What is the purpose of an EIGRP Feasible Successor (FS)?

The purpose of an EIGRP Feasible Successor (FS) is to maintain a backup route if the primary route fails. In Enhanced Interior Gateway Routing Protocol (EIGRP), after establishing a primary route to a destination, the routing protocol identifies a backup route that meets certain criteria known as the feasibility condition. This condition ensures that the backup route is loop-free when compared to the primary route.

The Feasible Successor serves as an alternate path, allowing for rapid failover in case the primary route becomes unavailable. Since EIGRP uses a protocol called Diffusing Update Algorithm (DUAL), it can quickly swap to the Feasible Successor without needing to re-establish the routing tables or engage in a full routing update, thus minimizing downtime and optimizing network performance.

The other choices relate to different functions of a routing protocol: direct neighbor communication pertains to neighbor discovery and hello packets; optimizing memory usage is more about the overall routing table management; and synchronizing metric values is more associated with the overall process of maintaining consistent state among routers, rather than the specific function of identifying a backup route.