Transparent Interconnection of Lots of Links (TRILL): Fine-Grained Labeling
RFC 7172
| Document | Type |
RFC - Proposed Standard
(May 2014; No errata)
Updates RFC 6325
|
|
|---|---|---|---|
| Authors | Donald Eastlake , Mingui Zhang , Puneet Agarwal , Radia Perlman , Dinesh Dutt | ||
| Last updated | 2015-10-14 | ||
| Replaces | draft-eastlake-trill-rbridge-fine-labeling | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text html pdf htmlized bibtex | ||
| Reviews | |||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | No shepherd assigned | ||
| IESG | IESG state | RFC 7172 (Proposed Standard) | |
| Action Holders |
(None)
|
||
| Consensus Boilerplate | Unknown | ||
| Telechat date | |||
| Responsible AD | Ted Lemon | ||
| IESG note | Erik Nordmark (nordmark@acm.org) is the Document Shepherd. | ||
| Send notices to | (None) | ||
| IANA | IANA review state | Version Changed - Review Needed | |
| IANA action state | RFC-Ed-Ack | ||
Internet Engineering Task Force (IETF) D. Eastlake 3rd
Request for Comments: 7172 M. Zhang
Updates: 6325 Huawei
Category: Standards Track P. Agarwal
ISSN: 2070-1721 Broadcom
R. Perlman
Intel Labs
D. Dutt
Cumulus Networks
May 2014
Transparent Interconnection of Lots of Links (TRILL):
Fine-Grained Labeling
Abstract
The IETF has standardized Transparent Interconnection of Lots of
Links (TRILL), a protocol for least-cost transparent frame routing in
multi-hop networks with arbitrary topologies and link technologies,
using link-state routing and a hop count. The TRILL base protocol
standard supports the labeling of TRILL Data packets with up to 4K
IDs. However, there are applications that require a larger number of
labels providing configurable isolation of data. This document
updates RFC 6325 by specifying optional extensions to the TRILL base
protocol to safely accomplish this. These extensions, called fine-
grained labeling, are primarily intended for use in large data
centers, that is, those with more than 4K users requiring
configurable data isolation from each other.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7172.
Eastlake, et al. Standards Track [Page 1]
RFC 7172 TRILL: Fine-Grained Labeling May 2014
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Eastlake, et al. Standards Track [Page 2]
RFC 7172 TRILL: Fine-Grained Labeling May 2014
Table of Contents
1. Introduction ....................................................4
1.1. Terminology ................................................5
1.2. Contributors ...............................................5
2. Fine-Grained Labeling ...........................................5
2.1. Goals ......................................................6
2.2. Base Protocol TRILL Data Labeling ..........................7
2.3. Fine-Grained Labeling (FGL) ................................7
2.4. Reasons for VL and FGL Coexistence .........................9
3. VL versus FGL Label Differences ................................10
4. FGL Processing .................................................11
4.1. Ingress Processing ........................................11
4.1.1. Multi-Destination FGL Ingress ......................11
4.2. Transit Processing ........................................12
4.2.1. Unicast Transit Processing .........................12
4.2.2. Multi-Destination Transit Processing ...............12
4.3. Egress Processing .........................................13
4.4. Appointed Forwarders and the DRB ..........................14
4.5. Distribution Tree Construction ............................14
4.6. Address Learning ..........................................15
4.7. ESADI Extension ...........................................15
5. FGL TRILL Interaction with VL TRILL ............................15
5.1. FGL and VL Mixed Campus ...................................15
5.2. FGL and VL Mixed Links ....................................17
5.3. Summary of FGL-Safe Requirements ..........................18
6. IS-IS Extensions ...............................................19
7. Comparison with Goals ..........................................19
8. Allocation Considerations ......................................20
8.1. IEEE Allocation Considerations ............................20
8.2. IANA Considerations .......................................20
9. Security Considerations ........................................20
Appendix A. Serial Unicast ........................................22
Appendix B. Mixed Campus Characteristics ..........................23
B.1. Mixed Campus with High Cost Adjacencies ...................23
B.2. Mixed Campus with Data Blocked Adjacencies ................24
Acknowledgements ..................................................25
References ........................................................25
Normative References ...........................................25
Informative References .........................................26
Eastlake, et al. Standards Track [Page 3]
RFC 7172 TRILL: Fine-Grained Labeling May 2014
1. Introduction
The IETF has standardized the Transparent Interconnection of Lots of
Links (TRILL) protocol [RFC6325], which provides a solution for
least-cost transparent routing in multi-hop networks with arbitrary
topologies and link technologies, using [IS-IS] [RFC6165] [RFC7176]
link-state routing and a hop count. TRILL switches are sometimes
called RBridges (Routing Bridges).
The TRILL base protocol standard supports the labeling of TRILL Data
packets with up to 4K IDs. However, there are applications that
require a larger number of labels of data for configurable isolation
based on different tenants, service instances, or the like. This
document updates [RFC6325] by specifying optional extensions to the
TRILL base protocol to safely accomplish this. These extensions,
called fine-grained labeling, are primarily intended for use in large
data centers, that is, those with more than 4K users requiring
configurable data isolation from each other.
This document describes a format for allowing a data label of
24 bits, known as a "fine-grained label", or FGL. It also describes
coexistence and migration from current RBridges, known as "VL" (for
"VLAN Labeled") RBridges, to TRILL switches that can support FGL
("Fine-Grained Labeled") packets. Because various VL implementations
might handle FGL packets incorrectly, FGL packets cannot be
introduced until either all VL RBridges are upgraded to what we will
call "FGL-safe", which means that they will not "do anything bad"
with FGL packets, or all FGL RBridges take special precautions on any
port by which they are connected to a VL RBridge. FGL-safe
requirements are summarized in Section 5.3.
It is hoped that many RBridges can become FGL-safe through a software
upgrade. VL RBridges and FGL-safe RBridges can coexist without any
disruption to service, as long as no FGL packets are introduced.
If all RBridges are upgraded to FGL-safe, FGL traffic can be
successfully handled by the campus without any topology restrictions.
