What is full-mesh iBGP?

• A. By default, each BGP speaker is assumed to have a neighbor statement for all other iBGP speakers in the AS
• B. iBGP automatically builds mesh through route reflectors
• C. Full mesh is not possible in iBGP
• D. Only EBGP routers participate in full mesh

Answer: (A)

Full-mesh iBGP means every router in the same AS that runs iBGP forms a direct peering session with every other iBGP router in that AS. This setup is required because iBGP does not re-advertise routes learned from one iBGP neighbor to other iBGP neighbors. Without a direct iBGP session to each peer, a router wouldn’t learn routes learned by its iBGP peers, leading to incomplete visibility of the network’s routes. By connecting all iBGP speakers in a full mesh, each router receives the full set of routes from its iBGP peers and can propagate them consistently across the AS.

There’s a scalability trade-off, since the number of iBGP sessions grows quadratically with the number of routers. To address this, operators can use route reflectors: a reflector and its clients don’t need to peer with every other iBGP router, yet routes can still be distributed throughout the cluster. However, route reflectors are an optional design choice, not the default behavior.

The other statements are not correct because iBGP inside an AS does participate in full-mesh configurations, and full mesh is indeed possible (though not always practical at large scale). EBGP peers are external to the AS, so the requirement for full-mesh applies to iBGP within the same AS, not only to EBGP.