What protocol is used when communication is required between autonomous systems?

• A. BGP
• B. RIP
• C. OSPF
• D. EIGRP

Answer: (A)

Routing between autonomous systems requires a protocol that can carry reachability information across different administrative domains and support policy-based routing. Border Gateway Protocol is designed for this purpose: it exchanges route advertisements between ASes, uses AS_PATH to indicate the sequence of ASes a route traverses (which helps prevent loops and informs routing decisions), and runs over TCP for reliable transport. It also lets each network apply its own policies at the edge, making it scalable for inter-domain routing across many organizations.

The other protocols are interior gateway protocols used within a single autonomous system. RIP is a simple distance-vector protocol limited by hop count, OSPF is a link-state protocol typically organized into areas for rapid convergence inside an AS, and EIGRP (Cisco’s implementation) is an enhanced distance-vector protocol used inside an AS. They aren’t designed to manage routing between different administrative domains, which is why BGP is the appropriate choice for inter-AS communication.