What does BGP use as its transport protocol?

• A. IP
• B. SCTP
• C. UDP
• D. TCP

Answer: (D)

BGP relies on a reliable, connection-oriented transport to ensure that routing updates are delivered intact and in the correct order between neighboring routers. This is provided by TCP, which establishes a session between two BGP speakers on port 179 and guarantees reliable delivery, sequencing, and flow control. Because BGP messages (updates, withdrawals, keepalives) must be processed in a consistent sequence to maintain accurate routing state, a transport like UDP or IP alone— which offers no reliability or ordering—would risk missing or reordering critical information. While SCTP also offers reliability, the standard BGP specification uses TCP. The use of TCP also enables optional session authentication features (like TCP MD5) to protect the BGP session.