Successor, Feasible Successor and Feasible Distance

Successor -A successor route is the best route to a destination network. EIGRP uses the successor route to forward traffic to a destination. This is the router which is stored in the routing table. A successor route is based on the advertised distance from the neighbor. A successor route is a least-cost route to the destination network. A feasible successor is the backup route for a successor route.

Feasible Distance – Feasible Distance is the lowest calculated metric to reach the destination network. It is the metric planned in the routing table entry as the second number inside the brackets. The Feasible Distance is also known as the metric for the route.

Now examine the topology that we have used in the previous article in figure 1 and routing table for R1 in Figure 2, you can see that EIGRP’s best path for the 192.168.2.0/24 network is through router R3, and the feasible distance for network 192.168.2.0/24 is 258816. This is the metric that was calculated in the previous lesson.10.10.10.2 is the IP address of Gig 0/1 interface of R3.

Feasible Successors (FS), Feasibility Condition, and Reported Distance

Feasible Successor – The FS route is an alternative route to a particular network. It can be used instantly if the successor route fails for any reason. We can use the feasible successor route without causing a routing loop. For using the feasible successor the feasibility condition is required to match. The feasible successor route is stored in the EIGRP topology table.

Feasible Successor is the loop-free backup path to the same network as the successor, and it satisfies the Feasibility Condition. R2’s successor for the 192.168.2.0/24 network is R3, providing the path with the lowest metric to the destination network. You can see that R2 is providing an alternative path, but is it and Feasible Successor? For a Feasible Successor, R2 must first meet the Feasibility Condition.

The feasibility condition included when neighbor’s reported distance to a network is less than the local routers feasible distance to the same destination network. The reported distance is the metric that a router reports to a neighbor about its own cost to that network. If the condition met, it provides a loop-free path. If the FS is stored in the topology table, the DUAL converged quickly after a change in the topology, because the backup path has no need for recomputing.