What is EIGRP Feasible Successor?

Understanding EIGRP Feasible Successor

An EIGRP Feasible Successor is a precomputed backup route in Cisco's Enhanced Interior Gateway Routing Protocol (EIGRP) that satisfies the feasibility condition and can be installed immediately if the primary route, the successor, fails. Because it is already loop-free and stored in the topology table, EIGRP achieves very fast convergence without running a new route computation.

EIGRP uses the Diffusing Update Algorithm (DUAL) and two key metrics: the Feasible Distance (FD), the best total metric to a destination, and the Reported Distance (RD), the metric a neighbor advertises to reach that destination. A neighbor qualifies as a feasible successor when its Reported Distance is strictly less than the current Feasible Distance. This feasibility condition guarantees the backup path does not loop back through the local router. The successor (lowest FD) is installed in the routing table, while qualifying feasible successors wait in the topology table.

This mechanism matters for network resilience and availability, a core security property. When the primary link drops, EIGRP can switch to a feasible successor in well under a second because no query/recompute process is needed, minimizing traffic loss and outage windows. Without a feasible successor available, EIGRP must enter the active state and send queries to neighbors (a slower diffusing computation), increasing convergence time and the chance of disruption that attackers or failures could exploit.

For example, a router reaches a remote subnet through Router A with a Feasible Distance of 3000. Router B advertises a Reported Distance of 2500 to the same subnet. Since 2500 is less than the FD of 3000, Router B qualifies as a feasible successor and is held in the topology table. When the link to Router A fails, the router instantly promotes Router B's path to the routing table, restoring connectivity with negligible downtime.