames................................48
4.4.2.2 TRILL ESADI Frames................................48
4.4.2.3 TRILL Data Frames.................................49
4.4.3 Receipt of a Control Frame..........................50
4.5 IGMP, MLD, and MRD Learning...........................50
4.6 End Station Address Details...........................51
4.6.1 Learning End Station Addresses......................52
4.6.2 Forgetting End Station Addresses....................53
4.6.3 Shared VLAN Learning................................54
4.7 RBridge Ports.........................................55
4.7.1 RBridge Port Configuration..........................55
4.7.2 RBridge Port Structure..............................56
4.7.3 BPDU Handling.......................................58
4.7.3.1 Receipt of BPDUs..................................59
4.7.3.2 Root Bridge Changes...............................59
4.7.3.3 Transmission of BPDUs.............................59
4.7.4 Dynamic VLAN Registration...........................60
5. RBridge Addresses, Parameters, and Constants...........61
6. Security Considerations................................63
6.1 VLAN Security Considerations..........................63
7. Assignment Considerations..............................65
7.1 IANA Considerations...................................65
7.2 IEEE Registration Authority Considerations............65
8. Normative References...................................66
9. Informative References.................................67
Appendix A: Incremental Deployment Considerations.........69
A.1 Link Cost Determination...............................69
A.2 Appointed Forwarders and Bridged LANs.................69
A.3 Wiring Closet Topology................................71
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Table of Contents Continued
A.3.1 The RBridge Solution................................72
A.3.2 The VLAN Solution...................................72
A.3.3 The Spanning Tree Solution..........................72
A.3.4 Comparison of Solutions.............................73
Appendix B: Trunk and Access Port Configuration...........74
Appendix C: Multipathing..................................75
Appendix D: Determination of VLAN and Priority............77
Appendix Z: Revision History..............................78
Changes from -03 to -04...................................78
Changes from -04 to -05...................................79
Changes from -05 to -06...................................80
Changes from -06 to -07...................................80
Changes from -07 to -08...................................81
Changes from -08 to -09...................................83
Changes from -09 to -10...................................84
Disclaimer................................................85
Additional IPR Provisions.................................85
Authors' Addresses........................................86
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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: Detailed RBridge Port Model...................57
Figure A.1: Link Cost of a Bridged Link...................69
Figure A.2: Wiring Closet Topology........................71
Figure C.1: Multi-Destination Multipath...................75
Figure C.2: Known Unicast Multipath.......................76
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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] 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 [PAS].
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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 which appear later:
4 > 3 > 2
1.3 Terminology and Notation in this document
"TRILL" normally refers to the protocol specified herein while
"RBridge" refers to the devices that implement that protocol. The
second letter in Rbridge is case insensitive. Both Rbridge and
RBridge are correct.
This document uses Hexadecimal Notation for MAC addresses. Each
octet (that is, 8-bit byte) is represented by two hexadecimal digits
giving the value of the octet as an unsigned integer and successive
octets are separated by a hyphen. This document consistently uses
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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 three categories,
TRILL frames, control frames, and native frames, as follows:
o "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. There are two sub-categories of control
frames as follows:
- "high level control" frames are those with a destination
address of 01-80-C2-00-00-00 (BPDU) or 01-80-C2-00-00-21
(VRP).
- "low level control" frames are all other control frames.
o "TRILL" frames are those (1) with a multicast destination
address allocated to the TRILL protocol (see Section 7.2) and
(2) non-control frames with the TRILL Ethertype. There are
three sub-categories of TRILL frames. RBridges do not generate
and silently discard on receipt any TRILL frames which do not
match one of these sub-categories as follows:
- "TRILL IS-IS" frames have the All-IS-IS-RBridges multicast
address as their destination address.
- "TRILL ESADI" frames have the TRILL Ethertype and the All-
ESADI-RBridges multicast address as the encapsulated
destination address.
- "TRILL data" frames have the TRILL Ethertype but are not
TRILL ESADI frames.
o "native" frames are all frames other than TRILL and control
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].
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
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CSNP - Complete Sequence Number PDU
DA - Destination Address
EAP - Extensible Authentication Protocol
ECMP - Equal Cost Multi-Path
EISS - Extended Internal Sublayer Service
ESADI - End Station Address Distribution Instance
DRB - Designated RBridge
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
MLD - Multicast Listener Discovery
MRD - Multicast Router Discovery
MVRP - Multiple VLAN Registration Protocol
NSAP - Network Service Access Point
PDU - Protocol Data Unit
PPP - Point-to-Point Protocol
RBridge - Routing Bridge
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RPF - Reverse Path Forwarding
SA - Source Address
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. 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 4.3.1).
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. RB1 is known as the "ingress RBridge" and RB2
is known as the "egress RBridge". To save room in the TRILL header,
a dynamic nickname acquisition protocol is run among the RBridges to
select a 2-octet nickname for each RBridge, unique within the campus,
which is 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 the IS-IS [ISO10589] election protocol to elect a
"Designated RBridge" (DRB) on each bridged LAN ("link"). 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
specifies 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
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.)
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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]),
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 end station address distribution instance (ESADI) of
IS-IS MAY be used by an RBridge that is the appointed VLAN-x
forwarder on one or more links 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 learning 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 IS-IS also
supports cryptographic authentication of its messages [RFC5304]. But
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 an ESADI of IS-IS is optional, as is
learning from these announcements.
(See Section 4.6 for further end station address details.)
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
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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 point-to-point or
shared media, hubs and IEEE 802.1D 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 from RB1 to 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 the egress RBridge for that
destination MAC address is known to the ingress RBridge.
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 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 4.3) 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]. Use of VLANs requires configuration.
The default method of determining the VLAN of a frame sent by an end
station is based on the port on which it is initially received. End
stations can also explicitly insert this information in a frame.
IEEE 802.1Q bridges can be configured to support multiple VLANs over
a single 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]. 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 4.2.3.1 for further discussion of TRILL IS-IS operation
on a link beyond that in the subsections below.)
