EIGRP Feasible Successor

EIGRP's DUAL (Diffusing Update Algorythm) uses a cost metric of links to choose routes. Without going into how those costs are calculated, we can talk about successors and feasible successors.

Advertised distance is the cost that is being passed or advertised by the neighbor router. In my diagram, I am looking at how R1 can get to the 192.168.0.0 network. There are three paths, and each advertise a cost. The top neighbor is advertising 3000, the middle 2500, and the bottom 4100. These paths are added to the EIGRP topology table.

Feasible distance (FD) is once R1 has taken those advertised distances and added the local link cost. FD is used to pick the best route to the destination. In our diagram the lowest FD is the top route at 4000 (AD + Local Link Cost or 3000 + 1000). This route would be pulled from the topology table and inserted in our routing table.

EIGRP can also have a Feasible Successor (FS). This is a loop free route used as a "backup" immediatly if the successor (main route) goes down. By doing this immediatly, the network does not need to reconverge to choose a new best path. To qualify as a FS, the next hop router must have an ADVERTISED DISTANCE of less than the current successor's Feasible Distance.

In our diagram, the top route has been chosen as successor, so now we look at the other two options for feasible successor. Even though the bottom route would have the lowest FD total, it does not qualify for a feasible successor because the advertised distance of 4100 is higher than the successor's FD of 4000. Looking now at the middle route, the advertised distance, 2500, is lower than the successor's feasible distance of 4000. The middle route qualifies and will be chosen as the Feasible Successor.