Replace a full mesh of internal sessions

Fewer control sessions do not imply a new data-forwarding path.

• BGP, the Border Gateway Protocol, exchanges information about routes to network prefixes. An autonomous system (AS) is a network under a common routing administration. IBGP is BGP between routers inside the same AS; EBGP is between ASes. A BGP session is a control connection, not a physical cable or the path taken by user packets (§§1,4).
• Ordinary IBGP does not redistribute an IBGP-learned route to other internal peers. A full mesh therefore connects every pair of internal speakers. With n speakers, it has n(n−1)/2 distinct sessions: three speakers need three; a chosen six-speaker mesh needs fifteen (§1; RFC 4271 §9.2).
• RFC 4456 permits a route reflector (RR) to advertise selected IBGP-learned routes to designated peers. In the three-router example, C reflects between clients A and B, allowing the A–B session to be omitted. The picture removes that session, not an underlying network link (§4).
• A chosen single-RR cluster with six routers, all five other routers configured as clients, uses five RR-to-client sessions instead of fifteen mesh sessions. This is an illustrative topology, not the session count of every deployment. Redundancy, non-client peering and additional clusters change the count (§§5–7).
• Reflection distributes route information. It does not force user packets to pass through the RR, change the AS number, create bandwidth, or guarantee the same path selection as a full mesh. NEXT_HOP and topology concerns are examined later (§§3,10–11).