2.3.1 Link VLAN Assumptions
In a network with a mix of bridges and RBridges, certain
configurations of bridges or ports may prevent some nodes in some
VLANs from communicating with each other.
TRILL requires that on each link in the campus there is at least one
VLAN that gives full connectivity to all the RBridges on that link
and that the RBridges are configured to know which VLAN that is if it
is not the default VLAN 1.
Since there will be only one appointed forwarder for any VLAN, say
VLAN A, on a link, if bridges are configured to cause VLAN A to be
partitioned on a link, some 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 A frame turns into a VLAN B frame).
TRILL detects this case through IS-IS Hellos, by inserting the
initial VLAN tag into the Hello. TRILL includes mechanisms to detect
VLAN mapping within a link and takes steps to ensure that there is at
most a single appointed forwarder on the link, to avoid possible
frame duplication or loops.
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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] port facilities and RBridges can be
incrementally deployed into an existing bridged LAN.
2.4.1 RBridge and 802.1 Layering
RBridges make use of 802.1Q port VLAN processing and 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]). The only exceptions are those protocols related to high
level control frames including spanning tree. RBridge do not use
spanning tree and do not block ports in the way that spanning tree
blocks ports. (There may in the future be additional lower level
802.1 protocols that require different handling in an RBridge than in
an 802.1 bridge.) 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 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 | | | |
| Processing (BPDU, VRP) | | | 802.1Q |
| | | | Port VLAN |
+-----------++-------------+ | | 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]. In
RBridges, high level control frames are processed above the ISS
interface.
The upper interface to the port VLAN processing corresponds to the
Extended Internal Sublayer Service (EISS) in [802.1Q]. In RBridges,
all 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 current IEEE [802.1Q]
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 unmanaged, zero configuration bridges, then their
RBridge replacements will not require configuration.
The RBridge campus will work best if all IEEE 802.1 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
The 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 which 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)
A single Ethertype is granted to a protocol and, under IEEE
guidelines, it is the protocol's responsibility to structure itself
to support future revisions. Adhering to this guideline, there is a
two bit Version field in the TRILL header. Version zero of TRILL is
specified in this document. An RBridge that sees a message with a
Version value it does not understand MUST silently discard the
message because it may not be able to parse it.
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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 zero on transmission
and are 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 RBridge for this tree. In particular:
o M = 0 (FALSE) - The egress RBridge nickname contains the nickname
of the egress Rbridge for a known unicast TRILL data frame;
o M = 1 (TRUE) - The egress RBridge nickname field contains the
nickname of the RBridge that is the root of a distribution tree.
This tree is selected by the ingress RBridge for a TRILL data
frame or by the source RBridge for a TRILL ESADI (end station
address distribution instance) frame.
3.5 TRILL Header Options
The TRILL Protocol includes an option capability in the TRILL Header.
The Op-Length header field gives the length of the options in units
of 4 octets which allows up to 124 octets of options area. If Op-
Length is zero there are no options present. If options are present,
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 the frame even it if doesn't understand the
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 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 RBridges 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 almost certainly 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. Each RBridge that is
about to forward a frame to another RBridge MUST check this field and
discard the frame if this field is zero. If this field is greater
than or equal to 1, it MUST be decremented in the forwarded frame.
For known unicast frames, the ingress RBridge MUST set the Hop Count
to at least the number of RBridge hops it expects to the egress
RBridge and SHOULD set it in excess of that number to allow for
alternate routing later in the path.
For multi-destination frames, the Hop Count MUST be set by the
ingress RBridge (or source RBridge for an end station address
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distribution TRILL IS-IS frame) to at least the expected number of
hops to the most distant RBridge. To accomplish this, RBridge RBn
calculates, for each branch from RBn of the 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 to the minimum 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 abbreviations for each
RBridge's 48-bit IS-IS System ID to achieve a more compact encoding.
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 a nickname 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 (end
station address distribution instance) frames, M == 1. The egress
RBridge nickname field contains the nickname of the root RBridge
of the distribution tree selected to be used to forward the frame.
This root MUST NOT be changed by transit RBridges.
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3.7.2 Ingress RBridge Nickname
The ingress RBridge nickname is set to the nickname of the ingress
RBridge for all TRILL data frames and to the nickname of the source
RBridge for all 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 the core TRILL IS-
IS instance as follows:
o The nickname being used by an RBridge is carried in an IS-IS TLV
(type-length-value data element) along with a priority of use
value. Each RBridge chooses its own nickname.
o The nickname value MAY be configured. An RBridge that has been
configured with a nickname value will have priority for that
nickname value 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
the 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 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 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 different nickname.
o To minimize the probability of nickname collisions, when an
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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
their nickname more than once before the situation is resolved.
To minimize the probability of a new RBridge usurping a nickname
already in use, an RBridge whose nickname is not configured SHOULD
wait to acquire the link state database from a neighbor before it
announces its own nickname.
An RBridge that will not act as an ingress, egress, or tree root need
not have a nickname.
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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
the nickname values of the egress and ingress RBridges,
respectively, unless the frame is multi-destination in which case
the Egress Nickname specified 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", also known as a "C-tag" (formerly known as a Q-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 a "VLAN tagged" [802.1Q].
(802.1Q 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], Rbridges SHOULD be implemented so as to
allow use of the full range of VLAN IDs from 0x001 through 0xFFE.
VLAN ID zero is the null VLAN identifier and indicates that no VLAN
is specified while VLAN ID 0xFFF is reserved. Rbridges MAY support a
smaller number of simultaneously active VLAN IDs than the total
number of different VLAN IDs they allow.
The VLAN ID 0xFFF is reserved and 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], 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] permits mapping some adjacent pairs of priority
levels into a single priority level with and without drop
eligibility. RBridges MAY also implement such configuration 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. (Such frames might not be sent out the same port if
multipath is implemented. See Appendix C.) Differing priorities can
cause frame re-ordering.
