Route target filtering is a feature which enhances the optimisation of route reflection in an MPLS VPN network. When enabled a PE and a route reflector negotiate which route targets are of interest to PE so the route reflector only sends routes for VPNs that the PE is associated with. This process is a very efficient way of controlling routing table distribution and in certain scenarios can allow specific core routers to operate as route reflectors for specific VPN ranges.
A specific NLRI (SAFI-132 route-target) is used as the control plane within MP-BGP to signal to the route reflector which route targets the PE is interested in. The PE simply gleans this information from its existing BGP import statements configured on its local VRFs and announces it’s configured route targets using the SAFI-132 address family NLRI; when received the route reflector responds with the routes for those specific VPNs.
The PE is configured with the family route-target statement. The prefix-limit defines how many SAFI-132 route targets are allowed to be received which in this case is 100 route-target prefixes. The teardown statement defines the percent at which syslog messages are generated for the percentage of route targets received against the maximum allowed. The idle-timeout is the period of time to which the BGP neighbour is withdrawn when the maximum amount of route-targets are received.
The route reflector is simply configured with the route-target family.