In EIGRP, what is a feasible successor?

• A. A route with the highest metric in the routing table.
• B. A route that can replace the current route if the current route fails and is guaranteed not to loop.
• C. A route learned from a static neighbor only.
• D. A route that has the same metric as the feasible distance.

Answer: (B)

A feasible successor in EIGRP is a backup route that can take over if the current best path fails and is guaranteed not to loop. This safety comes from the feasibility condition: the route learned through a neighbor must have an advertised distance that is strictly less than this router’s current feasible distance to the destination. In practice, if your best path to a destination has a certain metric (the feasible distance), a neighboring route can be considered a feasible successor only if the neighbor reports a smaller metric for that destination. That way, even if the primary path drops, you can immediately switch to the backup without risking a routing loop.

For example, if the main path to a destination has a metric of 10, and a neighbor advertises a route to the destination with a metric of 9, that neighbor qualifies as a feasible successor and can be used if the primary path fails. If the neighbor’s advertised distance were not less than the feasible distance, it would not be a feasible successor.

This idea is why the correct description emphasizes a backup route that can replace the current one and is loop-free. Routes that are learned from static neighbors, routes with the highest metric, or routes that merely have the same metric as the feasible distance don’t meet the feasibility condition and aren’t considered feasible successors.