ASpath

ASPath is a Border Gateway Protocol (BGP) path attribute that records the autonomous systems (AS) through which a route has traversed from its origin to the current advertising edge. It is an optional transitive attribute carried in BGP UPDATE messages. The value is a sequence of AS numbers. When a route is originated in an AS, the origin AS is listed at the end of the path; as the route is advertised to neighboring ASes, each forwarding AS prepends its own AS number to the left of the path. For example, a route originated in AS 65001 and learned by AS 65002 and then by AS 65003 might appear as 65003 65002 65001 on its way to a downstream peer.

ASPath serves several purposes in routing. It provides a mechanism for loop detection: if the local AS number appears anywhere in the path, the route is rejected to prevent routing loops. It is also a key factor in BGP route selection, where shorter AS_PATH length is typically preferred, subject to other attributes and policies such as LOCAL_PREF, MED, and route filtering. Operators use AS_PATH manipulation, or prepending, to influence upstream path selection for traffic engineering purposes.

The AS_PATH attribute is not a comprehensive security measure. While origin validation (for example via RPKI) can verify the origin AS for a prefix, AS_PATH content itself is not always authenticated and can be manipulated or misrepresented by misconfigurations or intentional policies. Understanding ASPath helps network operators analyze routing behavior and implement policies across multiple autonomous systems.