Community Strings

A community string is a string of numbers (and you thought it was characters) that can be used to tag a prefix. This tag can then be used for things like:

• Entry point control— Because the MED, in many cases, isn't used in path determination (because the AS path of two routes must be the same for the MED to be compared), there is a system where a router receiving a prefix with a given community string set will set its local preference.

• Propagating Quality of Service (QoS) information— An arrangement could be made between two BGP peers so that tagging a prefix with a given community string results in the packets destined to that subnet being treated differently.

Community strings are set and checked using route maps. (See the section, "Filtering with Route Maps," later in this appendix for more on this topic.)