BGP Large Communities

Introduction

BGP (RFC 4271) 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.

A BGP Communities attribute (RFC 1997) is a variable-length attribute consisting of a set of one or more four-octet values, each of which specifies a community. A common use of the individual values of this attribute splits this 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, the BGP Communities attribute cannot accommodate the encoding, because a two-octet word cannot fit a four-octet ASN.

To address this shortcoming, a BGP Large Communities attribute is encoded as an unordered set of one or more 12-octet values, each consisting of a four-octet Global Administrator field and two 4-octet operator-defined fields. Each operator-defined file can be used to denote properties or actions significant to the operator of the AS assigning the values.

BGP Large Communities attribute details

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                      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


Field	Description
Global Administrator	A four-octet namespace identifier. Allows 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.
Local Data Part 1	A four-octet operator-defined value. Represents a function identifier.
Local Data Part 2	A four-octet operator-defined value. Represents a parameter value.

Important

The use of Reserved ASNs (0 [RFC 7607], 65535, and 4294967295 [RFC 7300] is not a best practice.

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.

In canonical notation, each Large Community value can be summarized as "ASN:Function:Parameter". The function of a community can be divided into two categories for the treatment of routes:

Informational Communities: Labels for attributes such as the origin of the route announcement, the nature of the relation with an External BGP (eBGP) neighbor, or the intended propagation audience.
Action Communities: Added as labels to request that a route be treated in a particular way in an AS

For more information on the use of BGP Large Communities, see RFC 8195.