Successor and feasible successor explained

Two terms that you will often encounter in the EIGRP world are the successor and feasible successor. Here are the definitions of these two terms:

  • successor – the route with the best metric to reach a particular network. This route will be placed in the routing table.
  • feasible successor – alternative routes to a particular network that can be used immediately if the currently best route (the successor) fails, without causing a routing loop. These routes are stored in the EIGRP topology table.

Not all alternative routes to a particlar network will become feasible successors. In order for a route to become a feasible successor, the following condition must be met:

the neighbor’s advertised distance (AD) for the route must be less than the successor’s feasible distance (FD).

The definition above can be more easily understood with an example:

successor feasible successor example

We have a network of five routers, all running EIGRP. R5 has advertised the 10.0.0.0/24 subnet. R1 has three paths to reach the 10.0.0.0/24 subnet:

  • R2 > R5 – let’s say that this is the best route (the successor route). This route will be placed in R1’s routing table, with the metric of 30.
  • R3 > R2 > R5 – for a route to become a feasible successor, the neighbor’s advertised distance (AD) for the route must be less than the successor’s feasible distance (FD). This is not the case here – R3 has advertised the metric of 50 to reach 10.0.0.0/24, which is greater that the feasible distance of R1 (30).
  • R4 > R5 – this route will become a feasible successor route, since R4’s advertised distance is less than the successor’s feasible distance (25 < 30). This route will be placed in R1’s topology table and can be used immediately if the best route fails.

Subscribe to our Newsletter

Like us on Facebook

top
Geek University 2019