BGP Large Communities Attribute
A 12-byte BGP community tag that fits four-byte AS numbers, so every network can label routes without collisions.
Heitz, J., Ed., Snijders, J., Ed., Patel, K., Bagdonas, I., and N. Hilliard, "BGP Large Communities Attribute", RFC 8092, DOI 10.17487/RFC8092, February 2017, <https://www.rfc-editor.org/info/rfc8092>.
@misc{rfc8092,
series = {Request for Comments},
number = 8092,
howpublished = {RFC 8092},
publisher = {RFC Editor},
doi = {10.17487/RFC8092},
url = {https://www.rfc-editor.org/info/rfc8092},
author = {J. Heitz and J. Snijders and K. Patel and I. Bagdonas and N. Hilliard},
title = {{BGP Large Communities Attribute}},
pagetotal = 8,
year = 2017,
month = feb,
} RFC text
Contents
Internet Engineering Task Force (IETF) J. Heitz, Ed.
Request for Comments: 8092 Cisco
Category: Standards Track J. Snijders, Ed.
ISSN: 2070-1721 NTT
K. Patel
Arrcus
I. Bagdonas
Equinix
N. Hilliard
INEX
February 2017
BGP Large Communities Attribute
Abstract
This document describes the BGP Large Communities attribute, an
extension to BGP-4. This attribute provides a mechanism to signal
opaque information within separate namespaces to aid in routing
management. The attribute is suitable for use with all Autonomous
System Numbers (ASNs) including four-octet ASNs.
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 7841.
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/rfc8092.
Copyright Notice
Copyright (c) 2017 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.
Table of Contents
1. Introduction ....................................................2
2. Requirements Language ...........................................3
3. BGP Large Communities Attribute .................................3
4. Aggregation .....................................................4
5. Canonical Representation ........................................4
6. Error Handling ..................................................5
7. Security Considerations .........................................5
8. IANA Considerations .............................................6
9. References ......................................................6
9.1. Normative References .......................................6
9.2. Informative References .....................................6
Acknowledgments ....................................................7
Contributors .......................................................7
Authors' Addresses .................................................8
1. Introduction
BGP [RFC4271] implementations typically support a routing policy
language to control the distribution of routing information. Network
operators attach BGP communities to routes to associate particular
properties with these routes. These properties may include
information such as the route origin location, or specification of a
routing policy action to be taken, or one that has been taken, and is
applied to all routes contained in a BGP Update Message where the
Communities Attribute is included. Because BGP communities are
optional transitive BGP attributes, BGP communities may be acted upon
or otherwise used by routing policies in other Autonomous Systems
(ASes) on the Internet.
A BGP Communities attribute is a variable-length attribute consisting
of a set of one or more four-octet values, each of which specify a
community [RFC1997]. Common use of the individual values of this
attribute type split this single 32-bit value into two 16-bit values.
The most significant word is interpreted as an Autonomous System
Number (ASN), and the least significant word is a locally defined
value whose meaning is assigned by the operator of the AS in the most
significant word.
Since the adoption of four-octet ASNs [RFC6793], the BGP Communities
attribute can no longer accommodate the above encoding, as a two-
octet word cannot fit a four-octet ASN. The BGP Extended Communities
attribute [RFC4360] is also unsuitable. The six-octet length of the
Extended Community value precludes the common operational practice of
encoding four-octet ASNs in both the Global Administrator and the
Local Administrator sub-fields.
To address these shortcomings, this document defines a BGP Large
Communities attribute encoded as an unordered set of one or more
twelve-octet values, each consisting of a four-octet Global
Administrator field and two four-octet operator-defined fields, each
of which can be used to denote properties or actions significant to
the operator of the AS assigning the values.
2. Requirements Language
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].
3. BGP Large Communities Attribute
This document defines the BGP Large Communities attribute as an
optional transitive path attribute of variable length. All routes
with the BGP Large Communities attribute belong to the communities
specified in the attribute.
Each BGP Large Community value is encoded as a 12-octet quantity, as
follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Global Administrator |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Local Data Part 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Local Data Part 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Global Administrator: A four-octet namespace identifier.
Local Data Part 1: A four-octet operator-defined value.
Local Data Part 2: A four-octet operator-defined value.
The Global Administrator field is intended to allow different ASes to
define BGP Large Communities without collision. This field SHOULD be
an ASN, in which case the Local Data Parts are to be interpreted as
defined by the owner of the ASN. The use of Reserved ASNs (0
[RFC7607], 65535 and 4294967295 [RFC7300]) is NOT RECOMMENDED.
There is no significance to the order in which twelve-octet Large
Community Attribute values are encoded in a Large Communities
attribute, A BGP speaker can transmit them in any order.
Duplicate BGP Large Community values MUST NOT be transmitted. A
receiving speaker MUST silently remove redundant BGP Large Community
values from a BGP Large Community attribute.
4. Aggregation
If a range of routes is aggregated, then the resulting aggregate
should have a BGP Large Communities attribute that contains all of
the BGP Large Communities attributes from all of the aggregated
routes.
