Which protocol is primarily used for routing between different autonomous systems?

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

Answer: (B)

Routing between different autonomous systems relies on a protocol designed for inter-domain routing, where policy and scalability are essential. That protocol is BGP. It exchanges routing information between autonomous systems and uses TCP for reliable delivery, which helps maintain stable routes across the large and diverse Internet. BGP carries path attributes like AS_PATH, NEXT_HOP, and LOCAL_PREF that let networks implement routing policies, control which paths are preferred, and prevent routing loops. This combination of policy control, scalability, and Internet-wide applicability is why BGP is the standard for inter-AS routing.

Inside a single autonomous system, interior gateway protocols handle routing. OSPF and EIGRP (as well as RIP) are designed for internal use, focusing on efficient path calculation within one AS, not for exchanges between different organizations. OSPF is a link-state protocol, EIGRP is an advanced distance-vector protocol (Cisco-originated), and RIP uses simple hop counts—each suitable for inside-network routing but not the inter-domain routing role fulfilled by BGP.