Route reflectors

The inability of BGP to advertise a prefix learned from one IBGP peer to another can lead to scalability issues within an AS. The formula n(n-1)/2 provides the number of sessions required where n represents the number of routers. A full mesh topology of 10 routers requires 45 sessions. IBGP scalability becomes an issue for large networks.

RFC 1966 specifies that IBGP peering can be configured so that it reflects routes to another IBGP peer. The router reflecting route is known as a route reflector (RR) and the router receiving reflected routes is a route reflector client. Three basic rules apply to route reflectors and route reflection

  1. If an RR receives an NLRI from a non-RR client, the RR advertises the NLRI to an RR client. It does not advertise the NLRI to a nonroute reflector client.

  2. If an RR receives an NLRI from an RR client, it advertises the NLRI to RR and non-RR clients. Even the RR client that sent the advertisement receives a copy of the route, but it discards the NLRI.

  3. If an RR receives a route from an EBGP peer, it advertises the route to RR and non-RR clients.