iBGP requires that all routers be configured to establish a logical connection with all other iBGP routers. The logical connection is a TCP link between all iBGP-speaking routers. The routers in each TCP link become BGP peers. In large networks, the number of iBGP-meshed peers can become very large. Network administrators can use route reflectors to reduce the number of required mesh links between iBGP peers. Some routers are selected to become the route reflectors to serve several other routers that act as route-reflector clients. Route reflectors allow a router to advertise or reflect routes to clients. The route reflector and its clients form a cluster. All client routers in the cluster peer with the route reflectors within the cluster. The route reflectors also peer with all other route reflectors in the internetwork. A cluster can have more than one route reflector.

In Figure 4-12, without route reflectors, all iBGP routers are configured in an iBGP mesh, as required by the protocol. When Routers A and G become route reflectors, they peer with Routers C and D; Router B becomes a route reflector for Routers E and F. Routers A, B, and G peer among each other.

Figure 4-12 Route Reflectors

Routers A and G are configured to peer with each other and with Routers B, C, and D. The configuration of Routers C and D is different from the configuration of the rest of the routers; they are configured to peer with Routers A and G only. All route reflectors in the same cluster must have the same cluster ID number.

Note

The combination of the route reflector and its clients is called a cluster. In Figure 4-12, Routers A, G, C, and D form a cluster. Routers B, E, and F form another cluster.

Router B is the route reflector for the second cluster. Router B peers with Routers A and G and with Routers E and F in its cluster. Routers E and F are route-reflector clients and peer only with Router B. If Router B goes down, the cluster on the right goes down because no second route reflector is configured.