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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 or the VLAN tag
information associated with a TRILL ESADI (end station address
distribution instance) 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] (see Appendix D and
[802.1Q] 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. Thus, in TRILL data frames, the Inner.VLAN tag always
specifies a VLAN ID.
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 IS-IS Hellos,
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 (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] frame depends on the configuration
of the bridge port (see Section 4.7.2).
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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 the link. Any received frame for which the FCS
check fails MUST be discarded. 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 a class of internal RBridge failures such as
memory errors.
4.2 Link State Protocol (IS-IS)
TRILL uses an extension of IS-IS [ISO10589] as the routing protocol
since it has the following advantages:
o it runs directly over Layer 2, so therefore 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;
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,
usually by taking a 6-octet ID owned by the DRB, and appending
another octet. The only constraint is that the 7-octet 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.
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4.2.2 IS-IS Instances
TRILL implements separate IS-IS instances 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 a special multicast destination
address, either AllL1ISs or AllL2ISs. When they are TRILL
encapsulated, these multicast addresses appear as the Inner.MacDA and
the Outer.MacDA will be either unicast or the All-RBridges multicast
address.
Within TRILL, there is a mandatory core IS-IS instance across all
Rbridges in the campus as described in Section 4.2.3. This core
instance uses TRILL IS-IS frames which are distinguished by having a
multicast destination address of All-IS-IS-RBridges. TRILL IS-IS
frames have the IS-IS NSAP protocol type and do not have a TRILL
Header.
In addition, there can be optional end station address distribution
instances (ESADIs) between the RBridges on each supported VLAN as
described in Section 4.2.4. They are similar to TRILL data frames
where the encapsulated frame is an IS-IS protocol frame but are
distinguished by the presence of an Inner.MacDA of All-ESADI-
RBridges.
4.2.3 Core TRILL IS-IS
All Rbridges must participate in the core TRILL IS-IS instance. Core
instance 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.)
4.2.3.1 Core IS-IS Link Protocol
RBridges send TRILL IS-IS Hello frames on a link in order to discover
RBridge neighbors. As with Layer 3 IS-IS, one RBridge is elected DRB
(Designated RBridge), based on configured priority (most significant
field), and system ID. The DRB, as described in Section 4.2.3.2,
designates the VLAN to be used on the link for inter-RBridge
communication and appoints itself or other RBridges on the link as
appointed forwarder (see Section 4.2.3.3) for VLANs on the link.
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4.2.3.1.1 Core Hello VLAN Tagging
By default, RBridges tag TRILL IS-IS Hello frames with VLAN 1.
Because a link may be a bridged LAN with different connectivity for
different VLANs, and since an RBridge may be configured so that it
cannot use VLAN 1 on a port, Hellos may need to be sent out a port
with additional and/or other VLANs for connectivity and safety.
An RBridge RBn maintains for each port the same VLAN information as a
customer IEEE [802.1Q] bridge, including the set of VLANs enabled for
output through that port (see Section 4.7.2). 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 Hellos to be used by all RBridges on the link to
communicate all TRILL frames, except some Hellos as described
below. 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 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.
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 only
contain enabled VLANs for the port, possibly all enabled VLANs.
On each of its ports an RBridge sends Hellos Outer.VLAN tagged with
each VLAN in a set of VLANs. For each port, this set depends on the
RBridge's DRB status and the above VLAN parameters. All RBridges send
Hellos Outer.VLAN tagged with the Designated VLAN, unless that VLAN
is not enabled. In addition, the DRB sends Hellos Outer.VLAN tagged
with each enabled VLAN in its Announcing VLANs set. All non-DRB
RBridges send 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, Hellos are sent as follows:
If it is DRB
intersection ( Enabled VLANs,
union ( Designated VLAN, Announcing VLANs ) )
If it is not DRB
intersection ( Enabled VLANs,
union ( Designated VLAN,
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intersection ( Forwarding VLANs, Announcing VLANs ) ) )
Configuring the Announcing VLANs set to be null minimizes the number
of Hellos. In that case, Hellos are only tagged with the Designated
VLAN.
The number of 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
Hellos tagged with all its Enabled VLAN tags and any non-DRB RBridge
RBn will send Hellos 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 Hellos. In particular, non-DRB RBridges
could send Hellos on enabled VLANs for which they are not an
appointed forwarder and which are not the Designated VLAN. This would
not cause harm, other than a further communications and processing
burden.)
When an RBridge port comes up, until it has heard a Hello from a
higher priority RBridge, it considers itself to be DRB on that port
and sends Hellos on that basis. Similarly, even if it has at some
time recognized some other RBridge on the link as DRB, if on that
port it receives no Hellos from an RBridge with higher priority as
DRB for a long enough time, as specified by IS-IS, it will revert to
believing itself DRB. Note that an RBridge RBn does not defer to the
DRB listed in a Hello, even if that claimed DRB is higher priority,
if the Hello was sent by an RBridge with lower priority than RBn.
4.2.3.1.2 TRILL IS-IS Hello Contents
A TRILL IS-IS Hello includes the following information, in addition
to the standard IS-IS Hello header information. The actual encoding
of this information and the IS-IS Type or sub-Type values for the TLV
or sub-TLV data elements are specified in a separate document.
1. The VLAN ID of the Designated VLAN for the link.
2. In connection with VLAN mapping (see Section 4.2.3.1.3):
2.a A copy of the Outer.VLAN ID with which the Hello was tagged.
2.b A flag which, if set, indicates that the sender has detected
VLAN mapping on the link, within the past five Hello times.
3. The set of VLANs for which end station service is enabled on the
port. If this is missing or null, it implies that the port is
configured as a trunk port (see Section 4.7.1). This MAY be
omitted if the sender is DRB.
4. A flag which, if set, indicates that the sender believes it is
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appointed forwarder for the VLAN and port on which the Hello was
sent.
