Which protocol uses AS numbers for inter-domain routing?

• A. OSPF
• B. IS-IS
• C. RIP
• D. BGP

Answer: (D)

AS numbers identify separate administrative domains and are the mechanism used to route between those domains. The protocol that uses these numbers for inter-domain routing is BGP. BGP is a path-vector protocol that advertises networks with the list of ASes they traverse (the AS_PATH). This lets different autonomous systems exchange reachability while applying policies, preventing routing loops and enabling complex inter-domain routing decisions.

Within a single organization or ISP, interior gateway protocols like OSPF and IS-IS operate inside one AS and focus on topology inside that domain; they don’t rely on AS numbers for inter-domain routing. RIP is an older, simpler distance-vector protocol used for smaller networks and also functions inside a single AS. So, when routing decisions cross autonomy boundaries, BGP is the protocol designed to handle that using AS numbers.