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 two-octet words. 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 Autonomous System in the most significant word. Since the adoption of four-octet ASNs , 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 is also unsuitable, as the protocol limit of six octets cannot accommodate both a four-octet Global Administrator value and a four-octet Local Administrator value, which precludes the common operational practice of encoding a target ASN in the Local Administrator field. To address these shortcomings, this document defines a Large BGP Communities attribute encoded as one or more twelve-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 BGP Communities attribute as an optional transitive path attribute of variable length. All routes with the Large BGP Communities attribute belong to the community specified in the attribute. The attribute consists of one or more twelve-octet values. Each twelve-octet Large BGP Communities value represents three four-octet values, as follows:

A four-octet namespace identifier. This SHOULD be an Autonomous System Number. 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 BGP 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 BGP Communities are encoded in the BGP path attribute payload. A BGP speaker can transmit them in any order. Duplicate Large BGP Communities SHOULD NOT be transmitted. A receiving speaker SHOULD silently remove duplicate Large BGP Communities from a BGP UPDATE message.

If a range of routes is aggregated, then the resulting aggregate should have a Large BGP Communities attribute which contains all of the Large BGP Communities attributes from all of the aggregated routes.

Large BGP Communities MUST be represented as three separate unsigned integers in decimal notation, without leading zeros, in the following order: Global Administrator, Local Data 1, Local Data 2. Numbers MUST not be omitted, even when zero. For example: 64496:4294967295:2 or 64496:0:0 or (64496, 111, 222).

The following Global Administrator values are reserved: 0 (the first ASN) , 65535 (UINT16_MAX) and 4294967295 (the last ASN) . Operators SHOULD NOT use these Global Administrator values. Although this document does not define any Special-Use Large BGP Communities, the Global Administrator values specified above could be used if there is a future need for them.

The error handling of Large BGP Communities is as follows: A Large BGP Communities attribute SHALL be considered malformed if its length is not a non-zero multiple of 12. A BGP UPDATE message with a malformed Large BGP Communities attribute SHALL be handled using the approach of "treat-as-withdraw" as described in section 2. The Large BGP Communities Global Administrator field may contain any value, and a Large BGP 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 .

This extension to BGP has similar security implications as BGP 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 Communities 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 Communities 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 Communities: Cisco IOS XR ExaBGP GoBGP BIRD OpenBGPD pmacct The latest implementation news is tracked at http://largebgpcommunities.net/.

IANA has made an Early Allocation of the value 32 (LARGE_COMMUNITY) in the "BGP Path Attributes" registry under the "Border Gateway Protocol (BGP) Parameters" group and is now asked to make that Permanent.

The authors would like to thank Ruediger Volk, Russ White, Acee Lindem, Shyam Sethuram, Jared Mauch, Joel M. Halpern, Jeffrey Haas, John Heasley, 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, and David Freedman for their support, insightful review and comments.