What criteria does EIGRP use to determine which routes are eligible to become a Feasible Successor?

EIGRP (Enhanced Interior Gateway Routing Protocol) uses specific criteria to determine which routes can be considered Feasible Successors (FS). For a route to become eligible as a Feasible Successor, its Advertised Distance (AD) must be less than the Feasible Distance (FD) of the current Successor route. This relationship ensures that the Feasible Successor can provide a viable alternative path if the current primary route fails.

The formula is based on the concept that the AD of a route represents the distance advertised by another router, while the FD represents the best metric to reach the destination known by the local router. By ensuring the AD of the potential Feasible Successor is lower than the current Feasible Distance, EIGRP can quickly switch to this backup route without needing to re-calculate metrics, thus maintaining network reliability and efficiency.

This criterion emphasizes the idea that for EIGRP, redundancy is built into the routing decision-making process, allowing for seamless failover in case of route changes or failures. It makes EIGRP an efficient and reliable routing protocol used in many enterprise networks.