5 Replies Latest reply: Oct 23, 2018 7:16 AM by Kamil RSS

    Question about a Feasible Successor logic.



      Hello guys,


           I would like to understand how come is it possible that according to the above diagram Router-B might be considered as a Feasible Successor (FS)

      for route if his RD (reported distance) as it shows is possibly higher than the actual FD (feasible distance) to aforementioned route ?


           This image comes from a specific test-exam (INE) for CCNP Route and in the explanation it was told that there are two feasible successors for this route

      which had meant Router-A and Router-B per this diagram. It can also be inferred that a Router-C is a Successor here (based on presented Administrative Distances

      and next on the metrics).


           Finally I seem to understand why Router-A could is considered as a FS as his RD is lower than the RD of Router-C so there is no way that it can be lower than

      Router D's Feasible Distance to the network Though why Router-B is said to be a FS is beyond me (to me he could be but it is not certain under

      given circumstances).





        • 1. Re: Question about a Feasible Successor logic.



          I implemented a similar topology:


          You must have in count the following:


          * The network advertised by Router1 ( in your picture, ROUTER A) by EIGRP to another routers is EXTERNAL (remember the "redistribution" from RIP), for that reason, the AD=170

          * The network advertised by EIGRP by Router2 (in your picture, ROUTER B) and Router3 (in your picture, ROUTER C) by EIGRP to another routers is INTERNAL, for that reason, the AD=90

          The routing table in Router4 (In your case, ROUTER D):


          Router4#show ip route eigrp

          D [90/30720] via, 00:14:48, FastEthernet0/0

                                  [90/30720] via, 00:14:44, FastEthernet0/0

         is subnetted, 1 subnets

          D EX [170/2560002816] via, 00:42:53, FastEthernet0/0


          In my case, the route to the subnet has two equal-cost paths (same metric) because the configuration of bandwidths and delays are by default in both routers.


          Now, in your case, you have different RD:


          Router B

          EIGRP RD = 800


          Router C

          EIGRP RD = 500


          In this case, Router C is a sucessor. Router B can be feasible sucessor



          Router A

          EIGRP RD = 200

          Maybe the EIGRP RD of Router A is lower than EIGRP RD of Router C, but remember, the route advertised by Router A is EXTERNAL (AD=170), for that reason, the Router A can be feasible sucessor and not sucessor.

          If I apply the following:


          Router4#show ip route eigrp

          D EX [170/2560002816] via, 00:00:18, FastEthernet0/0

             is subnetted, 1 subnets

          D EX [170/2560002816] via, 00:53:30, FastEthernet0/0


          You can see the router as external Eigrp route.


          Best regards!

          • 2. Re: Question about a Feasible Successor logic.

            Thank you a lot for a reply buddy. I actually do understand the concept of internal and external EIGRP routes and that they intentionally have different AD's to allow EIGRP to easily prefer internal routes over their external friends


            The only issue I was really having is for how router's 2 advertisement of (in your case) if it was reporting a higher metric components in its update * could not be considered as a routing loop in case if the distance from Rtr-4 to Rtr-2 and to Rtr-3 would be anything that is less than 300.


            I believe that this would have given us a total Feasible Distance for a Successor Route of 501 - 799 and now considering the Feasibility condition the route going over router-2 would have a higher RD than FD of a successor route over router-3 which would only keep the route passing through router-2 as a reference inside of  EIGRP's topology database (viewable using all-links keyword but useless in general afaik).


            Finally I would like to thank you for taking your time and creating this topology and actually putting into action what I have asked for friend.

            I am hoping that my logic is right and I do agree with all that you have stated in your reply buddy as this is the way I understand Successors and Feasible Successors as well.


            * Let's assume you have used an offset list for this particular route that will allow this route to have RD of 800 when advertised from router-2 and RD of 500 for router's 3 route advertisement (as it appears on my diagram).

            • 3. Re: Question about a Feasible Successor logic.



              The comparison is not between RDs, the feasibililty condition is the comparison between lower FD and RD in each router.

              It must meet: RD < FD


              The lower metric to is known as sucessor route: Its metric is the lower Feasible Distance. As in your picture, there is not information about it (Bandwidth and Delay values in the interfaces), then, its value is unknown explicity.


              As you know about external and internal routes EIGRP, then only Router B or Router C can have the lower feasible distance. Now, Router C has the lower feasible distance (by the RD lower value), but I can't conclude that Router B will meet the feasibility condition or not (because I must compare the FD value with the RD value of Router B)


              Best regards!

              • 4. Re: Question about a Feasible Successor logic.



                It is possible, because router D will derive its FD by adding the best RD (in this case RD of Router C of 500) to the metric of its link to the neighbour advertising the best path. In your case, the uplink to Routers B and C is the same, so it will have the same metric for Router D. Let's assume that this link's metric is 250. Then FD from router D will be equal to 750 and Router B's RD of 800 will be higher than the FD. In case the link cost is 400, then Router D's FD will be 900, in which case Router B's RD of 800 will be lower than the FD and it will  be considered a feasible successor.

                • 5. Re: Question about a Feasible Successor logic.

                  Thank you again I did know that RD needs to be lower than the FD of a successor route to consider the Feasibility Condition to be met.

                  The issue I was having was just to confirm my sanity as I thought there was something wrong with my way of reasoning. Furthermore at least now I know that the way I understood the concepts is correct




                  p.s. I am sorry if my explanations were unclear, I do not ask too many questions on forums but I guess I better get used to it

                         Hopefully I can eventually help someone to solve their problems too at some point.