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.