The RTC is a mechanism allows a router to advertise route target membership information to its BGP peers to indicate interest in receiving only VPN routes tagged with specific route target extended communities. After receiving this information, peers restrict the advertised VPN routes to only those requested, which minimizes the control plane load in terms of protocol traffic and possibly routing information base (RIB) memory.
MP-BGP carries the route target membership information, using an address family identifier (AFI) value of 1 and subsequent address family identifier (SAFI) value of 132. For two routers to exchange RT membership network layer reachability information (NLRI), they must advertise the corresponding AFI/SAFI to each other during capability negotiation. MP-BGP allows RT membership NLRI to be propagated, loop-free, within an AS and between ASs using well-known BGP route selection and advertisement rules.
Outbound route filtering (ORF) can also be used for RT-based route filtering, but ORF messages have a limited scope of distribution (to direct peers or neighbors), and, therefore, do not automatically create pruned inter-cluster and inter-AS route distribution trees.