EIGRP For The CCNA: (Feasible) Successors And Tables

In Part 1 of this EIGRP tutorial series, we covered the basics of adjacency success.  Now it’s time to tackle two route types unique to EIGRP.

Successors and feasible successors are a vital part of EIGRP, and understanding when they come into play and where you can find information on them is an important part of your CCENT and CCNA exam success.  EIGRP keeps information stored in three separate tables:

  • The route table contains the best route to each remote network the router knows about.
  • The topology table keeps all known valid and loop-free routes to those same networks.
  • The neighbor table contains exactly what you think it would contain — information regarding all EIGRP neighbors.

The route and topology tables play a vital role in EIGRP’s rapid recovery from a lost route.   In the following illustration, R1 has two paths to R4.  EIGRP has determined the path through to R4 through R2 to be the superior route.  That route, the successor, will be placed into the route and topology tables.

EIGRP has determined the path to R4 through R3 to be free of routing loops, but it’s considered to not be as good a route as the one through R2.   It should be used in the successor becomes unavailable, though.  That makes this route a feasible successor and it is placed into the topology table only.

EIGRP Successor and Feasible Successors

(More on how EIGRP made these determinations is coming up.)

One of the great things about EIGRP is that any valid backup routes are calculated in advance of actually being needed.   EIGRP has calculated two valid paths from R1 to R4.  One is the primary route (the successor) and the other is a backup route (the feasible successor).  Should the primary route be lost, EIGRP doesn’t hesitate; the feasible successor is immediately named the new successor and is used to route traffic.

EIGRP Feasible Successor In Action

I have a feeling we’ll see the successor and feasible successors in action during the massive amount of lab work we’re about to perform.   We’ll get started in the very next installment of this tutorial by configuring EIGRP on a hub-and-spoke network.

Take some free CCNA practice exams while you’re here!