BGP community attribute: attribute that can be used to group destinations with a common property.
The community is an optional transitive attribute. In RFC 1997, the goal of communicates is stated as follows:
‘To facilitate and simplify the control of routing information this document suggests a grouping of destinations so that the routing decision can also be based on the identity of a group.’
With the community attribute, the autonomous system administrator can group destination prefixes. After having done this, other policies can refer to these groups.
The community attribute is a 32 bit value that is usually represented in two part format where each part is 2 bytes long. The first part is the AS number and the second part is the community number. For example, community 750 in AS 65000 would give the community 65000:750.
Aside from making up your own communities, there are three well-known communities:
NO_EXPORT: routes carrying this community should not be advertised beyond a BGP confederation boundary. The RFC also states that a standalone AS that is not part of a confederation should be considered a confederation. So in short, it should not be advertised to EBGP peers.
NO_ADVERTISE: routes carrying this community must not be advertised to any peers (IBGP or EBGP).
NO_EXPORT_SUBCONFED: routes carrying this community should not be advertised across EBGP sessions. This includes EBGP sessions inside confederations.