The existence of FGL traffic is known to all FGL RBridges because
some RBridge (say, RB3) that might source or sink FGL traffic will
advertise interest in one or more fine-grained labels in its
contribution to the link state (its LSP). If any VL RBridges remain
at the point when any RBridge announces that it might source or sink
FGL traffic, the adjacent FGL-safe RBridges MUST ensure that no FGL
packets are forwarded to their VL RBridge neighbor(s). The details
are specified in Section 5.1 below.
Eastlake, et al. Standards Track [Page 4]
RFC 7172 TRILL: Fine-Grained Labeling May 2014
1.1. Terminology
The terminology and acronyms of [RFC6325] are used in this document
with the additions listed below.
DEI - Drop Eligibility Indicator [802.1Q].
FGL - Fine-Grained Labeling or Fine-Grained Labeled or
Fine-Grained Label.
FGL-edge - An FGL TRILL switch advertising interest in an FGL
label.
FGL link - A link where all of the attached TRILL switches are
FGL.
FGL-safe - A TRILL switch that can safely be given an FGL data
packet, as summarized in Section 5.3.
RBridge - Alternative name for a TRILL switch.
TRILL switch - Alternative name for an RBridge.
VL - VLAN Labeling or VLAN Labeled or VLAN Label.
VL link - A link where any one or more of the attached RBridges
are VL.
VL RBridge - A TRILL switch that supports VL but is not FGL-safe.
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].
1.2. Contributors
Thanks for the contributions of the following:
Tissa Senevirathne and Jon Hudson
2. Fine-Grained Labeling
The essence of Fine-Grained Labeling (FGL) is that (a) when frames
are ingressed or created they may incorporate a data label from a set
consisting of significantly more than 4K labels, (b) TRILL switch
ports can be labeled with a set of such fine-grained data labels,
Eastlake, et al. Standards Track [Page 5]
RFC 7172 TRILL: Fine-Grained Labeling May 2014
and (c) an FGL TRILL Data packet cannot be egressed through a TRILL
switch port unless its fine-grained label (FGL) matches one of the
data labels of the port.
Section 2.1 lists FGL goals. Section 2.2 briefly outlines the more
coarse TRILL base protocol standard [RFC6325] data labeling.
Section 2.3 outlines FGL for TRILL Data packets. Section 2.4
discusses VL and FGL coexistence.
2.1. Goals
There are several goals that would be desirable for FGL TRILL. They
are briefly described in the list below in approximate order by
priority, with the most important first.
1. Fine-Grained
Some networks have a large number of entities that need
configurable isolation, whether those entities are independent
customers, applications, or branches of a single endeavor or some
combination of these or other entities. The labeling supported by
[RFC6325] provides for only 2**12 - 2 valid identifiers or labels
(VLANs). A substantially larger number is required.
2. Silicon
Fine-grained labeling (FGL) should, to the extent practical, use
existing features, processing, and fields that are already
supported in many fast path silicon implementations that support
the TRILL base protocol.
3. Base RBridge Interoperation
To support some incremental conversion scenarios, it is desirable
that not all RBridges in a campus using FGL be required to be FGL
aware. That is, it is desirable if RBridges not implementing the
FGL features can exchange VL TRILL Data packets with FGL TRILL
switches.
4. Alternate Priority
Under some circumstances, it would be desirable for traffic from
an attached non-TRILL network to be handled, while transiting a
TRILL network, with a different priority from the priority of the
original native frames. This could be accomplished by the ingress
TRILL switch assigning a different priority to the FGL TRILL Data
packet resulting from ingressing the native frames. The original
priority should be restored on egress.
Eastlake, et al. Standards Track [Page 6]
RFC 7172 TRILL: Fine-Grained Labeling May 2014
2.2. Base Protocol TRILL Data Labeling
This section provides a brief review of the [RFC6325] TRILL Data
packet VL Labeling and changes the description of the TRILL Header by
moving the point at which the TRILL Header ends. This change in
description does not involve any change in the bits on the wire or in
the behavior of VL TRILL switches.
VL TRILL Data packets have the structure shown below:
+-------------------------------------------+
| Link Header (depends on link technology) |
| (if link is an Ethernet link, the link |
| header may include an Outer.VLAN tag) |
+-------------------------------------------+
| TRILL Header |
| +---------------------------------------+ |
| | Initial Fields and Options | |
| +---------------------------------------+ |
| | Inner.MacDA | (6 bytes) |
| +-----------------------------+ |
| | Inner.MacSA | (6 bytes) |
| +-----------------------+-----+ |
| | Ethertype 0x8100 | (2 bytes) |
| +-----------------------+ |
| | Inner.VLAN Label | (2 bytes) |
| +-----------------------+ |
+-------------------------------------------+
| Native Payload |
+-------------------------------------------+
| Link Trailer (depends on link technology) |
+-------------------------------------------+
Figure 1: TRILL Data with VL
In the base protocol as specified in [RFC6325], the 0x8100 value is
always present and is followed by the Inner.VLAN field, which
includes the 12-bit VL.
2.3. Fine-Grained Labeling (FGL)
FGL expands the variety of data labels available under the TRILL
protocol to include a fine-grained label (FGL) with a 12-bit high
order part and a 12-bit low order part. In this document, FGLs are
denoted as "(X.Y)", where X is the high order part and Y is the low
order part of the FGL.
Eastlake, et al. Standards Track [Page 7]
RFC 7172 TRILL: Fine-Grained Labeling May 2014
FGL TRILL Data packets have the structure shown below.
+-------------------------------------------+
| Link Header (depends on link technology) |
| (if link is an Ethernet link, the link |
| header may include an Outer.VLAN tag) |
+-------------------------------------------+
| TRILL Header |
| +---------------------------------------+ |
| | Initial Fields and Options | |
| +---------------------------------------+ |
| | Inner.MacDA | (6 bytes) |
| +-----------------------------+ |
| | Inner.MacSA | (6 bytes) |
| +-----------------------+-----+ |
| | Ethertype 0x893B | (2 bytes) |
| +-----------------------+ |
| | Inner.Label High Part | (2 bytes) |
| +-----------------------+ |
| | Ethertype 0x893B | (2 bytes) |
| +-----------------------+ |
| | Inner.Label Low Part | (2 bytes) |
| +-----------------------+ |
+-------------------------------------------+
| Native Payload |
+-------------------------------------------+
| Link Trailer (depends on link technology) |
+-------------------------------------------+
Figure 2: TRILL Data with FGL
For FGL packets, the inner Media Access Control (MAC) address fields
are followed by the FGL information using 0x893B. There MUST be two
occurrences of 0x893B, as shown. Should a TRILL switch processing an
FGL TRILL Data packet notice that the second occurrence is actually
some other value, it MUST discard the packet. (A TRILL switch
transiting a TRILL Data packet is not required to examine any fields
past the initial fixed fields and options, although it may do so to
support Equal-Cost Multi-Path (ECMP) or distribution tree pruning.)