5. A flag which, if set, indicates that the sender's port was
configured as an access port. The value of this flag for the DRB
controls and when it is asserted by the DRB all ports on that link
which recognize the DRB act as access ports.
6. If the sender is DRB, the Rbridges (including itself) that it
appoints as forwarders for that link and the VLANs for which it
appoints them.
7. TRILL connectivity over which TRILL data, ESADI, and non-Hello
TRILL IS-IS frames will be sent is only established on the
Designated VLAN. Establishing such connectivity requires exchange
of Hellos containing the IS Neighbor TLV, so that TLV MUST be
included in Hellos sent on the Designated VLAN. The Neighbor TLV
MAY be send on other VLANs but neighbor status is only established
and updated based on Hellos on the Designated VLAN.
It is anticipated that many links between RBridges will be point-to-
point in which case a pseudonode merely adds to the complexity. If
the DRB specifies the pseudonode ID as all zeros, this indicates that
the RBridges on the link are just to 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 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 R1 declares no pseudonode, then
there will be only 2 LSPs: RB1 and RB2 each reporting connectivity to
each other.
A DRB SHOULD NOT create a pseudonode for its link unless it has seen
at least two simultaneous adjacencies on the link at some point since
it last re-booted or in certain cases for access ports (see Section
4.7.1).
4.2.3.1.3 VLAN Mapping Within a Link
IEEE [802.1Q] 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.
For example, a bridge port can be configured to strip certain 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
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[802.1Q], in the aggregate they perform manipulations not permitted
within a single customer 802.1Q bridge. Since RBridge ports have the
same VLAN capabilities as customer 802.1Q bridges, this can occur
even in the absence of bridges. (VLAN mapping is referred to in IEEE
802 as VID translation.)
RBridges include the Outer.VLAN ID inside a TLV within each Hello
message. When a Hello is received, they 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 a VLAN mapping is detected, the RBridge detecting it sets a flag
in all Hellos it sends on the link for the subsequent five Hellos
times. This notifies the DRB if the detecting RBridge is not the DRB.
The DRB then assures that only one RBridge (either the DRB itself or
some RBridge it appoints) is appointed forwarder for any VLANs on the
link. This avoids loops and duplication of frames with different VLAN
tags.
4.2.3.2 Designated RBridge
TRILL IS-IS elects one RBridge for each 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 data and ESADI frames and all non-Hello TRILL
IS-IS frames. TRILL IS-IS Hellos are sent on this VLAN but are
usually also sent on others (see Section 4.2.3.1.1).
o If the link is represented in the IS-IS topology as a pseudonode,
chooses a pseudonode ID and announces that in its Hello 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
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 5 Hello intervals (to ensure it is DRB).
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o Continues sending IS-IS Hellos on all its enabled VLANs that have
been configured in the Announcing VLANs set of the DRB, which
defaults to all enabled VLANs.
4.2.3.3 Appointed VLAN-x Forwarder
The appointed VLAN-x forwarder for a link is responsible for the
following:
o Loop avoidance:
- Inhibiting itself for a configurable time from 30 to zero
seconds, which defaults to 30 second, after it sees a root
bridge change on the link (see Section 4.7.3.2). An inhibited
appointed forwarder for a port drops any native frames it
receives and does not transmit and native frames in the VLAN
for which it is appointed.
- Inhibiting itself, as descrived above, for VLAN-x if, within
the past five Hello times, it has received a Hello on VLAN-x in
which the sender asserts that it is appointed forwarder for
VLAN-x.
- 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, as appropriate, forwarding it in native and/or
encapsulated form.
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.
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.
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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 the messages in the TRILL IS-IS end
station address distribution instance (ESADI) 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 a TRILL IS-IS ESADI.
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.
o Including a "port number" in its Hellos, and if it sees its own
Hello on port p, where the port number in the received Hello is
"q", then if q>p, not forwarding traffic to/from port p, as
already provided in IS-IS.
4.2.3.4 Core TRILL IS-IS LSP Information
The information in the TRILL IS-IS LSP for the mandatory core
instance 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
except for neighbors reached through certain access ports (see
Section 4.7.1).
2. If an RBn is to be able to act as an ingress, egress, or tree
root, the nickname of RBridge RBn (2 octets) and the unsigned
8-bit priority for RBn to have that nickname (see Section 3.7.3).
3. The TRILL Header Versions supported by RBridge RBn (4 bits).
4. The priority of RBn for becoming a distribution tree root and the
number of additional distribution trees it wants computed for the
campus (each 16 bits, see Section 4.3).
5. The list of RBridge nicknames that RBn might select for a
distribution tree root when RBn injects a multi-destination frame
into the campus. These tree roots MUST be from the set of roots
for the distribution trees which all RBridges in the campus are
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computing (see Section 4.3). Using this field RBridges can
efficiently build receipt filters to avoid multicast loops (see
Section 4.3.1). If the list is empty or not provided, RBn MUST
only select the highest priority distribution tree root for the
campus.
6. 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 4.6.3.) In addition, the LSP contains the following
information on a per-VLAN basis:
6.1 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 which does not do IP multicast control snooping MUST
set both of these bits (see Section 4.3.3). 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
broadcast.
6.2 Per VLAN Other Multicast flag. This is a flag bit that
indicates that the RBridge wishes to receive non-IP derived
multicast for that VLAN. It defaults to true (one). Within
each VLAN, all non-IP derived multicast traffic MUST be sent
to an RBridge that asserts this flag.
6.3 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 4.2.3.3.)
6.4 Optionally, per VLAN Layer 2 multicast addresses derived from
IPv4 IGMP or IPv6 MLD notification messages received from
attached end nodes on that VLAN, indicating the location of
listeners for these multicast addresses (see Section 4.3.4).
6.5 Per VLAN ESADI participation flag, priority, and holding time.
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If this flag is one, it indicates that the RBridge wishes to
receive such TRILL ESADI frames (see Section 4.2.4.1).
6.6 Per VLAN appointed forwarder status lost counter (see Section
4.6.2).
