How does BGP establish an adjacency?

• A. By UDP handshake with neighbor.
• B. By ARP resolution of neighbor MAC address.
• C. You must configure BGP on each router; BGP forms a TCP relationship with each neighbor and uses keepalive messages to maintain state.
• D. By flooding updates to all routers in the same AS.

Answer: (C)

BGP adjacency is established by a TCP session between two BGP speakers. BGP runs over TCP (port 179) to get reliable, ordered delivery of its control messages. When two routers can reach each other at the IP layer, they open a TCP connection and exchange BGP OPEN messages, which include their BGP version, AS numbers, and other parameters. If these meet the expected criteria, the session is considered established and the peers exchange KEEPALIVE messages at the negotiated hold time to keep the session alive. Only after this adjacency is up do they exchange UPDATE messages to share routing information. The other options don’t fit because UDP is not used for BGP sessions, ARP resolution is a link-layer step not about BGP adjacency, and BGP does not flood updates to all routers in an AS—updates are exchanged over established peering sessions with specific neighbors.