Which attribute in BGP helps prevent routing loops by indicating the traversed Autonomous Systems?

The AS Path attribute in BGP (Border Gateway Protocol) is crucial for preventing routing loops by maintaining a record of the Autonomous Systems (AS) that a route has traversed. When a router receives a BGP update containing a route, it checks the AS Path to determine if it has already seen that route in any path it has traversed. If the router's own AS number appears in the AS Path, it recognizes that the route has looped back to it and will reject that route to avoid creating a routing loop.

The AS Path not only serves to prevent loops but also can influence route selection, as BGP prefers shorter paths with fewer AS hops. This attribute thus plays a vital role in maintaining the stability and efficiency of BGP routing.

Other attributes in BGP, while important for various functions, do not specifically address the prevention of routing loops as effectively as the AS Path does. For instance, the Next Hop indicates the next reachable router for a route but does not provide information about the AS traversed. Local Preference and Weight influence route selection within an AS but are not concerned with inter-AS paths, so they don't directly help in loop prevention.