7. Optionally, a list of VLAN groups where address learning is shared
across that VLAN group (see Section 4.6.3). Each VLAN group is a
list of VLAN IDs, with 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.
4.2.4 TRILL ESADI IS-IS
RBridges that are the appointed VLAN-x forwarder for a link MAY
participate in the TRILL end station address distribution instance
(ESADI) of IS-IS for that VLAN. But all transit RBridges MUST
properly forward TRILL ESADI frames as if they were multicast TRILL
data frames.
Because of this forwarding, it appears to an IS-IS ESADI at an
RBridge that it is directly connected by a shared virtual link to all
other RBridges in the campus running that instance. 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
exactly as multicast TRILL data frames and no IS-IS processing is
invoked due to such forwarding.
4.2.4.1 TRILL ESADI Participation
An RBridge participating in an end station address distribution
instance (ESADI) does not send any additional Hellos. The information
available in the core TRILL IS-IS link state database is sufficient
to determine the DRB on the virtual link for each VLAN's ESADI. In
particular, the core 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
instances, 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. A
participating RBridge that determines that some other RBridge should
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be DRB on such a virtual link and has not received or sent a CSNP in
at least the DRB holding time MAY also send a 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
TRILL ESADI LSPs or CSNPs on the virtual link.
4.2.4.2 TRILL ESADI Information
The information in the LSP for a optional TRILL 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 4.6). In order to make it practical to
optionally provide for customer 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 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.
Each RBridge RBn may advertise in the core instance link state
database its priority to be chosen as a tree root and the number of
additional distribution trees it specifies that every RBridge in the
campus must compute if RBn is the highest priority tree root. The
priority is a 16-bit unsigned integer that defaults, for a zero
configuration RBridge to 0x8000. The number of distribution trees to
be computed is a 16-bit unsigned integer giving the number of trees
to be computed in addition to the one rooted at the highest priority
root. The number of additional trees defaults, for a zero
configuration RBridge, to one.
The RBridge with highest priority to be tree root is determined by
the numerically lowest priority field or, if priority fields are
equal, by the numerically lowest system ID. A tree is always
calculated rooted at this highest priority RBridge and that RBridge
specifies to all RBridges in the campus the total number of
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additional distribution trees to be calculated. If it indicates that
K-1 additional trees are to be calculated, then they are rooted at
the 2nd through the Kth highest priority RBridges. Thus every RBridge
calculates the same set of K distribution trees.
Every RBridge RBn defaults, in the zero configuration case, to using
a single distribution tree for multi-destination frames. For this
purpose, it orders the trees being computed for the campus in order
of increasing cost from RBn to the root RBridge of that tree and, if
cost is equal, by decreasing priority to be a tree root and selects
the first tree in that ordering.
If RBn is to multi-path multi-destination frames, it can be
configured with the number of different trees it would like to use,
say J. RBn selects the first trees in its priority-of-use ordering,
up to the minimum of J and K number of trees. However, RBn MUST
announce, in its LSP, an intention to use any particular trees by
listing the tree root, unless it is content to only use the highest
priority tree root in the campus.
4.3.1 Distribution Tree Calculation and Checks
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 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.
When a node RBn has two or more minimal equal cost paths toward the
Root RBi, a deterministic tiebreaker is needed to guarantee that all
Rbridges calculate the same distribution tree. This is obtained by
selecting the path that goes to the parent that has the lower 7-octet
IS-IS ID.
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. 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.
To further avoid temporary multicast loops during topology changes,
RBridges MUST do a sanity check that a multi-destination frame
arrives on the expected link. This is called the Reverse Path
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Forwarding Check and is done as follows. When RBn calculates the RBi
tree, for each adjacency in the RBi tree, RBn lists the possible
ingress RBridge nicknames on that adjacency. The only ingress
RBridges that appear on any of the adjacencies are RBridges that have
explicitly stated, in their LSP, that they may select RBi as a
distribution tree root or ingress RBridges that list no roots on
adjacencies for the distribution tree with the highest priority root.
If a multi-destination frame is received on a particular adjacency,
marked as the RBi-tree, then RBn MUST NOT forward it if the ingress
RBridge is not listed in the allowed list of ingress RBridges for
that adjacency for that tree.
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.
4.3.2 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 several cases: (1) IGMP [RFC3376],
MLD [RFC2710], and MRD [RFC4286] messages, where these are to be
delivered only to links with IP Multicast routers; (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; and (3) other multicast traffic not derived
from an IP address that is only delivered to links for which the
appointed forwarder has the Other Multicast requested flag set. All
of these cases are scoped per-VLAN.
Let's assume that RBridge RBn knows that adjacencies (a, c, and f)
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 whether there are
one or more RBridges downstream with the Other Multicast flag set,
and
o the set of Layer 2 multicast addresses derived from IP multicast
groups for which there are receivers downstream.
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4.3.3 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. 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 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 4.5
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.)
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].
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4.3.4 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 4.3.1);
o else, if the frame is an IGMP or MLD announcement message or an
MRD query message, then the 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, if the frame is for a Layer 2 multicast address not derived
from an IP multicast group, then the frame is forwarded onto
adjacencies in the RBi-tree that indicate there are downstream
RBridges in VLAN-x with the Other Multicast flag set;
o else (the inner frame is for 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, or process the frame.
TRILL ESADI frames will be delivered only to RBridges that are
appointed forwarders for their VLAN. Such frames will be multicast
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 unless the RBridge has cleared its Other
Multicast bit for that VLAN and has no appointed forwarders
downstream in the tree with the Other Multicast bit set.
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4.4 Frame Processing Behavior
This section describes RBridge behavior for all varieties of received
frames, including how they are forwarded when appropriate. Section
4.4.1 covers native frames, Section 4.4.2 covers TRILL frames, and
section 4.4.3 covers control frames. Processing may be organized or
sequenced in a different way than described here as long as the
result is the same.
