Large BGP Communities

BGP implementations typically support a routing policy language to control the distribution of routing information. Network operators attach BGP communities to routes to identify intrinsic properties of 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 may apply to an individual route or to a group of routes. 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. BGP Communities Attributes are four-octet values split into two individual two-octet words. The most significant word is usually 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 Autonomous System in the most significant word. Since the adoption of four-octet ASNs , the BGP Communities Attribute can no longer accommodate this encoding, as the specification in contains only four octets. This does not allow operators to specify any locally significant values. To address these shortcomings, this document defines a Large Community BGP Attribute encoded as one or more 12-octet values, each consisting of a four-octet ASN and two four-octet operator-defined values, each of which can be used to denote properties or actions significant to that ASN.

This document creates the Large Communities BGP path attribute as an optional transitive attribute of variable length. All routes with the Large Communities attribute belong to the community specified in the attribute. The attribute consists of one or more 12-octet values. Each 12-octet Large Communities value represents three 4-octet values, as follows:

A four-octet namespace identifier. This SHOULD be an Autonomous System Number assigned by IANA. A four-octet operator-defined value. A four-octet operator-defined value. The Global Administrator field is intended to allow different Autonomous Systems to define Large Communities without collision. Implementations MUST allow the operator to specify any value for the Global Administrator field. There is no significance to the order in which Large Communities are encoded in a path attributes field and a receiving speaker MAY retransmit them in an order different from which it received them. Duplicate Large Communities SHOULD NOT be transmitted. A receiving speaker SHOULD silently remove duplicate Large Communities from a BGP UPDATE message. There are no routing semantics implied by the Global Administrator field.

If a range of routes is aggregated and the resulting aggregates attribute section does not carry the ATOMIC_AGGREGATE attribute, then the resulting aggregate should have a Large Communities path attribute which contains all of the large communities from all of the aggregated routes.

BGP Communities are usually represented in routing policy languages as two individual two-octet unsigned integers separated by a colon; for example, 64496:12345. BGP Large Communities implementations MUST represent Large Communities in a manner similar to their representation of BGP Communities . Large Communities MUST be represented as three separate four-octet unsigned integers in decimal format with no leading zeros. These integers MUST NOT be omitted, even when zero. For example, 64496:4294967295:2 or 64496:0:0. Vendors MAY provide other textual representations. For example, a vendor's routing policy language may use a separator other than a colon or may require keywords or characters prepending or postpending the Large Communities attribute. Such differences are permitted. However, each implementation MUST make a representation available that depicts the integers in decimal and in the following order: Global Administrator, Local Data Part 1, Local Data Part 2.

The Large BGP Community attribute values in the following ranges are reserved:

The error handling of Large Communities is as follows: A Large Communities BGP Path Attribute with a length of zero MUST be ignored upon receipt and removed when sending. A Large Communities attribute SHALL be considered malformed if its length is not a non-zero multiple of 12 bytes. A BGP UPDATE message with a malformed Large Communities attribute SHALL be handled using the approach of "treat-as-withdraw" as described in section 2. The BGP Large Communities Global Administrator field may contain any value, and a Large Communities attribute MUST NOT be considered malformed if the Global Administrator field contains an unallocated, unassigned or reserved ASN or is set to one of the reserved Large BGP Community values defined in . A receiving speaker MUST NOT consider duplicate Large Communities attributes in a BGP UPDATE message to be malformed.

This extension to BGP has similar security implications as BGP Communities and BGP Extended Communities . This document does not change any underlying security issues associated with any other BGP Communities mechanism. Specifically, an AS relying on the Large BGP Community attribute carried in BGP must have trust in every other AS in the path, as any intermediate Autonomous System in the path may have added, deleted or altered the Large BGP Community attribute. Specifying the mechanism to provide such trust is beyond the scope of this document. Network administrators should note the recommendations in Section 11 of BGP Operations and Security .

This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in . The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist. As of today these vendors have produced an implementation of Large BGP Community: Cisco IOS XR ExaBGP GoBGP BIRD The latest implementation news is tracked at http://largebgpcommunities.net/.

IANA has assigned value 30 (LARGE_COMMUNITY Attribute) in the "BGP Path Attributes" sub-registry under the "Border Gateway Protocol (BGP) Parameters" registry.

The authors would like to thank Ruediger Volk, Russ White, Acee Lindem, Shyam Sethuram, Jared Mauch, Joel M. Halpern, Nick Hilliard, Jeffrey Haas, John Heasley, Gunter van de Velde, Marco Marzetti, Eduardo Ascenco Reis, Mark Schouten, Paul Hoogsteder, Martijn Schmidt, Greg Hankins, Acee Lindem, 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 and Tom Scholl for their support, insightful review and comments.