BGP Large Communities

BGP 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. BGP Communities attributes are a variable length attribute consisting of a set of one or more four-octet values, each of which specify a community . 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 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. The six-octet length of the Extended Community value precludes the common operational practise 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 Autonomous System assigning the values.

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:

A four-octet namespace identifier. 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 BGP Large Communities without collision. This field SHOULD be an Autonomous System Number (ASN). The use of Reserved ASNs (0 , 65535 and 4294967295 ) 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.

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

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.

The error handling of BGP Large Communities is as follows: 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. A BGP Large Communities attribute SHALL NOT be considered malformed due solely to presence of duplicate community values. 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. 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.

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 BGP Large 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 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 .

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 BGP Large Communities: Cisco IOS XR ExaBGP GoBGP BIRD OpenBGPD pmacct Quagga 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 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

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.