What is the AS-Path attribute?

• A. The next hop IP address
• B. The origin code
• C. The MED value
• D. A list of the full path of BGP autonomous system numbers necessary to reach a destination network

Answer: (D)

AS_PATH is the sequence of autonomous systems a BGP route has traversed from its origin to your router. In UPDATE messages, this path is recorded as a list of AS numbers, with each AS that the route passes through appending its ID to the end. This makes it easy to detect routing loops (if your own AS appears in the path) and to influence routing decisions (shorter AS_PATHs are typically preferred unless policies say otherwise). Administrators can also lengthen a path on purpose by prepending AS numbers to steer inbound traffic.

The other attributes serve different purposes: the next hop IP address tells you where to forward packets next, the origin code indicates where the route originated, and the MED value is used for external path selection to influence inbound traffic, but none of these describe the AS_PATH itself.