Eastlake, et al. Standards Track [Page 8]
RFC 7172 TRILL: Fine-Grained Labeling May 2014
The two bytes following each 0x893B have, in their low order 12 bits,
fine-grained label information. The upper 4 bits of those two bytes
are used for a 3-bit priority field and one Drop Eligibility
Indicator (DEI) bit as shown below.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
+--+--+--+---+--+--+--+--+--+--+--+--+--+--+--+--+
|priority|DEI| label information |
+--+--+--+---+--+--+--+--+--+--+--+--+--+--+--+--+
Figure 3: FGL Part Data Structure
The priority field of the Inner.Label High Part is the priority used
for frame transport across the TRILL campus from ingress to egress.
The label bits in the Inner.Label High Part are the high order part
of the FGL, and those bits in the Inner.Label Low Part are the low
order part of the FGL. The priority field of the Inner.Label Low
Part is remembered from the data frame as ingressed and is restored
on egress.
The appropriate FGL value for an ingressed or locally originated
native frame is determined by the ingress TRILL switch port as
specified in Section 4.1.
2.4. Reasons for VL and FGL Coexistence
For several reasons, as listed below, it is desirable for FGL TRILL
switches to be able to handle both FGL and VL TRILL Data packets.
o Continued support of VL packets means that, by taking the
precautions specified herein, in many cases such arrangements as
VL TRILL switches easily exchanging VL packets through a core of
FGL TRILL switches are possible.
o Due to the way TRILL works, it may be desirable to have a
maintenance VLAN or FGL [RFC7174] in which all TRILL switches in
the campus indicate interest. It will be simpler to use the same
type of label for all TRILL switches for this purpose. That
implies using VL if there might be any VL TRILL switches in the
campus.
o If a campus is being upgraded from VL to FGL, continued support of
VL allows long-term support of edges labeled as VL.
Eastlake, et al. Standards Track [Page 9]
RFC 7172 TRILL: Fine-Grained Labeling May 2014
3. VL versus FGL Label Differences
There are differences between the semantics across a TRILL campus for
TRILL Data packets that are data labeled with VL and FGL.
With VL, data label IDs have the same meaning throughout the campus
and are from the same label space as the C-VLAN IDs used on Ethernet
links to end stations.
The larger FGL data label space is a different space from the VL data
label space. For ports configured for FGL, the C-VLAN on an
ingressed native frame is stripped and mapped to the FGL data label
space with a potentially different mapping for each port. A similar
FGL-to-C-VLAN mapping occurs per port on egress. Thus, for ports
configured for FGL, the native frame C-VLAN on one link corresponding
to an FGL can be different from the native frame C-VLAN corresponding
to that same FGL on a different link elsewhere in the campus or even
a different link attached to the same TRILL switch. The FGL label
space is flat and does not hierarchically encode any particular
number of native frame C-VLAN bits or the like. FGLs appear only
inside TRILL Data packets after the inner MAC addresses.
It is the responsibility of the network manager to properly configure
the TRILL switches in the campus to obtain the desired mappings.
Such configuration is expected to be automatic in many cases, based
on configuration databases and orchestration systems.
With FGL TRILL switches, many things remain the same because an FGL
can appear only as the Inner.Label inside a TRILL Data packet. As
such, only TRILL-aware devices will see a fine-grained label. The
Outer.VLAN that may appear on native frames and that may appear on
TRILL Data packets if they are on an Ethernet link can only be a
C-VLAN tag. Thus, ports of FGL TRILL switches, up through the usual
VLAN and priority processing, act as they do for VL TRILL switches:
TRILL switch ports provide a C-VLAN ID for an incoming frame and
accept a C-VLAN ID for a frame being queued for output. Appointed
Forwarders [RFC6439] on a link are still appointed for a C-VLAN. The
Designated VLAN for an Ethernet link is still a C-VLAN.
FGL TRILL switches have capabilities that are a superset of those for
VL TRILL switches. FGL TRILL switch ports can be configured for FGL
or VL, with VL being the default. As with a base protocol [RFC6325]
TRILL switch, an unconfigured FGL TRILL switch port reports an
untagged frame it receives as being in VLAN 1.
Eastlake, et al. Standards Track [Page 10]
RFC 7172 TRILL: Fine-Grained Labeling May 2014
4. FGL Processing
This section specifies ingress, transit, egress, and other processing
details for FGL TRILL switches. A transit or egress FGL TRILL switch
determines that a TRILL Data packet is FGL by detecting that the
Inner.MacSA is followed by 0x893B.
4.1. Ingress Processing
FGL-edge TRILL switch ports are configurable to ingress native frames
as FGL. Any port not so configured performs the previously specified
[RFC6325] VL ingress processing on native frames resulting in a VL
TRILL Data packet. (There is no change in Appointed Forwarder logic
(see Section 4.4).) An FGL-safe TRILL switch may have only VL ports,
in which case it is not required to support the capabilities for FGL
ingress described in this section.
FGL-edge TRILL switches support configurable per-port mapping from
the C-VLAN of a native frame, as reported by the ingress port, to an
FGL. FGL TRILL switches MAY support other methods to determine the
FGL of an incoming native frame, such as methods based on the
protocol of the native frame or based on local knowledge.
The FGL ingress process MUST copy the priority and DEI (Drop
Eligibility Indicator) associated with an ingressed native frame to
the upper 4 bits of the Inner.Label Low Order part. It SHOULD also
associate a possibly different mapped priority and DEI with an
ingressed frame, but a TRILL switch might not be able to do so
because of implementation limitations. The mapped priority is placed
in the Inner.Label High Part. If such mapping is not supported, then
the original priority and DEI MUST be placed in the Inner.Label
High Part.
