EIGRP Feasible Successor Routes

EIGRP uses the term successor and feasible successor to denote the best path and backup path for a particular route. The successor is the route with the Computed Distance (CD). The CD is calculated by taking the advertising routers Reported Distance (RD), which is a neighbors CD to reach the particular subnet, and then adding the cost for the local router to reach the advertising router.

There is also a term known as Feasible Distance (FD). The FD is the lowest metric observed for a given route since the last time it went from active to passive. The FD is used when converging the topology to avoid temporary routing loops by comparing the FD with the RD of a given path. If the RD is higher than the FD it means that there is a possibility that the route could point back to the local router and would therefore cause a temporary routing loop to occur if used. Note that it is only a possibility that it would be a loop and EIGRP is designed with this in mind: it will rather black-hole traffic, than cause a temporary routing loop.

So, let's look at an example. Below is a diagram of an EIGRP network with 3 routers R1 through R3.

EIGRP topology with IP address notations

And below here we have the same topology with the EIGRP metric noted for R3 to reach the network 172.16.1.0/24

EIGRP topology with metric notations

Note: the composite metric is calculated by taking the lowest bandwidth (in kilobits) on the path and the cumulative delay (in 10s of microseconds) on the path in a formula that looks like this: 256*(BW+DLY).

From the perspective of router R3, the network 172.16.1.0/24 is reachable via successor route 10.0.0.5 with Computed Distance of 3072 and through the feasible successor route 172.16.2.1 with a Computed Distance of 3328.

Below is the topology output as seen on R3.

R3#show ip eigrp topology all-links
EIGRP-IPv4 Topology Table for AS(1)/ID(172.16.2.2)
<output omitted>
P 172.16.1.0/24, 1 successors, FD is 3072, serno 37
        via 10.0.0.5 (3072/2816), GigabitEthernet1.13
        via 172.16.2.1 (3328/3072), GigabitEthernet1.23

The output states "1 successors" meaning only one route has the lowest metric and will be entered into the routing table. The Computed Distance and the Reported Distance can be seen in the output in the parentheses (CD/RD).

This feasible route will be a backup route for the network 172.16.1.0/24 in case the current best route, directly through R1, should fail - effectively enabling EIGRP to failover to the backup route without having to mark the route as active and sending out queries to its adjacent neighbors.

This feature also allows EIGRP to do unequal cost load distribution. It can do this because it knows of a loop-free path to the destination with a lower cost than the best path. So, by setting the variance under the routing process, you can influence how much the cost of the best path and the lesser path may vary for them to be used in unequal load distribution. Again, I don't think unequal cost load distribution is a desired feature in most networks - otherwise it would have been a feature of some other routing protocol by now.