5. Canonical Representation
The canonical representation of BGP Large Communities is three
separate unsigned integers in decimal notation in the following
order: Global Administrator, Local Data 1, Local Data 2. Numbers
MUST NOT contain leading zeros; a zero value MUST be represented with
a single zero. Each number is separated from the next by a single
colon. For example: 64496:4294967295:2, 64496:0:0.
BGP Large Communities SHOULD be represented in the canonical
representation.
6. Error Handling
The error handling of BGP Large Communities is as follows:
o A BGP Large Communities attribute SHALL be considered malformed if
the length of the BGP Large Communities Attribute value, expressed
in octets, is not a non-zero multiple of 12.
o A BGP Large Communities attribute SHALL NOT be considered
malformed due to presence of duplicate Large Community values.
o A BGP UPDATE message with a malformed BGP Large Communities
attribute SHALL be handled using the approach of "treat-as-
withdraw" as described in Section 2 of [RFC7606].
The BGP Large Communities Global Administrator field may contain any
value, and a BGP Large Communities attribute MUST NOT be considered
malformed if the Global Administrator field contains an unallocated,
unassigned, or reserved ASN.
7. Security Considerations
This document does not change any underlying security issues
associated with any other BGP Communities mechanism. Specifically,
an AS relying on the BGP Large Communities attribute carried in BGP
must have trust in every other AS in the path, as any intermediate AS
in the path may have added, deleted, or altered the BGP Large
Communities attribute. Specifying the mechanism to provide such
trust is beyond the scope of this document.
BGP Large Communities do not protect the integrity of each community
value. Operators should be aware that it is possible for a BGP
speaker to alter BGP Large Community Attribute values in a BGP Update
Message. Protecting the integrity of the transitive handling of BGP
Large Community attributes in a manner consistent with the intent of
expressed BGP routing policies falls within the broader scope of
securing BGP, and is not specifically addressed here.
Network administrators should note the recommendations in Section 11
of "BGP Operations and Security" [RFC7454].
8. IANA Considerations
IANA has assigned the value 32 (LARGE_COMMUNITY) in the "BGP Path
Attributes" subregistry under the "Border Gateway Protocol (BGP)
Parameters" registry.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<http://www.rfc-editor.org/info/rfc4271>.
[RFC7606] Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K.
Patel, "Revised Error Handling for BGP UPDATE Messages",
RFC 7606, DOI 10.17487/RFC7606, August 2015,
<http://www.rfc-editor.org/info/rfc7606>.
9.2. Informative References
[RFC1997] Chandra, R., Traina, P., and T. Li, "BGP Communities
Attribute", RFC 1997, DOI 10.17487/RFC1997, August 1996,
<http://www.rfc-editor.org/info/rfc1997>.
[RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
Communities Attribute", RFC 4360, DOI 10.17487/RFC4360,
February 2006, <http://www.rfc-editor.org/info/rfc4360>.
[RFC6793] Vohra, Q. and E. Chen, "BGP Support for Four-Octet
Autonomous System (AS) Number Space", RFC 6793,
DOI 10.17487/RFC6793, December 2012,
<http://www.rfc-editor.org/info/rfc6793>.
[RFC7300] Haas, J. and J. Mitchell, "Reservation of Last Autonomous
System (AS) Numbers", BCP 6, RFC 7300,
DOI 10.17487/RFC7300, July 2014,
<http://www.rfc-editor.org/info/rfc7300>.
[RFC7454] Durand, J., Pepelnjak, I., and G. Doering, "BGP Operations
and Security", BCP 194, RFC 7454, DOI 10.17487/RFC7454,
February 2015, <http://www.rfc-editor.org/info/rfc7454>.
[RFC7607] Kumari, W., Bush, R., Schiller, H., and K. Patel,
"Codification of AS 0 Processing", RFC 7607,
DOI 10.17487/RFC7607, August 2015,
<http://www.rfc-editor.org/info/rfc7607>.
Acknowledgments
The authors would like to thank Ruediger Volk, Russ White, Acee
Lindem, Shyam Sethuram, Jared Mauch, Joel M. Halpern, Jeffrey Haas,
Gunter van de Velde, Marco Marzetti, Eduardo Ascenco Reis, Mark
Schouten, Paul Hoogsteder, Martijn Schmidt, Greg Hankins, Bertrand
Duvivier, Barry O'Donovan, Grzegorz Janoszka, Linda Dunbar, Marco
Davids, Gaurab Raj Upadhaya, Jeff Tantsura, Teun Vink, Adam
Davenport, Theodore Baschak, Pier Carlo Chiodi, Nabeel Cocker, Ian
Dickinson, Jan Baggen, Duncan Lockwood, David Farmer, Randy Bush, Wim
Henderickx, Stefan Plug, Kay Rechthien, Rob Shakir, Warren Kumari,
Gert Doering, Thomas King, Mikael Abrahamsson, Wesley Steehouwer,
Sander Steffann, Brad Dreisbach, Martin Millnert, Christopher Morrow,
Jay Borkenhagen, Arnold Nipper, Joe Provo, Niels Bakker, Bill Fenner,
Tom Daly, Ben Maddison, Alexander Azimov, Brian Dickson, Peter van
Dijk, Julian Seifert, Tom Petch, Tom Scholl, Arjen Zonneveld, Remco
van Mook, Adam Chappell, Jussi Peltola, Kristian Larsson, Markus
Hauschild, Richard Steenbergen, David Freedman, Richard Hartmann,
Geoff Huston, Mach Chen, and Alvaro Retana for their support,
insightful review, and comments.
