BGP does not advertise a route learned from one iBGP router to another. A route is advertised via iBGP only if it is learned from the iBGP router that first advertised it. For example, assume that Router A advertises a route, and Router B learns about that route. Router B cannot advertise that route to Router C; Router C must learn the route from Router A. In other words, an iBGP router cannot advertise a route it learned from another iBGP router to a third iBGP router. Because of this restriction, if you have multiple routers connected to different AS networks, all of the routers must be fully "meshed."
One solution to this problem is to use route reflectors . Route reflectors ease the advertisement restriction by allowing a BGP router to reflect BGP routes it learns about to a third BGP router. Let's assume we don't have a link between Router 2 and Router 3 in Figure 10-3. The following configuration shows how to set up a route reflector on Router 1 that propagates iBGP routes between Router 2 and Router 3:
router bgp 500 neighbor 10.10.2.1 remote-as 500 neighbor 10.10.2.1 route-reflector-client neighbor 10.10.3.1 remote-as 500 neighbor 10.10.3.1 route-reflector-client
With this configuration, Router 1 can advertise Router 2's iBGP routes to Router 3 and Router 3's routes to Router 2.