4.1.1. Multi-Destination FGL Ingress
If a native frame that has a broadcast, multicast, or unknown MAC
destination address is FGL ingressed, it MUST be handled in one of
the following two ways. The choice of which method to use can vary
from frame to frame, at the choice of the ingress TRILL switch.
1. Ingress as a TRILL multi-destination data packet (TRILL Header M
bit = 1) on a distribution tree rooted at a nickname held by an
FGL RBridge or by the pseudonode of an FGL link. FGL TRILL Data
packets MUST NOT be sent on a tree rooted at a nickname held by a
VL TRILL switch or by the pseudonode of a VL link.
Eastlake, et al. Standards Track [Page 11]
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2. Serially TRILL unicast the ingressed frame to the relevant egress
TRILL switches by using a known unicast TRILL Header (M bit = 0).
An FGL ingress TRILL switch SHOULD unicast a multi-destination
TRILL Data packet if there is only one relevant egress FGL TRILL
switch. The relevant egress TRILL switches are determined by
starting with those announcing interest in the frame's (X.Y)
label. That set SHOULD be further filtered based on multicast
listener and multicast router attachment LSP announcements if the
native frame was a multicast frame.
Using a TRILL unicast header for a multi-destination frame when it
has only one actual destination RBridge almost always improves
traffic spreading and decreases latency as discussed in Appendix A.
How to decide whether to use a distribution tree or serial unicast
for a multi-destination TRILL Data packet that has more than one
destination TRILL switch is beyond the scope of this document.
4.2. Transit Processing
Any FGL TRILL switch MUST be capable of TRILL Data packet transit
processing. Such processing is fairly straightforward as described
in Section 4.2.1 for known unicast TRILL Data packets and in
Section 4.2.2 for multi-destination TRILL Data packets.
4.2.1. Unicast Transit Processing
There is very little change in TRILL Data packet unicast transit
processing. A transit TRILL switch forwards any unicast TRILL Data
packet to the next hop towards the egress TRILL switch as specified
in the TRILL Header. All transit TRILL switches MUST take the
priority and DEI used to forward a packet from the Inner.VLAN label
or the FGL Inner.Label High Part. These bits are in the same place
in the packet.
An FGL TRILL switch MUST properly distinguish flows if it provides
ECMP for unicast FGL TRILL Data packets.
4.2.2. Multi-Destination Transit Processing
Multi-destination TRILL Data packets are forwarded on a distribution
tree selected by the ingress TRILL switch, except that an FGL ingress
TRILL switch MAY TRILL unicast such a frame to all relevant egress
TRILL switches, all as described in Section 4.1. The distribution
trees do not distinguish between FGL and VL multi-destination
packets, except in pruning behavior if they provide pruning. There
is no change in the Reverse Path Forwarding Check.
Eastlake, et al. Standards Track [Page 12]
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An FGL TRILL switch (say, RB1) having an FGL multi-destination frame
for label (X.Y) to forward on a distribution tree SHOULD prune that
tree based on whether there are any TRILL switches on a tree branch
that are advertising connectivity to label (X.Y). In addition, RB1
SHOULD prune multicast frames based on reported multicast listener
and multicast router attachment in (X.Y).
Pruning is an optimization. If a transit TRILL switch does less
pruning than it could, there may be greater link utilization than
strictly necessary but the campus will still operate correctly. A
transit TRILL switch MAY prune based on an arbitrary subset of the
bits in the FGL label, for example, only the High Part or only the
Low Part of the label.
4.3. Egress Processing
Egress processing is generally the reverse of ingress progressing
described in Section 4.1. An FGL-safe TRILL switch may have only VL
ports, in which case it is not required to support the capabilities
for FGL egress described in this section.
An FGL-edge TRILL switch MUST be able to convert, in a configurable
fashion, from the FGL in an FGL TRILL Data packet it is egressing to
the C-VLAN ID for the resulting native frame with different mappings
on a per-port basis. The priority and DEI of the egressed native
frame are taken from the Inner.Label Low Order Part. A port MAY be
configured to strip output VLAN tagging.
It is the responsibility of the network manager to properly configure
the TRILL switches in the campus to obtain the desired mappings.
FGL egress is similar to VL egress, as follows:
1. If the Inner.MacDA is All-Egress-RBridges, special processing
applies, based on the payload Ethertype (for example, End-Station
Address Distribution Information (ESADI) [RFC6325] or RBridge
Channel [RFC7178]), and if the payload Ethertype is unknown, the
packet is discarded. If the Inner.MacDA is not
All-Egress-RBridges, then either item 2 or item 3 below applies,
as appropriate.
2. A known unicast FGL TRILL Data packet (TRILL Header M bit = 0)
with a unicast Inner.MacDA is egressed to the FGL port or ports
matching its FGL and Inner.MacDA. If there are no such ports, it
is flooded out of all FGL ports that have its FGL, except any
ports for which the TRILL switch has knowledge that the frame's
Inner.MacDA cannot be present on the link out of that port.
Eastlake, et al. Standards Track [Page 13]
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3. A multi-destination FGL TRILL Data packet is decapsulated and
flooded out of all ports that have its FGL, subject to multicast
pruning. The same processing applies to a unicast FGL TRILL Data
packet with a broadcast or multicast Inner.MacDA that might be
received due to serial unicast.
An FGL TRILL switch MUST NOT egress an FGL packet with label (X.Y) to
any port not configured with that FGL, even if the port is configured
to egress VL packets in VLAN X.
FGL TRILL switches MUST accept multi-destination TRILL Data packets
that are sent to them as TRILL unicast packets (packets with the
TRILL Header M bit set to 0). They locally egress such packets, if
appropriate, but MUST NOT forward them (other than egressing them as
native frames on their local links).
4.4. Appointed Forwarders and the DRB
There is no change in adjacency [RFC7177], DRB (Designated RBridge)
election, or Appointed Forwarder logic [RFC6439] on a link,
regardless of whether some or all the ports on the link are for FGL
TRILL switches, with one exception: implementations SHOULD provide
that their default priority for a VL RBridge port to be the DRB is
less than their default priority for an FGL RBridge to be the DRB.
