RFC 1997 (rfc1997) - Page 2 of 5

BGP Communities Attribute

RFC 1997               BGP Communities Attribute             August 1996


Terms and Definitions

   Community
      A community is a group of destinations which share some common
      property.

      Each autonomous system administrator may define which communities
      a destination belongs to.  By default, all destinations belong to
      the general Internet community.

Examples

   A property such as "NSFNET sponsored/AUP" could be added to all AUP
   compliant destinations advertised into the NSFNET.  NSFNET operators
   could define a policy that would advertise all routes, tagged or not,
   to directly connected AUP compliant customers and only tagged routes
   to commercial or external sites. This would insure that at least one
   side of a given connection is AUP compliant as a way of enforcing NSF
   transit policy guidelines.

   In this example, we have just eliminated the primary motivation for a
   complex policy routing database that is used to generate huge prefix
   and AS path based filter rules.  We have also eliminated the delays
   caused by the out-of-band maintenance of this database (mailing in
   NACRs, weekly configuration runs, etc.)

   A second example comes from experience with aggregation.  It is often
   useful to advertise both an aggregate prefix and the component more-
   specific prefixes that were used to form the aggregate to optimize
   "next hop" routing.  These component prefixes are only useful to the
   neighboring BGP peer or perhaps the autonomous system of the
   neighboring BGP peer, so it is desirable to filter this information.
   By specifying a community value that the neighboring peer or peers
   will match and filter on, these more specific routes may be
   advertised with the assurance that they will not propagate beyond
   their desired scope.

COMMUNITIES attribute

   This document creates the COMMUNITIES path attribute is an optional
   transitive attribute of variable length.  The attribute consists of a
   set of four octet values, each of which specify a community.  All
   routes with this attribute belong to the communities listed in the
   attribute.

   The COMMUNITIES attribute has Type Code 8.





Chandra, et. al.            Standards Track