Contributors
The following people contributed significantly to the content of the
document:
John Heasley
NTT Communications
Email: heas@shrubbery.net
Adam Simpson
Nokia
Email: adam.1.simpson@nokia.com
Authors' Addresses
Jakob Heitz (editor)
Cisco
170 West Tasman Drive
San Jose, CA 95054
United States of America
Email: jheitz@cisco.com
Job Snijders (editor)
NTT Communications
Theodorus Majofskistraat 100
Amsterdam 1065 SZ
The Netherlands
Email: job@ntt.net
Keyur Patel
Arrcus, Inc.
Email: keyur@arrcus.com
Ignas Bagdonas
Equinix
80 Cheapside
London EC2V 6EE
United Kingdom
Email: ibagdona.ietf@gmail.com
Nick Hilliard
INEX
4027 Kingswood Road
Dublin 24
Ireland
Email: nick@inex.ie
AI ELI5
On this page
§1Why Communities Needed to Grow
Network operators attach BGP communities to routes to associate properties with them, such as where the route originated or a routing policy action to take. Because communities are optional transitive attributes, other Autonomous Systems (ASes) on the Internet may act on them in their own routing policies.
The original RFC 1997 attribute is a set of four-octet values. Common use splits each value into two 16-bit words: the top word is an ASN and the bottom word is a value the AS owner defines. Once four-octet ASNs arrived (RFC 6793), a two-octet word could no longer hold the ASN. The Extended Communities attribute (RFC 4360) has a six-octet value, which also cannot hold a four-octet ASN in both its Global Administrator and Local Administrator sub-fields.
The old label had two small boxes, and a big network number would not fit in the first one. So a new, roomier label was needed.
§3The Large Community Value
Large Communities is an optional transitive path attribute of variable length. All routes carrying it belong to the communities it lists. Each value is a 12-octet quantity made of three 32-bit fields: Global Administrator (a namespace identifier), Local Data Part 1 and Local Data Part 2 (both operator-defined).
The Global Administrator field exists so different ASes can define communities without collision. It SHOULD be an ASN, and then the owner of that ASN decides what the two local parts mean. Using reserved ASNs (0, 65535 and 4294967295) is NOT RECOMMENDED.
The label has three number slots. The first says whose label it is, and the other two are free for that network to use however it likes.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
GLOBAL-ADMIN | Global Administrator | 4 bytes | A four-octet namespace identifier, normally an ASN. | |
LOCAL-1 | Local Data Part 1 | 4 bytes | A four-octet operator-defined value. | |
LOCAL-2 | Local Data Part 2 | 4 bytes | A four-octet operator-defined value. |
§4Ordering, Duplicates and Aggregation
The order of the 12-octet values inside the attribute has no significance, and a speaker can transmit them in any order. Duplicate values MUST NOT be transmitted, and a receiving speaker MUST silently remove redundant duplicates.
When a range of routes is aggregated, the aggregate should carry a Large Communities attribute containing all the Large Communities from all the aggregated routes.
It does not matter what order the labels are in, and a repeated label is just thrown away. If routes are bundled together, the bundle keeps everyone's labels.
§5Canonical Text Representation
The canonical way to write a Large Community is three unsigned decimal integers in the order Global Administrator, Local Data 1, Local Data 2, separated by single colons. Numbers MUST NOT have leading zeros, and zero is written as a single 0. Examples from the RFC are 64496:4294967295:2 and 64496:0:0.
Large Communities SHOULD be represented in this canonical form.
Everyone writes the three numbers the same way, with colons between them, so people can read each other's labels.
§6Error Handling
An attribute is malformed if its length in octets is not a non-zero multiple of 12. Duplicate values do not make it malformed. A BGP UPDATE message with a malformed Large Communities attribute is handled with treat-as-withdraw, as described in Section 2 of RFC 7606.
The Global Administrator field may hold any value. An attribute MUST NOT be considered malformed because that field has an unallocated, unassigned or reserved ASN.
If a label is cut off and does not have complete groups of three numbers, the route in that message is treated as taken back. Odd-looking owner numbers are fine though.
§7Security Considerations
The RFC does not change security issues of other BGP community mechanisms. An AS that relies on Large Communities must trust every other AS in the path, because any intermediate AS may have added, deleted or altered the attribute. Large Communities do not protect the integrity of each value, and providing such protection is part of the broader task of securing BGP, which is out of scope. Administrators should note Section 11 of RFC 7454.
Any network along the way can change the labels, so you have to trust the networks in between.
§8IANA Assignment
IANA assigned path attribute type value 32, named LARGE_COMMUNITY, in the BGP Path Attributes subregistry of the Border Gateway Protocol (BGP) Parameters registry.
The official list of label types now has an entry, number 32, for this new kind of label.