9.1 Route Reflectors
9.1.2 Internal peers without route reflectors
A typical network without route reflectors is illustrated in the figure; router A (RTA), router B (RTB), and router C (RTC) form an IBGP full mesh. Each router acts as a BGP peer with the other two routers.

When RTA receives an update from an external peer, it will pass the update on to its two internal peers, RTB and RTC. RTB and RTC, in turn, will pass on the update to their external peers.

RTB will not pass on the update to RTC, because RTC is an internal peer and the update received by RTB also comes from and internal peer. Without the internal BGP session between RTA and RTC, RTC would never get the update; hence, the full mesh is necessary.