When a route becomes unreachable, what does the BGP Update carry?

• A. New Path
• B. Keepalive
• C. Withdrawn Routes
• D. Notification

Answer: (C)

In BGP, updates carry changes to what routes are reachable, split into two parts: Withdrawn Routes and NLRI. When a route becomes unreachable, that prefix is listed in the Withdrawn Routes portion of the Update. This tells peers to remove the route from their routing tables. The NLRI section may carry new or changed routes, but the action of signaling an unreachable route is specifically done by withdrawing it. Keepalive messages simply maintain the session when nothing changes, and a Notification is used to signal an error or fault in the session. So, the information that communicates that a previously advertised route is no longer reachable is Withdrawn Routes.