EIGRP Terminologies

In this post, I will explain some terminologies related to EIGRP, include:

• Feasible distance and reported/administrative distance (FD and AD).
• Successors and feasible successor routes.

1.Feasible Distance and Reported Distance - FD and RD


Feasible Distance (FD): is the metric of best route from local router to destination subnet. Best route is the route with the lowest metric.

Reported Distance (RD): is the metric advertised by the neighbor/next hop for that route. Local router knows this information because they exchange their topology tables together. 

In this example, let consider the route from R1 to 1.1.1.0/24.
There are two routes from R1 to reach 1.1.1.0/24:

1. R1-R2-R3-dest: metric is 10+15+7 = 32.
2. R1-R4-R5-dest: metric is 12+9+12 = 33.

In this scenario, R1 will choose 1st route with lower metric. The FD in here is 32 because it is the best metric.

Reported Distance is the value of next hop router to destination, in here, next hop router of R1 is R2, and RD of this route is the metric calculated from R2 to destination: 15+7 = 22.

2. Successor and Feasible successor routes

Successor route is the best route choosen to install into routing table, in above example is R1-R2-R3 route.
Feasible successor route is the backup route for successor route in case feasible successor route down.

By default, EIGRP only keeps one best route to install to routing table, but it still keeps the less-than-optimal routes in topology table.

To become a feasible route, a route must be qualified the stuff called feasibility condition. The condition is:

• The route must be loop-free. This means, the RD of that route from local router must less than the local best metric (FD).

So, in above diagram, what is the feasible successor route of the best route from R1 to 1.1.1.0/24?
As explained before, the best route from R1 to 1.1.1.0/24 is R1-R2-R3 with FD is 32. Another route to reach 1.1.1.0/24 is R1-R4-R5 with metric is 33.
If R1-R4-R5 route wants to become feasible successor route, its RD must be less than current FD of R1.
In this example, the RD of R1-R4-R5 is calculated from R4 to 1.1.1.0/24 so it is 21.

With this RD value (21 < 32), this route is feasible successor route.