EIGRP-Successors and Distance

The terminology describing the distance or metric as seen by a router and its neighbors in EIGRP can be confusing.  Lets start first with the Feasible Distance (FD) and Reported Distance (RD).

Feasible Distance: This is the calculated metric at the local router to a destination subnet
Reported Distance: This is the metric advertised, or reported, by an EIGRP neighbor to reach the advertised route.
It is easier to understand this concept with a diagram:

(These are hypothetical values)

R1's feasible distance to reach its directly attached fa0/0 subnet is 28,610.  When it advertises this route to R2 through either a topology exchange or a bounded update, it will advertise the metric as 28,610.  At R2, this is the reported distance.  R2 then takes into account the bandwidth and delay of its serial link to R1 in order to reach the 10.0.1.0/24 network.  This calculated metric is R2's feasible distance, or in this example 1,258,478.


The Feasibility Condition: Successor Routes and Feasible Successor Routes
Next, we have the terms Successor Route and Feasible Successor route.  A successor route is a route with the lowest metric to a destination subnet.  A Feasible Successor route is essentially a backup route, but it has to meet a certain condition in order to be eligible in EIGRP.  This "eligibility" is determined by what is called the "Feasibility Condition".  In order for a route to be considered a feasible route, it must have a RD that is lower than the successor route's FD.

Again, this is something that is easier to understand by seeing it in a network diagram.  I have again used hypothetical values RD/FD values to make it easier to understand the concept.

As you can see in the diagram, R2 is advertising a RD of 2,020,000.  R3 is advertising a RD of 220,000.  The FD from R4 to R1 is 280,000.  In order to meet the Feasibility Condition, a route's RD must be less then the successor route's FD.  R2's route clearly exceeds this with a RD of 2,020,000.  R3's RD on the other hand, is 220,000, which is less than the FD of the successor route on R4-280,000.  The route through R3 to R1's subnet is then considered a feasible successor route.

Terminology aside, in plain English: R3's route can be considered a valid backup route, since the metric R3 is reporting is lower then R4's best route metric.

In this instance, the successor route (R4-R1) would be installed in the routing table, and the feasible successor would be visible under a show ip eigrp topology


Remember though-there can in fact be more then one successor route.  Depending on the load balancing options configured on the router (the default is 4 equal metric routes), if four routes are found to have the best metric, and have equal metrics, all four of those routes will be successor routes and will be installed in the routing table.

There can also be multiple feasible successor routes, provided they meet the feasibility condition.  Feasible successor routes are not installed in the routing table, but can be seen in the EIGRP topology table.

 I'll explain the configuration details of equal and unequal load balancing in a later post.