This will assure that, in the unconfigured case, an FGL RBridge will
be elected DRB when using that implementation.
4.5. Distribution Tree Construction
All distribution trees are calculated as provided for in the TRILL
base protocol standard [RFC6325] as updated by [RFC7180], with the
exception that the default tree root priority for a nickname held by
an FGL TRILL switch or an FGL link pseudonode is 0x9000. As a
result, they will be chosen in preference to VL nicknames in the
absence of configuration. If distribution tree roots are configured,
there MUST be at least one tree rooted at a nickname held by an FGL
TRILL switch or by an FGL link pseudonode. If distribution tree
roots are misconfigured so there would not be such a tree, then the
highest priority FGL nickname to be a tree root is used to construct
an additional tree, regardless of configuration. (VL TRILL switches
will not know about this additional distribution tree but, through
the use of Step (A) or (B) in Section 5.1, no VL TRILL switch should
ever receive a multi-destination TRILL Data packet using this
additional tree.)
Eastlake, et al. Standards Track [Page 14]
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4.6. Address Learning
An FGL TRILL switch learns addresses from the data plane on ports
configured for FGL based on the fine-grained label rather than the
native frame's VLAN. Addresses learned from ingressed native frames
on FGL ports are logically represented by { MAC address, FGL, port,
confidence, timer }, while remote addresses learned from egressing
FGL packets are logically represented by { MAC address, FGL, remote
TRILL switch nickname, confidence, timer }.
4.7. ESADI Extension
The TRILL ESADI (End-Station Address Distribution Information)
protocol is specified in [RFC6325] as optionally transmitting MAC
address connection information through TRILL Data packets between
participating TRILL switches over the virtual link provided by the
TRILL multi-destination packet distribution mechanism. In [RFC6325],
the VL to which an ESADI packet applies is indicated only by the
Inner.VLAN label, and no indication of that VL is allowed within the
ESADI payload.
ESADI is extended to support FGL by providing for the indication of
the FGL to which an ESADI packet applies only in the Inner.Label of
that packet, and no indication of that FGL is allowed within the
ESADI payload.
5. FGL TRILL Interaction with VL TRILL
This section discusses mixing FGL-safe and VL TRILL switches in a
campus. It does not apply if the campus is entirely FGL-safe or if
there are no FGL-edges. Section 5.1 specifies what behaviors are
needed to render such mixed campuses safe. See also Appendix B for a
discussion of campus characteristics when these behaviors are in use.
Section 5.2 gives details of link-local mixed behavior.
It is best, if possible, for VL TRILL switches to be upgraded to
FGL-safe before introducing FGL-edges (and therefore FGL data
packets).
5.1. FGL and VL Mixed Campus
By definition, it is not possible for VL TRILL switches to safely
handle FGL traffic, even if the VL TRILL switch is only acting in the
transit capacity. If a TRILL switch can safely transit FGL TRILL
Data packets, then it qualifies as FGL-safe but will still be assumed
to be VL until it advertises in its LSP that it is FGL-safe.
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VL frames are required to have 0x8100 at the beginning of the data
label, where FGL frames have 0x893B. VL TRILL switches conformant to
[RFC6325] should discard frames with this new value after the inner
MAC addresses. However, if they do not discard such frames, they
could be confused and egress them into the wrong VLAN (see Section 9
below) or persistently reorder them due to miscomputing flows for
ECMP, or they could improperly prune their distribution if they are
multi-destination so that they would fail to reach some intended
destinations. Such difficulties are avoided by taking all practical
steps to minimize the chance of a VL TRILL switch handling an FGL
TRILL Data packet. These steps are specified below.
FGL-safe switches will report their FGL capability in LSPs. Thus,
FGL-safe TRILL switches (and any management system with access to the
link-state database) will be able to detect the existence of TRILL
switches in the campus that do not support FGL.
Once a TRILL switch advertises an FGL-edge, any FGL-safe TRILL switch
(RB1 in this discussion) that observes, on one of its ports, a VL
RBridge on the link out of that port, MUST take Step (A) or (B) below
for that port and also take Step (C) further below. ("Observes"
means that it has an adjacency to the VL TRILL switch that is in any
state other than Down [RFC7177] and holds an LSP fragment zero for
it, showing that it is not FGL-safe.) Finally, for there to be full
FGL connectivity, the campus topology must be such that all FGL TRILL
switches are reachable from all other FGL TRILL switches without
going through a VL TRILL switch.
(A) If RB1 can discard any FGL TRILL Data packet that would be output
through a port where it observes a VL RBridge, while allowing the
output of VL TRILL Data packets through that port, then
A1. RB1 MUST so discard all FGL TRILL Data output packets that
would otherwise be output through the port, and
A2. For all adjacencies out of that port (even adjacencies to
other FGL RBridges or a pseudonode) in the Report state
[RFC7177], RB1 MUST report that adjacency cost as 2**23
greater than it would have otherwise reported, but not more
than 2**24 - 2 (the highest link cost still usable in least-
cost path calculations and distribution tree construction).
This assures that if any path through FGL-safe TRILL switches
exists, such a path will be computed.
(B) If RB1 cannot discard any FGL TRILL Data packet that would be
output through a port where it observes a VL RBridge while
allowing VL TRILL Data packets, then RB1 MUST, for all
adjacencies out of that port (even adjacencies to other FGL-safe
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RBridges or a pseudonode) in the Report state [RFC7177], report
the adjacency cost as 2**24 - 1. As specified in IS-IS
[RFC5305], that cost will stop the adjacency from being used in
least-cost path calculations, including distribution tree
construction (see Section 2.1 of [RFC7180]) but will still leave
it visible in the topology and usable, for example, by any
traffic engineered path mechanism.
(C) The roots for all distribution trees used for FGL TRILL Data
packets must be nicknames held by an FGL-safe TRILL switch or by
a pseudonode representing an FGL link. As provided in
Section 4.5, there will always be such a distribution tree.
