When a device running BGP receives a BGP route and finds that its AS number is in the AS_Path, which of the following methods will it take?

Question

Options

• AFirst notify the neighbor through a Notification message, and then discard the route.
• BForward directly to the peer
• CAfter deleting its own AS number, forward it to all neighbors.
• DDiscard directly

• A
  7% (4)
• B
  14% (8)
• C
  3% (2)
• D
  76% (45)

Explanation

BGP uses the AS_PATH attribute as a built-in loop prevention mechanism. When a router detects its own AS number already present in the AS_PATH of an incoming route update, it silently discards the route immediately, because receiving such a route means the advertisement has already passed through this AS and forwarding it further would create a routing loop.

Option A is wrong because BGP Notification messages are reserved for session-level errors (such as malformed messages or state machine violations) and trigger session teardown - they are not used to signal route filtering decisions to a neighbor. Option B is wrong because forwarding a loop-detected route would propagate the loop rather than prevent it, which defeats the entire purpose of the AS_PATH check. Option C is wrong because a BGP router never strips its own AS number out of the AS_PATH before forwarding; doing so would corrupt path information and re-introduce the loop vulnerability the mechanism is designed to eliminate.

Memory tip: AS_PATH is a "travel stamp" on the route. If you see your own stamp already on the passport, you know the route has been here before - just throw it away quietly, no questions asked.

No community discussion yet for this question.