Which protocol uses path vector information to determine routing decisions between autonomous systems?

• A. Border Gateway Protocol
• B. Open Shortest Path First
• C. RIP
• D. EIGRP

Answer: (A)

Path vector routing across autonomous systems relies on AS_PATH information carried with each route advertisement. This AS_PATH records the sequence of autonomous systems a route has traversed, which helps prevent routing loops and enables operators to apply routing policies between different networks. Border Gateway Protocol is designed exactly for this inter-domain environment; it exchanges routes between ASs and uses the AS_PATH data to determine which paths to advertise and prefer. That makes it the protocol that uses path vector information to govern routing decisions between autonomous systems. In contrast, OSPF is a link-state interior gateway protocol used inside a single AS, RIP uses a simple hop-count distance-vector metric, and EIGRP is a Cisco proprietary hybrid protocol that doesn’t rely on path-vector AS_PATH information for inter-AS routing.