All BGP updates include a BGP attribute called the communities path attribute. The communities path attribute allows ASs to “tag” prefix announcements. This tag can then be used by routing policies to modify the normal behavior for that prefix announcement. For example, an AS could choose to filter out all prefix announcements containing a community value that identifies the prefix as a customer-generated prefix, instead only announcing the summary prefix for all customer prefixes. It is important to note that the community path attribute is carried in BGP update messages, which allows ASs not directly connected to each other to share information about a prefix.

The format for community identifiers is defined in RFC 1997: “BGP Communities Attribute.” The community identifier is a 32-bit value, where the first two bytes of the value are the AS number and the second two bytes are an arbitrary value defined by the AS. This format can be represented as AA:NN, where AA is the AS number of the AS adding the community identifier to the community path attribute, and NN represents a user-defined policy value.

There are two types of BGP communities:

• well-known communities
• user-defined or private communities

The Vyatta router recognizes the following BGP well-known communities as per RFC 1997:

NO_EXPORT: Allows a BGP speaker to specify that redistribution should extend only to the neighbouring AS. All routes received carrying a communities attribute containing this value are not advertised outside a BGP confederation boundary (a stand-alone autonomous system that is not part of a confederation should be considered a confederation itself).

NO_ADVERTISE: All routes received carrying a communities attribute containing this value are not advertised to other BGP peers.

LOCAL_AS: All routes received carrying a communities attribute containing this value are not advertised to other BGP peers.

In additon, the Vyatta router recognizes the INTERNET community. The INTERNET community is not, strictly speaking, a well-known community. (It is not, for example, mentioned in RFC 1997.) You can use the INTERNET community when you would use a “permit all” statement; for example, at the end of a community list. (Like access lists, community lists have an implicit “deny all” statement at their end.) The “permit INTERNET” statement is equivalent to the “permit any” statement, matching any community in the IP community list.

BGP community list is a user defined BGP communities attribute list. BGP community list can be used for matching or manipulating BGP communities attribute in updates.

The Vyatta router also supports the following user-defined community lists:

• Standard
• Extended

Furthermore, standard and extended community lists support standard and expanded lists. A standard community list ranges from 1 through 99 and an expanded community list ranges from 100 through 199.

Standard Community List

• Standard list specifies standard community attributes.
• Expanded list specifies regular expression.

Extended Community List

• Standard list specifies route-target and site-of-origin.
• Expanded list specifies regular expression.