Using the increased adjacency cost specified in part A2 of Step (A)
above, VL links will be avoided unless no other path is available for
typical data center link speeds using the default link cost
determination method specified in Item 1 of Section 4.2.4.4 of
[RFC6325]. However, if links have low speed (such as about
100 megabits/second or less) or some non-default method is used for
determining link costs, then link costs MUST be adjusted such that no
adjacency between FGL-safe TRILL switches has a cost greater than
200,000.
To summarize, for a mixed TRILL campus to be safe once FGL-edges are
introduced, it is essential that the steps above be followed by
FGL-safe RBridges, to ensure that paths between such RBridges do not
include VL RBridges, and to ensure that FGL packets are never
forwarded to VL RBridges. That is, all FGL-safe switches MUST do
Step (A) or (B) for any port out of which they observe a VL RBridge
neighbor. Also, for full FGL connectivity, all FGL-safe TRILL
switches MUST do Step (C) and be connected in a single FGL contiguous
area.
5.2. FGL and VL Mixed Links
The usual DRB election operates on a link with mixed FGL and VL
ports. If an FGL TRILL switch port is a DRB, it can handle all
native traffic. It MUST appoint only other FGL TRILL switch ports as
Appointed Forwarder for any VLANs that are to be mapped to FGL.
For VLANs that are not being mapped to FGL, if Step (A) is being
followed (see Section 5.1), it can appoint either a VL or FGL TRILL
switch for a VLAN on the link to be handled by a VL. If Step (B) is
being followed, an FGL DRB MUST only appoint FGL Appointed
Forwarders, so that all end stations will get service to the FGL
campus. If a VL RBridge is a DRB, it will not understand that FGL
TRILL switch ports are different. To the extent that Step (B) is in
effect and a VL DRB handles native frames or appoints other VL TRILL
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switch ports on a link to handle native frames for one or more VLANs,
the end stations sending and receiving those native frames may be
isolated from the FGL campus. When a VL DRB happens to appoint an
FGL port as Appointed Forwarder for one or more VLANs, the end
stations sending and receiving native frames in those VLANs will get
service to the FGL campus.
5.3. Summary of FGL-Safe Requirements
The list below summarizes the requirements for a TRILL switch to be
FGL-safe.
1. For both unicast and multi-destination data, RB1 MUST NOT forward
an FGL packet to a VL neighbor RB2. This is accomplished as
specified in Section 5.1.
2. For both unicast and multi-destination data, RB1 MUST NOT egress a
packet onto a link that does not belong in that FGL.
3. For unicast data, RB1 must forward the FGL packet properly to the
egress nickname in the TRILL Header. This means that it MUST NOT
delete the packet because of not having the expected VLAN tag, it
MUST NOT insert a VLAN tag, and it MUST NOT misclassify a flow so
as to persistently misorder packets, because the TRILL fields are
now 4 bytes longer than in VL TRILL packets.
4. For multi-destination data, RB1 must forward the packet properly
along the specified tree. This means that RB1 MUST NOT falsely
prune the packet. RB1 is allowed not to prune at all, but it MUST
NOT prevent an FGL packet from reaching all the links with that
FGL by incorrectly refusing to forward the FGL packet along a
branch in the tree.
5. RB1 must advertise, in its LSP, that it is FGL-safe.
Point 1 above, for a TRILL switch to correctly support ECMP, and
point 2, for a TRILL switch to correctly prune distribution trees,
require that the TRILL switch properly recognize and distinguish
between the two Ethertypes that can occur immediately after the
Inner.MacSA in a TRILL Data packet.
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6. IS-IS Extensions
Extensions related to TRILL's use of IS-IS are required to support
FGL and must include the following:
1. A method for a TRILL switch to announce itself in its LSP as
FGL-safe (see Section 8.2).
2. A sub-TLV analogous to the Interested VLANs and Spanning Tree
Roots sub-TLV of the Router Capabilities TLV but indicating FGLs
rather than VLs. This is called the Interested Labels and
Spanning Tree Roots (INT-LABEL) sub-TLV in [RFC7176].
3. Sub-TLVs analogous to the GMAC-ADDR sub-TLV of the Group Address
TLV that specifies an FGL rather than a VL. These are called the
GLMAC-ADDR, GLIP-ADDR, and GLIPV6-ADDR sub-TLVs in [RFC7176].
7. Comparison with Goals
Comparing TRILL FGL, as specified in this document, with the goals
given in Section 2.1, we find the following:
1. Fine-Grained: FGL provides 2**24 labels, vastly more than the
4094 (4K) VLAN labels supported in TRILL as specified in
[RFC6325].
2. Silicon: Existing TRILL fast path silicon chips can perform base
TRILL Header insertion and removal to support ingress and egress.
In addition, it is believed that most such silicon chips can also
perform the native-frame-to-FGL mapping and the encoding of the
FGL as specified herein, as well as the inverse decoding and
mapping. Some existing silicon chips can perform only one of
these operations on a frame in one pass through the fast path;
however, other existing chips are believed to be able to perform
both operations on the same frame in one pass through their fast
path. It is also believed that most FGL TRILL switches will be
capable of having their ports configured to discard FGL packets.
Such a capability makes interoperation with VL TRILL switches
practical using Step (A) as opposed to Step (B) (see Section 5.1).
3. Base RBridge Interoperation: As described in Section 3, FGL is not
generally compatible with TRILL switches conformant to the base
specification [RFC6325]. In particular, a VL TRILL switch cannot
be an FGL TRILL switch because there is a risk that it would
mishandle FGL packets. However, a contiguous set of VL TRILL
switches can exchange VL frames, regardless of the presence of FGL
TRILL switches in the campus. The provisions of Section 5 support
reasonable interoperation and migration scenarios.
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4. Alternate Priority: The encoding specified in Section 2.3 and the
ingress/egress processing specified in Section 4 provide for a new
priority and DEI in the Inner.Label High Part and a place to
preserve the original user priority and DEI in the Low Part so
that it can be restored on egress.
8. Allocation Considerations
Allocations by the IEEE Registration Authority and IANA are listed
below.
8.1. IEEE Allocation Considerations
The IEEE Registration Authority has assigned Ethertype 0x893B for
TRILL FGL.
8.2. IANA Considerations
IANA has allocated capability flag 1 in the TRILL-VER sub-TLV
capability flags [RFC7176] to indicate that a TRILL switch is
FGL-safe.
