A network without route
reflectors is illustrated in the figure; RTA, RTB, and 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 passes the update on to its two internal peers,
RTB and RTC. RTB and RTC, in turn, 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 an internal peer. Without the IBGP session between
RTA and RTC, RTC would never get the update; hence, the logical full
mesh is necessary.