Which path attribute in BGP is used to track the sequence of ASes and helps prevent loops?

• A. AS-path
• B. Origin
• C. Next Hop
• D. Community

Answer: (A)

The AS_PATH attribute is used to track the sequence of ASes a BGP route has traversed and to prevent loops. As a route is advertised from one autonomous system to the next, each router prepends its own AS number to the left of the path. This creates a chain like [AS1 AS2 AS3 …], showing the exact path the route has taken. If a router sees its own AS already present in the path, it recognizes that forwarding the route further would create a loop, so it stops advertising that route. This mechanism directly provides loop prevention.

Beyond loop prevention, AS_PATH also helps with path selection, since many implementations prefer shorter AS_PATHs. Other attributes, such as Origin (where the route originated), Next Hop (the next hop toward the destination), and Community (policy tagging), do not perform loop tracking in the same way, which is why they aren’t the correct choice here.