Which BGP attribute is primarily used to determine the sequence of autonomous systems for a route?

Which BGP attribute is primarily used to determine the sequence of autonomous systems for a route?

• A. LOCAL_PREF
• B. AS_PATH
• C. MED
• D. NEXT_HOP

Answer: (B)

AS_PATH is the attribute that records the sequence of autonomous systems a BGP route has traversed. As a route is advertised from one AS to another, the advertising AS appends its own AS number to the path, creating an ordered list of the domains the route crosses. The routing decision often favors the path with the fewest AS hops, making the shorter AS_PATH the preferred route. This path list also helps prevent loops, since a router will reject routes that would imply returning to an AS it has already seen. LOCAL_PREF is used inside a single AS to choose among exit points to other ASes, not the sequence across multiple ASes. MED influences inbound traffic from neighboring ASes to a preferred entry point, but it doesn’t describe the traversal order. NEXT_HOP indicates the next-hop IP address to reach the destination, focusing on reachability rather than the sequence of ASes.

AS_PATH is the attribute that records the sequence of autonomous systems a BGP route has traversed. As a route is advertised from one AS to another, the advertising AS appends its own AS number to the path, creating an ordered list of the domains the route crosses. The routing decision often favors the path with the fewest AS hops, making the shorter AS_PATH the preferred route. This path list also helps prevent loops, since a router will reject routes that would imply returning to an AS it has already seen.

LOCAL_PREF is used inside a single AS to choose among exit points to other ASes, not the sequence across multiple ASes. MED influences inbound traffic from neighboring ASes to a preferred entry point, but it doesn’t describe the traversal order. NEXT_HOP indicates the next-hop IP address to reach the destination, focusing on reachability rather than the sequence of ASes.