9. Security Considerations
See [RFC6325] for general TRILL security considerations.
As with any communications system, end-to-end encryption and
authentication should be considered for sensitive data. In this
case, that would be encryption and authentication extending from a
source end station and carried through the TRILL campus to a
destination end station.
Confusion between a packet with VL X and a packet with FGL (X.Y) or
confusion due to a malformed frame is a potential problem if an FGL
TRILL switch did not properly check for the occurrence of 0x8100 or
0x893B immediately after the Inner.MacSA (see Sections 2.2 and 2.3)
and handle the frame appropriately.
[RFC6325] requires that the Ethertype immediately after the
Inner.MacSA be 0x8100. A VL TRILL switch that did not discard a
packet with some other value there could cause problems. If it
received a TRILL Data packet with FGL (X.Y) or with junk after the
Inner.MacSA that included X where a VLAN ID would appear, then:
1. It could egress the packet to an end station in VLAN X. If the
packet was a well-formed FGL frame, the payload of such an
egressed native frame would appear to begin with Ethertype 0x893B,
which would likely be discarded by an end station. In any case,
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such an egress would almost certainly be a violation of security
policy requiring the configurable separation of differently
labeled data.
2. If the packet was multi-destination and the TRILL switch pruned
the distribution tree, it would incorrectly prune it on the basis
of VLAN X. For an FGL packet, this would probably lead to the
multi-destination data packet not being delivered to all of its
intended recipients.
Possible problems with an FGL TRILL switch that (a) received a TRILL
Data packet with junk after the Inner.MacSA that included X where a
VLAN ID would appear and (b) did not check the Ethertype immediately
after the Inner.MacSA would be that it could improperly egress the
packet in VLAN X, violating security policy. If the packet was
multi-destination and was improperly forwarded, it should be
discarded by properly implemented TRILL switches downstream in the
distribution tree and never egressed, but the propagation of the
packet would still waste bandwidth.
To avoid these problems, all TRILL switches MUST check the Ethertype
immediately after the Inner.MacSA and, if it is a value they do not
know how to handle, either discard the frame or make no decisions
based on any data after that Ethertype. In addition, care must be
taken to avoid FGL packets being sent to or through VL TRILL switches
that will discard them if the VL TRILL switch is properly implemented
or mishandle them if it is not properly implemented. This is
accomplished as specified in Section 5.1.
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Appendix A. Serial Unicast
This informational appendix discusses the advantages and
disadvantages of using serial unicast instead of a distribution tree
for multi-destination TRILL Data packets. See Sections 4.1 and 4.3.
This document requires that FGL TRILL switches accept serial unicast,
but there is no requirement that they be able to send serial unicast.
Consider a large TRILL campus with hundreds of TRILL switches in
which, say, 300 end stations are in some particular FGL data label.
At one extreme, if all 300 end stations were on links attached to a
single TRILL switch, then no other TRILL switch would be advertising
interest in that FGL. As a result, it is likely that because of
pruning a multi-destination (say, broadcast) frame from one such end
station would not be sent to any another TRILL switch, even if put on
a distribution tree.
At the other extreme, assume that the 300 end stations are attached,
one each, to 300 different TRILL switches; in that case, you are
almost certainly better off using a distribution tree because if you
tried to serially unicast you would have to output 300 copies,
probably including multiple copies through the same port, and would
cause much higher link utilization.
Now assume that these 300 end stations are connected to exactly two
TRILL switches, say, 200 to one and 100 to the other. Using unicast
TRILL Data packets between these two TRILL switches is best because
the frames will follow least-cost paths, possibly with such traffic
spread over a number of least-cost paths with equal cost. On the
other hand, if distribution trees were used, each frame would be
constrained to the tree used for that frame and would likely follow a
higher cost route and only a single path would be available per tree.
Thus, this document says that unicast SHOULD be used if there are
exactly two TRILL switches involved.
The decision of whether to use a distribution tree or serial unicast
if the end stations are connected to more than two TRILL switches is
more complex. Which would be better would depend on many factors,
including network topology and application data patterns. How to
make this decision in such cases is beyond the scope of this
document.
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Appendix B. Mixed Campus Characteristics
This informational appendix describes the characteristics of a TRILL
campus with mixed FGL-safe and VL TRILL switches for two cases:
Appendix B.1 discusses the case where all FGL adjacencies with VL are
handled by Step (A) in Section 5.1, and Appendix B.2 discusses the
case where all FGL adjacencies with VL are handled by Step (B) in
Section 5.1.
B.1. Mixed Campus with High Cost Adjacencies
If the FGL TRILL switches use Step (A) in Section 5.1, then VL and
FGL TRILL switches will be able to interoperate for VL traffic.
Least-cost paths will avoid any FGL -> VL TRILL switch hops unless no
other reasonable path is available. In conjunction with Section 4.5,
there will be at least one distribution tree rooted at a nickname
held by an FGL TRILL switch or the pseudonode for an FGL link.
Furthermore, if the FGL TRILL switches in the campus form a single
contiguous island, this distribution tree will have a fully connected
sub-tree covering that island. Thus, any FGL TRILL Data packets sent
on this tree will be able to reach any other FGL TRILL switch without
attempting to go through any VL TRILL switches. (Such an attempt
would cause the FGL packet to be discarded as specified in part A1 of
Step (A).)
If supported, Step (A) is particularly effective in a campus with an
FGL TRILL switch core and VL TRILL switches in one or more islands
around that core. For example, consider the campus below. This
campus has an FGL core consisting of FGL01 to FGL14 and three VL
islands consisting of VL01 to VL04, VL05, and VL06 to VL14.
*VL01--*VL02
| |
*VL03--*VL04 *VL05
| | |
FGL01--FGL02--FGL03--FGL04--FGL05
| | | | |
FGL06--FGL07--FGL08--FGL09--FGL10
| | | | |
FGL11--FGL12--*VL06--*VL07---FGL13
| | | |
*VL08--*VL09--*VL10---FGL14
| | | |
*VL11--*VL12--*VL13--*VL14
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Assuming that the FGL TRILL switches in this campus all implement
Step (A), then end stations connected through a VL port can be
connected anywhere in the campus to VL or FGL TRILL switches and, if
in the same VLAN, will communicate. End stations connected through
an FGL port on FGL TRILL switches will communicate if their local
VLANs are mapped to the same FGL.
