Why is a full mesh of iBGP peers required?

Question

Options

• ATo ensure proper route propagation inside a transit AS.
• BTo ensure that all routers are directly connected.
• CTo ensure that all routers run iBGP inside a transit AS.
• DTo maximize control plane scalability.

• A
  91% (32)
• C
  3% (1)
• D
  6% (2)

Explanation

A full mesh of iBGP peers is required because iBGP does not re-advertise routes learned from one iBGP peer to another iBGP peer (the iBGP split-horizon rule). Without full mesh, some routers inside a transit AS would never learn routes from external BGP peers, breaking end-to-end route propagation across the AS.

• B is wrong because full mesh refers to logical peering sessions (TCP connections), not physical links - routers can be many hops apart and still peer via loopback addresses.
• C is wrong because full mesh doesn't just mean running iBGP on all routers; it specifically means every iBGP router must have a direct peering session with every other iBGP router.
• D is wrong because full mesh is actually the opposite of scalable - it requires O(n²) sessions, which is why route reflectors and confederations exist as alternatives.

Memory tip: Think of iBGP as a "no gossip" club - a router won't pass along what it heard from another club member, so every member must hear the news directly. Full mesh ensures everyone gets the announcement firsthand.

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