EIGRP succesor route

Unanswered Question
Jan 8th, 2008


I have a question about EIGRP:

In EIGRP you hace a succesor route (best route to a destination) and a feassible succesor route (as a bakcup route for a destination). So, when the succesor fails, the feassible succesor is used without the use of recomputation for a new route, then a new feassible should be choosed. If this is right, what event triggers the recomputation for choosing a new feassible succesor route?

I mean, at the end you'll have a recomputation process?


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Jon Marshall Tue, 01/08/2008 - 06:24


Not really. EIGRP will query for a new route if the current route is invalidated and there is no feasible successor in the topology table. So in your example the successor fails, the Feasible successor is installed and this new route is used.

If the feasible successor goes and there are no more feasible successor in the topology table the router will mark the route as active and query it's neighbor.


Kevin Dorrell Tue, 01/08/2008 - 07:29


As you rightly say, if the successor fails and there is a feasible successor, then the feasible successor can be installed and there is no need for anything to go active.

But at the same time, the advertised distance has changed, because you are using a different successor. The advertised distance (i.e. the metric at the successor) has probably gone up, but may even have gone down. Therefore the criteria for validating new feasible successors has changed.

So I guess there must be some recomputation going on, even if there is no active querying. What can you say about that recomputation?

Kevin Dorrell


ccnp_seeker Tue, 01/08/2008 - 09:12

Let me see if I get it...

I have a succesor and a feassible succesor, if the succesor fails then feassible succesor will be used...when the succesor (wich have failed before) comes up....it takes the role of active route? or the feasible remains working? until a recomputation occurrs...

royalblues Tue, 01/08/2008 - 13:33


There is no recomputation when the successor route is replaced by the FS route.

This is because the FD never changes (as the route never went active) and hence no new paths are installed as FS by Dual.

If the active route comes up, then it becomes successor again

Infact i had asked a question to the forum whether it would be beneficial to update the FD when a FS replaces the route




Jon Marshall Tue, 01/08/2008 - 23:13

Hi Kevin

I believe Narayan has explained it a little better than i did. If the route doesn't go active then there is no recomputation as far as i understand it, the router merely uses the FS and installs that route into it's routing table.

I'm going to have a good read of EIGRP over the next few days just make sure i have understood it correctly !!



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