Due to the high cost of FGL-to-VL adjacencies used in path
computations, VL TRILL switches are avoided on paths between FGL
TRILL switches. For example, even if the speed and default adjacency
cost of all the connections shown above were the same, traffic from
FGL12 to FGL13 would follow the 5-hop path FGL12 - FGL07 - FGL08 -
FGL09 - FGL10 - FGL13 rather than the 3-hop path FGL12 - VL09 - VL10
- FGL14.
B.2. Mixed Campus with Data Blocked Adjacencies
If the FGL TRILL switches use Step (B) in Section 5.1, then least-
cost and distribution tree TRILL Data communication between VL and
FGL TRILL switches is blocked, although TRILL IS-IS communication is
normal. This data blocking, although implemented only by FGL TRILL
switches, has relatively symmetric effects. The following paragraphs
assume that such data blocking between VL and FGL is in effect
throughout the campus.
A campus of mostly FGL TRILL switches implementing Step (B) with a
few isolated VL TRILL switches scattered throughout will work well in
terms of connectivity for end stations attached to those FGL
switches, except that they will be unable to communicate with any end
stations for which a VL switch is appointed forwarder. The VL TRILL
switches will be isolated and will only be able to route TRILL Data
to the extent that they happen to be contiguously connected to other
VL TRILL switches. Distribution trees computed by the FGL switches
will not include any VL switches (see Section 2.1 of [RFC7180]).
A campus of mostly VL TRILL switches with a few isolated FGL TRILL
switches scattered throughout will also work reasonably well as
described immediately above but with all occurrences of "FGL" and
"VL" swapped.
However, a campus so badly misconfigured that it consists of a
randomly intermingled mixture of VL and FGL TRILL switches using
Step (B) is likely to offer very poor data service, due to many links
being blocked for data.
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Acknowledgements
The comments and suggestions of the following, listed in alphabetic
order, are gratefully acknowledged:
Stewart Bryant, Spencer Dawkins, Adrian Farrel, Anoop Ghanwani,
Sujay Gupta, Weiguo Hao, Phanidhar Koganti, Yizhou Li, Vishwas
Manral, Rajeev Manur, Thomas Narten, Gayle Nobel, Erik Nordmark,
Pete Resnick, Olen Stokes, Sean Turner, Ilya Varlashkin, and
Xuxiaohu.
References
Normative References
[802.1Q] IEEE 802.1, "IEEE Standard for Local and metropolitan area
networks--Media Access Control (MAC) Bridges and Virtual
Bridged Local Area Networks", IEEE Std 802.1Q-2011,
August 2011.
[IS-IS] ISO/IEC 10589:2002, Second Edition, "Information
technology -- Telecommunications and information exchange
between systems -- Intermediate System to Intermediate
System intra-domain routeing information exchange protocol
for use in conjunction with the protocol for providing the
connectionless-mode network service (ISO 8473)", 2002.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, October 2008.
[RFC6325] Perlman, R., Eastlake 3rd, D., Dutt, D., Gai, S., and A.
Ghanwani, "Routing Bridges (RBridges): Base Protocol
Specification", RFC 6325, July 2011.
[RFC7176] Eastlake 3rd, D., Senevirathne, T., Ghanwani, A., Dutt,
D., and A. Banerjee, "Transparent Interconnection of Lots
of Links (TRILL) Use of IS-IS", RFC 7176, May 2014.
[RFC7177] Eastlake 3rd, D., Perlman, R., Ghanwani, A., Yang, H., and
V. Manral, "Transparent Interconnection of Lots of Links
(TRILL): Adjacency", RFC 7177, May 2014.
Eastlake, et al. Standards Track [Page 25]
RFC 7172 TRILL: Fine-Grained Labeling May 2014
[RFC7180] Eastlake 3rd, D., Zhang, M., Ghanwani, A., Manral, V., and
A. Banerjee, "Transparent Interconnection of Lots of Links
(TRILL): Clarifications, Corrections, and Updates"
RFC 7180, May 2014.
Informative References
[RFC6165] Banerjee, A. and D. Ward, "Extensions to IS-IS for Layer-2
Systems", RFC 6165, April 2011.
[RFC6439] Perlman, R., Eastlake, D., Li, Y., Banerjee, A., and F.
Hu, "Routing Bridges (RBridges): Appointed Forwarders",
RFC 6439, November 2011.
[RFC7174] Salam, S., Senevirathne, T., Aldrin, S., and D. Eastlake
3rd, "Transparent Interconnection of Lots of Links (TRILL)
Operations, Administration, and Maintenance (OAM)
Framework", RFC 7174, May 2014.
[RFC7178] Eastlake 3rd, D., Manral, V., Li, Y., Aldrin, S., and D.
Ward, "Transparent Interconnection of Lots of Links
(TRILL): RBridge Channel Support", RFC 7178, May 2014.
Authors' Addresses
Donald Eastlake 3rd
Huawei Technologies
155 Beaver Street
Milford, MA 01757
USA
Phone: +1-508-333-2270
EMail: d3e3e3@gmail.com
Mingui Zhang
Huawei Technologies Co., Ltd
Huawei Building, No.156 Beiqing Rd.
Z-park, Shi-Chuang-Ke-Ji-Shi-Fan-Yuan, Hai-Dian District
Beijing 100095
P.R. China
EMail: zhangmingui@huawei.com
Eastlake, et al. Standards Track [Page 26]
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Puneet Agarwal
Broadcom Corporation
3151 Zanker Road
San Jose, CA 95134
USA
Phone: +1-949-926-5000
EMail: pagarwal@broadcom.com
Radia Perlman
Intel Labs
2200 Mission College Blvd.
Santa Clara, CA 95054
USA
Phone: +1-408-765-8080
EMail: Radia@alum.mit.edu
Dinesh G. Dutt
Cumulus Networks
1089 West Evelyn Avenue
Sunnyvale, CA 94086
USA
EMail: ddutt.ietf@hobbesdutt.com
Eastlake, et al. Standards Track [Page 27]