Which statement about transport layer usage is true?

• A. BGP is the only routing protocol to use TCP as its transport layer.
• B. BGP uses UDP as its transport layer.
• C. BGP uses IP directly as its transport layer.
• D. BGP uses ICMP as its transport layer.

Answer: (A)

BGP runs on top of TCP, using a TCP session between two peers. This transport layer choice provides the reliable, ordered delivery that BGP needs to exchange its OPEN, UPDATE, KEEPALIVE, and NOTIFICATION messages. The connection-oriented nature of TCP ensures that routing information is delivered correctly and retransmitted if lost, which is essential for maintaining consistent routing tables across routers.

UDP would not fit because it offers no guarantees of delivery or order, which could lead to missing or out-of-sequence routing updates. IP operates at the network layer and does not provide the necessary transport semantics for BGP messages. ICMP is a diagnostic/error protocol, not a transport path for BGP communications.