6 Replies Latest reply: Sep 22, 2012 1:29 AM by cadetalain RSS

    Feasible Successor




      I'm currently studying EIGRP fror my CCNA exam, but I'm getting a little confused at how the FS gets calculated.  I understand it does not get install into the routing table but the topology table.  I read that the feasibility condition states that the AD from a neighbor router must be less than the metric of the successor route.  If that his the case why isn't in the successor as it has a lower metric so it is better?



        • 1. Re: Feasible Successor

          You have two distance: FD (feasible) and AD (advertised).

          The neighbour router send to another router only AD which is the metric to the network from neighbour router, without metric of the link between this routers.


          FD = AD + metric of the link between.

          • 2. Re: Feasible Successor

            Andrew wrote:

            If that his the case why isn't in the successor as it has a lower metric so it is better?

            couldn't get your question??

            • 3. Re: Feasible Successor
              • Advertised Distance = Distance from a routers neighbor to a network destination.
              • Feasible Distance = Advertised Distance + The cost between your router and a neighboring router to get to a destiantion network.





              So RouterB wil Advertise a Distance of 20 to get to network RouterA will add the cost between itself and Router B and have a Feasible Distance of 30.


              If RouterA has another path to get to network, let's say we have RouterD and RouterE, which are also connected to the network and router A, the Router will first check if the pathcost is the same.


              If so, both routes/paths will be used to send data to the network and put into the routing table, if not, it will see if the alternative route can be used as an possible alternative route if the first path through RouterB and RouterC fails.


              If the Advertised Distance of the alternate path is lower than the Feasible Distancee of the Succesors it is considered and alternative path.

              • 4. Re: Feasible Successor
                Shahryar Ali - CCDA / CCNA R&S

                Wel le me try to explain:


                For the route to become feasible successor it has to meet the following condition.


                "The distance advertised by the neighboring router(non successor) to reach that route (Reported distance) should be less than the FD(feasible distance) calculated by the successor"

                • 5. Re: Feasible Successor

                  The Feasible Distance includes the cost on the link between the router in question and the neighbor that is Advertising the route.


                  So router A is receiving an Advertised route from Router B.
                  Router B says I can get to that network in X (Bandwidth + Delay).


                  So router A, at this point now has an Adverstised Distance of X that it learned from B.  It takes X and adds to X what ever the metric (bandwitdth + Delay) is from A to B.  The sum is the Feasible Distance.   Both the FD/AD are placed into the Topology Table.


                  The Feasible Successor is any route (up to 16) that had a "AD" that is less than the current FD of the "successor" (the route currently in the Routing Table).

                  • 6. Re: Feasible Successor



                    The Feasible Successor is any route (up to 16)


                    I think you are confusing with the number of paths with same metric that can be installed in the routing table but the  FD is not installed in the RIB until the successsor fails.Is there a limit to the number of feasible successor? I must admit that i've never thought about it.


                    Now concerning the calculation, it's not really additive because if it has a lower bandwidth it will choose this one for the FD and add only its delay. Brian posted a very good doc about this here on CLN.