The DUAL query process for EIGRP

[Reprovoid]

   HI.I was Just reading about the DUAL algorithm.It says that a queried neighbouring router considers a route to be loop-free If It Is passive for the listed prefix.How does only knowing the prefix Is passive clarify whether It's loop free or not? Don't the routes In the querying routers topology table show which routes are feasible succesors and which are not already ? What other Information Is gained using the convergence process?

[Richard Burts]

There are some parts of your question that I am not clear about. But the most important part of the question is clear and has an answer. If the listed prefix at the queried neighbor is passive then the neighbor has already been through the process of assuring that the route is loop free. If the queried router has already used DUAL and checked it once it is not necessary to idr DUAL again and it is safe to use on the queried router and will be returned to the quering router for consideration as a possibly valid path.

HTH

Rick

[Reprovoid]

Hi.Thanks for taking look.Sorry I wasn't clear. What process does the queried router go through to assure It's loop free ? Does It note the neighbor from which the query came from and checks that the route doesn't pass through the querying router? I thought that a passive state Just means the prefix has a successor route and Is working.

[Richard Burts · Accepted solution]

I suggest that we start by getting clear about some terminology and some concepts. The querying router is the router where some path to a destination has failed. So the querying router sends a query to find another path. And it does not send the query down the path that failed. The queried router is some neighbor of the querying router and it is not on the path to the destination that failed. (If the queried router were on the path then it would already know about the failure and its route would no longer be in passive state). For the listed prefix to be in the passive state the queried router has already used DUAL to verify that the route is loop free.

So for the prefix to be in the passive state is sufficient for the router to believe that the route is loop free and there is not any need to run DUAL again. The queried router can simply reply to the query that it does have a path to the destination.

HTH

Rick

[Reprovoid]

Thank you! It's clear now.I appreciate your help