9.1 Route Reflectors
9.1.4 Route reflector operation
The IBGP peers of a route reflector fall under two categories: clients and nonclients. A route reflector and its clients form a cluster. All IBGP peers of the route reflector that are not part of the cluster are nonclients and must be fully meshed to all other IBGP routers. Never configure route reflector clients to peer with IBGP speakers outside their cluster as this can cause routing loops.

How is route reflection configured? The commands needed are surprisingly simple. In fact, only the route reflector servers themselves receive the configuration. Clients and nonclients do not even know that route reflection is occurring. To identify clients and clusters, use the neighbor command, which has the following syntax, on the route reflector server:

router(config-router)#neighbor IP-address route-reflector-client

In the figure, the three routers RTA, RTB, and RTC form a single cluster, with RTC acting as the route reflector server. Note that clients are considered part of the cluster only because the route reflector lists them as such. Thus, RTC is configured as follows:

RTC(config)#router bgp 100
RTC(config-router)#neighbor 1.1.1.1 remote-as 100
RTC(config-router)#neighbor 1.1.1.1 route-reflector-client
RTC(config-router)#neighbor 2.2.2.2 remote-as 100
RTC(config-router)#neighbor 2.2.2.2 route-reflector-client
RTC(config-router)#neighbor 4.4.4.4 remote-as 100
RTC(config-router)#neighbor 7.7.7.7 remote-as 100
RTC(config-router)#neighbor 8.8.8.8 remote-as 200

On the route reflector clients, you define only one IBGP peer, the route reflector server:

RTA(config)#router bgp 100
RTA(config-router)#neighbor 3.3.3.3 remote-as 100

Also in the figure, RTD is the route reflector for its clients RTE and RTF; RTG is a route reflector in a third cluster. The router reflectors RTC, RTD, and RTG maintain an IBGP full mesh with each other and with the clients in their respective clusters. The clients peer only with their servers.

If a route reflector, such as RTC, receives multiple routes for the same destination, it will pick the best path based on the BGP decision process described in Chapter 8, BGP. The router will then propagate that route inside the AS based on the following rules:

  • If the route is received from a nonclient peer, reflect to clients only.
  • If the route is received from a client peer, reflect to all nonclient peers and also to client peers, except the originator of the route.
  • If the route is received from an EBGP peer, reflect to all client and nonclient peers.
Interactive Lab Activity (Flash, 358 kB)
  In this lab you will configure IBGP routers to use ISP-B as a route reflector to work around IBGP's full-mesh requirements. All routers are already configured for IP and RIP. IBGP peer information has also been configured on all routers.