Frames with a bad FCS are discarded on receipt. Source address
information ( { VLAN, Outer.MacSA, port } ) is learned from any frame
with a unicast sources address (see Section 4.6).
4.4.1 Receipt of a Native Frame
If end station service is disabled on the port, 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 bridges do (see Appendix D
and Section 4.7.2). Once the VLAN is determined, if RB1 is not the
appointed forwarder for that VLAN on the port where the frame was
received, the frame is discarded. If it is appointed forwarder for
that VLAN and is not inhibited (see Section 4.2.3.3), then the native
frame is forwarded according to 4.4.1.1 if it is unicast and
according to 4.4.1.2 if it is multicast or broadcast.
4.4.1.1 Native Unicast Case
If the destination MAC address of the native frame is a unicast
address, the following steps are performed.
The Layer 2 destination address and VLAN are looked up in the ingress
RBridge's Encapsulation Database to find the egress RBridge RBm or
the local egress port or to discover that the destination is unknown.
One of the following three cases will apply:
1. If the destination is known to be on the same link from which the
native frame was received, the RBridge silently discards the
frame, since the destination should already have received it.
2. 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.MacSA of RB1 and 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
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for RBm. (RBm may be RB1 in which case processing then proceeds as
in 4.4.2.2.1.)
3. If a unicast destination address is unknown, RB1 handles the frame
as described in Section 4.4.1.2 for a broadcast frame except that
the Inner.MacDA is the original native frame's unicast destination
address.
4.4.1.2 Native Multicast and Broadcast Frames
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
4.5).
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 Outer.MacSA
of RB1 and 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 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 4.3)
The default is for RB1 to write into the egress nickname field the
nickname for the distribution tree, from the set of distribution
trees being computed by each RBridge in the campus, whose root is
least cost from RB1. However, RB1 MAY choose a different distribution
tree if RB1 has been configured to path-split multicast. In that
case RB1 MUST select a tree by specifying an RBridge that is a
distribution tree root (see Section 4.3). Also, RB1 MUST select a
tree that RB1 has announced (in RB1's own LSP) to be one of those
that RB1 may choose as a distribution tree or the tree with the
highest priority root if none is announced.
Although the Outer.MacDA is normally the All-Rbridges multicast
address if, for any particular frame sent out a particular port,
there is only one next hop RBridge of interest, the frame MAY be sent
with the unicast Outer.MacDA of the target RBridge. (Using a unicast
Outer.MacDA is of no benefit on a point-to-point link but may result
in substantial savings if the link is actually a bridged LAN with
many bridged branches and end stations, to all of which the frame may
get flooded if a multicast destination is used.)
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4.4.2 Receipt of a TRILL Frame
A TRILL frame has either a multicast Outer.MacDA allocated to TRILL
(see Section 7.2) or is a non-control frame with an outer TRILL
Ethertype. Processing proceeds in the following order:
If the Outer.MacDA is All-IS-IS-RBridges, the frame is handled as
described in Section 4.4.2.1.
If the Outer.MacDA is All-ESADI-RBridges, the frame is discarded.
If the Ethertype is not TRILL, the frame is discarded.
If the Outer.MacDA is a unicast address, the frame is discarded
unless that address is the address of the receiving Rbridge. (Such
discarded frames are most likely addressed to another RBridge on a
multi-access link and that other Rbridge will handle them.)
After the above checks, further processing of TRILL frames is
independent of the Outer.MacDA.
If the Version field in the TRILL Header is greater than 0, the frame
is discarded. The Inner.MacDA is then tested. If it is the All-ESADI-
Rbridges multicast address and RBn implements the ESADI feature,
processing proceeds as in Section 4.4.2.2 below. If it is any other
address or RBn does not implement the ESADI feature, processing
proceeds as in Section 4.4.2.3.
4.4.2.1 TRILL IS-IS Frames
If the frame protocol is not IS-IS NSAP, it is discarded. Otherwise,
it is processed by the core IS-IS instance on RBn and is not
forwarded.
4.4.2.2 TRILL ESADI Frames
The port on which the frame was received is first checked and the
frame discarded if there is no TRILL core IS-IS adjacency on that
port.
If M == 0, the frame is silently discarded. The egress nickname
designates the distribution tree. In this case, the frame is
forwarded as described in Section 4.4.2.3.2. 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 that
instance is enabled, the inner frame is decapsulated and provided to
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that local IS-IS instance.
4.4.2.3 TRILL Data Frames
The port on which the frame was received is first checked and the
frame discarded if there is no TRILL core 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.
The M flag is then checked. If it is zero, processing continues as
described in Section 4.4.2.3.1, if it is one, processing continues as
described in Section 4.4.2.3.2.
4.4.2.3.1 Known Unicast TRILL Data Frames
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, (2) locally processed if the RBridge itself is the
destination, or (3) processed as in the 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 4.7.1).
If RBn is a transit RBridge and the hop count is zero, the frame is
silently discarded. Otherwise the hop count is decremented by one and
the frame forwarded to the next hop RBridge towards the egress
RBridge, using the Forwarding Database. The Inner.VLAN is not
examined by a transit RBridge forwarding a know unicast TRILL data
frame.
4.4.2.3.2 Multi-Destination TRILL Data Frames
The Outer.MacSA is checked and the frame discarded if it is not a
tree adjacency for the tree indicated by the egress RBridge nickname
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or the reverse path forwarding check fails (see Section 4.3.1).
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 4.2.3.3,
and/or locally processed as appropriate.
If the hop count in the frame is zero, it is then silently discarded.
If non-zero, it is decreased (see Section 3.6) and the frame
forwarded down the tree specified by the egress RBridge nickname
pruned as described in Section 4.3.
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 normally
the All-Rbridges multicast address; however, if for any particular
frame transmitted on a particular port there is only one next hop
RBridge of interest, the frame MAY be sent with a unicast Outer.MacDA
of that next hop RBridge. (Using a unicast Outer.MacDA is of no
benefit on a point-to-point link but may result in substantial
savings if the link is actually a bridged LAN with many bridged
branches and end stations, to all of which the frame may get flooded
if a multicast destination is used.)
