3.7.d [iv] Communities, extended communities
excellent discussion here: http://www.cisco.com/web/about/ac123/ac147/archived_issues/ipj_6-2/bgp_communities.html
The BGP Community Attribute
Within an AS, all BGP-speaking routers run Internal BGP (iBGP) in a full mesh to prevent routing loops (route reflectors can be used to relax this rule). This means that every BGP-speaking router passes its prefixes to each of its iBGP neighbors. ASs that are adjacent typically run eBGP on directly connected routers. All BGP routers share their prefixes—that is, the network number, network mask, and BGP attributes with each other—allowing each to run its own best-path selection algorithm. As a prefix is passed between ASs, an attribute called the AS-PATH is updated with the corresponding ASN. The AS-PATH is used to prevent routing loops between eBGP neighbors.
A community is a BGP attribute that may be added to each prefix. Communities are transitive optional attributes , meaning BGP implementations do not have to recognize the attribute and at the network operator’s discretion carry it through an AS or pass it on to another AS. The community attribute can be thought of as simply a flat, 32-bit value that can be applied to any set of prefixes. It can be read as a 32-bit value or split into two portions, the first 2 bytes representing an ASN and the last 2 bytes as a value with a predetermined meaning.