As stated in RFC 1997, BGP communities provide a way, for a Service Provider to tag, or add information to a specific BGP announcement. The information attached to an announcement can be used to enforce, or maintain any number of dynamic policies.
A BGP community can define a property of a prefix, so a router can be configured to treat all prefixes with a certain community, or a set of communities, in a certain way. For example, to modify a prefix's local preference, or weight. To this end, a network engineer would not have to create a custom access list to match the prefixes and manually set the local preference. An advanced form of the technique can be used to tag inbound announcements, such that actions would take place based upon multiple communities, giving the customer a highly flexible traffic engineering tool.
One application of BGP communities has been described in RFC 1998. This RFC describes a scenario in which two ISPs provide each other with backup connectivity. Using communities, they can tag the prefixes for which they provide backup routing. The upstream provider can then apply a lower local preference to these routes, so in non-failure mode, traffic would not be routed via the backup path. Of course, this assumes some level of cooperation from the upstream provider, but the level of cooperation is less than would be required if the upstream provider would have to create explicit access lists for the prefixes or ASes involved. This is typically a very costly staffing expense.
Today, many transit providers have standard communities to influence how prefixes are accepted and treated within their networks. The most common communities used are for changing the local preference value (as used for the backup routing example described above), suppressing advertisements to certain peers, pre-pending the transit providers AS to the AS_PATH to a set of subset of peers or even selectively sending an announcement to a peer. Typically, the policies are listed by the provider on a website geared towards their customers, or as a comment in the aut-num object in a local RIR registry.
Please contact us, to find out how One Step Consulting can help your network take advantage of BGP Communities to better utilize your existing assets.