4.4.3 Receipt of a Control Frame
Low-level control frames received by an RBridge are handled within
the port where they are received as described in Section 4.7.
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 4.7.3 01-80-C2-00-00-00
VRP 4.7.4 01-80-C2-00-00-21
4.5 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 4.3.
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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].
4.6 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
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addresses and VLANs.
4.6.1 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.
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
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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.
4.6.2 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]. 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
IS-IS 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
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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 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 core TRILL IS-IS 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]) "Forward Delay". This parameter is in the
range of 4 to 30 seconds with a default value of 15 seconds.
4.6.3 Shared VLAN Learning
RBridges can map VLAN IDs into a smaller number of identifiers for
purposes of address learning, as [802.1Q] 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
a TRILL ESADI for the VLANs who's ID is not being retained.
o In cases where 4.6.2 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
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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.
4.7 RBridge Ports
Section 4.7.1 below describes the several general RBridge port
configuration bits, Section 4.7.2 give a logical port structure in
terms of frame processing, and Sections 4.7.3 and 4.7.4 describe the
handling of high-level control frames.
4.7.1 RBridge Port Configuration
There are three per port configuration bits as follows:
o Disable port bit. When this bit is set, all frames received are
discarded and no frames are sent, with the possible exception of
some low-level control frames that may be 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
control frames.)
A port with end station service disabled reports, in the Hellos it
sends out that port, that it has no VLANs for which it is provides
end station support. As a result, such a port will not be
appointed forwarder for any VLAN. Thus a port with end station
service disabled cannot contribute to the VLANs which the RBridge
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reports in its LSP as being "connected" to that RBridge. Unless
there is at least one port on an RBridge for which VLAN-x is
appointed forwarder, that RBridge does not normally advertise
itself in the link state as connected to VLAN-x. As a consequence,
it will not normally receive any traffic for VLAN-x except as
TRILL data frames to forward as a transit RBridge.
o TRILL traffic disable (access port) bit. If this bit is set, the
goal is to avoid sending TRILL fames, except TRILL Hellos, on the
port since it is intended for end station traffic (see Appendix
B). This bit is reported in Hellos sent out the port and the bit
for the DRB controls the link. If the DRB asserts the access port
bit in its Hello, then the link is an RBridge access link all
RBridge ports which acknowledge that DRB act as access ports. If
there are no TRILL IS-IS adjacencies on the access port, no
special action need be taken.
If there are adjacencies, and the RBridge is DRB, it normally does
not create a pseudonode for the link. In that case, no adjacencies
over the access link are reported in their LSPs by any of the
RBridges connected to the link. In this case no TRILL frames,
except Hellos, are sent out the access ports.
Alternatively, the DRB MAY choose to creates a pseudonode for the
access link. If it does create a pseudonode, it sets the IS-IS
overflow bit in the pseudonode. This will cause IS-IS routing to
avoid sending transit data on the link if any other path is
available but it will still be available as a path of last resort;
however, TRILL IS-IS frames will be sent over the link. In
addition, if a pseudonode is created by the DRB for an access
link, all the RBridges on the access link report connectivity to
the pseudonode as usual and the DRB reports connectivity in the
LSP it creates for the pseudonode.
4.7.2 RBridge Port Structure
An RBridge port can be modeled as having a structure, in its lower
levels, similar to that of an [802.1Q] bridge port as shown in Figure
4.3. 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 |
|| | | | Processing |
+---++------------++------+------------+------------+ <-- ISS
| 802.1/802.3 Low Level Control Frame |
| Processing, Port/Link Control Logic |
+------------++-------------------------------------+
||
|| +------------+
|| | 802.3 PHY |
++========+ (Physical +========= 802.3
| Interface) | Link
+------------+
Figure 4.3: 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 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 to this
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lower level port control logic corresponds to the Internal Sublayer
Service (ISS) in 802.1Q.
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 4.7.3. Handling of VRP frames is
described in Section 4.7.4.
Non-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 bridge. The upper interface to the port VLAN processing
corresponds to the Extended Internal Sublayer Service (EISS) in
802.1Q.
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
obtained by decapsulation but only output if the RBridge is an
uninhibited appointed forwarder for the frame's VLAN.
TRILL Hellos are handled per port and never forwarded. They can
affect the appointed forwarder and inhibition logic as well as the
RBridge's LSP.
TRILL IS-IS frames, other than Hellos, and TRILL data and ESADI
frames are pass 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 but
there are additional filters on some of them such as the Reverse Path
Forwarding check.
4.7.3 BPDU Handling
If RBridge campus topology were static, RBridges would be simply 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, these are a
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.
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4.7.3.1 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 4.7.3.3.
4.7.3.2 Root Bridge Changes
A change in the root bridge seen out a port may indicate a change in
bridged LAN topology including the possibility of the merger of two
bridged LANs or the like. During topology transients, bridges may go
into pre-forwarding states that block TRILL IS-IS Hellos. 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 30 and
zero 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 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 the time period while things are sorted out by BPDUs
within the merged bridged LAN and TRILL IS-IS Hellos between all the
RBridges involved.
4.7.3.3 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.
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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. Except for this optional capability,
RBridges MUST NOT send spanning tree BPDUs.
4.7.4 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 VLANs whose enablement is dynamic. 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] / [802.1ak] bridge.
An RBridge that supports a VRP sends GVRP/MVRP frames as an [802.1Q]
/ [802.1ak] bridge would send on each port that is not configured as
an RBridge trunk port. For this purpose, it sends VRP frames to
request traffic in the VLANs for which it is appointed forwarder and
traffic 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. RBridge Addresses, 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.
A new Ethertype must be assigned to indicate a TRILL encapsulated
frame.
Three Layer 2 multicast addresses must be assigned:
o All-RBridges for use as Outer.MacDA in TRILL ESDADI 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 A nickname and nickname selection priority.
o Priority to be a distribution tree root and desired number of
additional distribution trees for the campus, as discussed in
Section 4.3.
o The per RBridge parameters Aging Timer and Forward Delay, as
described in Section 4.6.
RBridges may be configured to have ESADIs (end station address
distribution instances) of TRILL IS-IS 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 per RBridge per VLAN Other Multicast bit, which defaults to true,
to request the receipt of non-IP derived multicast traffic.
The following RBridge per port parameters:
o The same parameters as for an 802.1Q port in terms of VLAN C-
tags and frame priority code points.
o Three per-port configuration bits: disable port, disable end
station service (trunk), and access port (see Section 4.7.1).
o Configuration for the optional send-BPDUs solution to the
wiring closet topology problem (see Section A.3.3) consists of
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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
instances to use IS-IS security [RFC5304] and ignore unauthenticated
TRILL IS-IS and ESADI frames received on a port. 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 the
optional TRILL ESADIs (see Section 4.6).
TRILL encapsulates native frames inside the TRILL Ethertype while
they are in transit between that frame's ingress RBridge and egress
RBridge(s). Thus, TRILL ignorant devices with firewall features and
which 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. Routers and
hosts appear to RBridges to be end stations and such frames will be
decapsulated before being sent to such devices. Thus they will not
see the TRILL Ethertype. Firewall devices which 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.
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
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.
Furthermore, if VLANs were used to keep some information off links
where it might be observed, this will no longer work with TRILL; 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
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security option should one be specified.
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.
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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, and
Version 0 Header Reserved bits.
The initial contents of the TRILL Version Registry is as follows:
Version Status
0 As specified in RFCthisdocument
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
0x2 Available for allocation by IETF Standards Action
0x1 Available for allocation by IETF Standards Action
The initial contents of the TRILL Nicknames Registry is as follows:
0x0000 Reserved to indicate no nickname specified
0x0001-0xFFBF Dynamically allocated within each TRILL campus
0xFFC0-0xFFFE Available for allocation by IETF Review
0xFFFF Permanently reserved
7.2 IEEE Registration Authority Considerations
The Ethertype <tbd> is assigned by the IEEE Registration Authority to
the TRILL Protocol.
The Layer 2 multicast MAC addresses <tbd1>, <tbd2>, and <tbd3> are
assigned by the IEEE Registration Authority for "All-Rbridges", "All-
IS-IS-Rbridges", and "All-ESADI-RBridges" respectively.
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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] "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.
R. Perlman, et al [Page 66]
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9. Informative References
[802.1AB] "IEEE Standard for Local and metropolitan area networks /
Station and Media Access Control Connectivity Discovery",
802.1AB-2005, 6 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.
[Arch] Gray, E., "The Architecture of an RBridge Solution to TRILL",
draft-ietf-trill-rbridge-arch-05.txt, February 2008, work in
progress.
[PAS] Touch, J., & R. Perlman, "Transparent Interconnection of Lots
of Links (TRILL) / Problem and Applicability Statement", draft-
ietf-trill-prob-05.txt, June 2008, work in progress.
[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.
[RP1999] Perlman, R., "Interconnection: Bridges, Routers, Switches,
R. Perlman, et al [Page 67]
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and Internetworking Protocols", Addison Wesley 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. Then two potential problems can occur as follows:
1. The requirement that native frames enter and leave a link via the
appointed forwarder for the link and VLAN of the frame can cause
congestion or suboptimal routing. (Similar problems can occur
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within 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 singly connected to a core bridge, the replacement of all of
the core bridges by RBridges without replacing the peripheral
bridges would generally improve performance without inducing any
appointed forwarder congestion. Solutions to this problem are
discussed below and a particular example explored in Section A.3.
2. TRILL traffic sent to the All-Rbridges or All-IS-IS-Rbridges
multicast addresses will typically be flooded throughout a bridged
LAN, which may create a greater burden than necessary. In cases
where there is actually only one RBridge next hop recipient of
interest, this problem can be eliminated by using the option of
unicasting the TRILL data or ESADI frame to that recipient rather
than multicasting it.
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 problem 1 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
4.2.3.2). 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
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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 evade the
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 respectively), 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
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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
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 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 arbitrarily 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
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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.
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 link and the RB2 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 point-
to-point 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 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
4.2.3.1 and 4.7.1)
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 4.7.1.
When designing RBridge configuration user interfaces, consideration
should be given to making it convenient to configure 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, the distribution tree rooted at RBy will predominantly use the
vertical links among RB1 through RB9 while that rooted at RBz will
predominantly use the horizontal. If RBy chooses itself as the
distribution tree root for half of this traffic and RBz 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 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 such the 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:
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 Z: Revision History
RFC Editor: Please delete this Appendix Z before publication.
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.
13. Add end station address learning section. Specify end station
address learning from decapsulated native frames.
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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 psuedo code.
8. Change to one DRB per physical link which 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 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.
16. Miscellaneous minor editing and terminology updates. Changed
Figure numbers to be relative to major section. Added Table
captions.
Changes from -07 to -08
1. Add "low" and "high" level control frame definitions to Section
1.2 and note concerning frames which would qualify as both
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"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) prohibition
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
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
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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 4.3, 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 which 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.
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.
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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 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 RBride 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 4.3 at the upper port layers and
correspondingly expand the accompanying text which 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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R. Perlman, et al [Page 85]
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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
Eastlake Enterprises
155 Beaver Street
Milford, MA 01757 USA
Phone: +1-508-634-2066
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
Nuova Systems
2600 San Tomas Expressway
Santa Clara, CA 95051 USA
Phone: +1-408-387-6123
Email: sgai@nuovasystems.com
Anoop Ghanwani
Brocade
1745 Technology Drive
San Jose, CA 95110 USA
Phone: +1-408-333-7149
Email: anoop@brocade.com
R. Perlman, et al [Page 86]
