8 Replies Latest reply: Jan 10, 2016 6:19 PM by daniel RSS

    EIGRP Successors and Feasible Successors?

    Jay.Kalmath

      Hi All,

       

      I am going through the EIGRP, i am not getting clear idea about Feasible Distance(FD) and Reported Distance(RD).Successsors and Feasible Successorsand one more qstn How the convergence will take place in the presence and absence of Feasible successor .

        • 1. Re: EIGRP Successors and Feasible Successors?
          Mithun Adhikary

          Hi Jay,

           

          I am not good in language, so I don't know whether I have wrote in a dialuted way which can help you but still I have tried.

           

          Feasible Distance(FD) - The Feasible Distance, or FD in short, is the historical minimum of the best metric towards that particular destination (with the history starting from scratch when the route neds to enter the Active state and then returns back to Passive state). This means that the FD may be equal to or lower than the current total best metric towards the destination, including the link to the next hop (or successor in EIGRP parlance).

           

          Reported Distance(RD) - Reported Distance, or RD, as advertised by a particular neighbor, is its current total distance to a particular destination. It is simply the neighbor's own distance to the destination. Our total distance via a particular neighbor is that neighbor's RD plus the metric of the link between us and that neighbor.

           

          The successor route is the best route to reach a subnet, based on the advertised distance (AD) from the neighbor plus the distance to reach that neighbor. This is the route which is installed in the routing table.

           

          The feasible successor route is a route which has a higher metric than the successor route to reach a subnet but meets the feasibility condition and can be used in the event that the successor route goes down. This route does NOT get installed in the routing table but is kept in the topology table.

           

          Successor - A successor route (think successful!) is the best route to a remote network. A successor route is used by EIGRP to forward traffic to a destination and is stored in the routing table. It is backed up by a feasible successor route that is stored in the topology table-if one is available.

           

          Feasible successor - A destination entry is moved from the topology table to the routing table when there is a feasible successor. A feasible successor is a path whose reported distance is less than the feasible distance, and it is considered a backup route.

           

          EIGRP will keep up to six feasible successors in the topology table. Only the one with the best metric (the successor) is placed in the routing table. The show ip eigrp topology command will display all the EIGRP feasible successor routes known to a router.

           

          The best route is determine by your metric to each network or device.

           

          The feasibility condition states that the AD from a neighbor must be less than the metric of the successor route (the feasible distance [FD]) because routing through a feasible successor when the AD > FD may cause a routing loop.

           

          All other routes (non-successor and non-feasible successor) are called possibilities are kept in the topology table but are only visible when u supply the all-links argument to the show ip eigrp topology command.

           

          Regards,

          Mithun

          • 2. Re: EIGRP Successors and Feasible Successors?
            bogdan CCSI #35055

            If you have a feasible successor the router will use this route after the successor router will go down. It will take few hundred of ms until you you will have the feasible successor route in the routing table.

            If you don't have a successor the router will start the query process to all his neigbors listed in the neighbor table to find out if somenone knows about the lost prefix.

            • 3. Re: EIGRP Successors and Feasible Successors?
              Jay.Kalmath

              Hi Mithun,

               

              Its really wonderfull explanation...Thanku..

              • 4. Re: EIGRP Successors and Feasible Successors?
                bogdan CCSI #35055

                To be more clear and to prove the  the behaviour when the FS route exist in the topology table let's look at the bellow topology:

                 

                Untitled.png

                 

                Now for network 11.11.11.0 R1 in the default setup has 2 routes with equal metric and will load balance over these 2.

                But I modified the dalay on R1 fa0/0 interface to let R1 use the successor R3 and feasible successor R2 for 11.11.11.0

                 

                R1#sh run int fa0/0

                interface FastEthernet0/0

                ip address 10.10.10.1 255.255.255.252

                delay 101

                duplex auto

                speed auto

                end

                 

                R1#sh ip eigrp topology 11.11.11.0/24

                IP-EIGRP (AS 1): Topology entry for 11.11.11.0/24

                  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 307200

                  Routing Descriptor Blocks:

                  12.12.12.2 (FastEthernet0/1), from 12.12.12.2, Send flag is 0x0

                      Composite metric is (307200/281600), Route is Internal

                      Vector metric:

                        Minimum bandwidth is 10000 Kbit

                        Total delay is 2000 microseconds

                        Reliability is 255/255

                        Load is 1/255

                        Minimum MTU is 1500

                        Hop count is 1

                  10.10.10.2 (FastEthernet0/0), from 10.10.10.2, Send flag is 0x0

                      Composite metric is (307456/281600), Route is Internal

                      Vector metric:

                        Minimum bandwidth is 10000 Kbit

                        Total delay is 2010 microseconds

                        Reliability is 255/255

                        Load is 1/255

                        Minimum MTU is 1500

                        Hop count is 1

                 

                You can see that now the route through 10.10.10.2 (R2 is FS router).

                 

                R1#traceroute 11.11.11.1

                Type escape sequence to abort.
                Tracing the route to 11.11.11.1

                  1 12.12.12.2 208 msec 96 msec 140 msec
                  2 11.11.11.1 96 msec *  160 msec

                 

                Let's shut down the interface fa0/0 from R3 and see how lonh it will last until R1 will delete the route through R3 and will use the router through R2. Note that a ping is running from R1 to 11.11.11.1.

                 

                 

                Untitled.png

                 

                You can see that when there is a fs the convergence is very very short.

                • 5. Re: EIGRP Successors and Feasible Successors?
                  daniel

                  This video helped me understand this concept. Except the part about FD and RD being the same thing. I think he is wrong about that. But how he explains it is the best way to comprehend this.

                   

                  EIGRP, Feasible Distance - YouTube

                  • 6. Re: EIGRP Successors and Feasible Successors?
                    shashank

                    hi this is wonder full explanation but i have one query that is that in routing table we have only one route to a particular destination  than why  its showing two for some networks and one for some networks in routing table (i make it bold for better understanding of my question)

                     

                    R1#show ip eigrp neighbors
                    IP-EIGRP neighbors for process 1
                    H Address Interface Hold Uptime SRTT RTO Q Seq
                      (sec) (ms) Cnt Num
                    2 172.16.124.2 Se3/0 128 00:04:29 1179 5000 0 18
                    1 172.16.124.3 Se3/0 132 00:04:29 984 5000 0 17
                    0 172.16.124.4 Se3/0 136 00:04:29 982 5000 0 15

                    R1#show ip eigrp topology
                    IP-EIGRP Topology Table for AS(1)/ID(10.1.3.1)

                    Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
                      r - reply Status, s - sia Status

                    P 10.2.0.0/22, 1 successors, FD is 2297856
                      via 172.16.124.2 (2297856/128256), Serial3/0
                    P 10.3.1.0/24, 2 successors, FD is 3370496
                      via 172.16.124.2 (3370496/2858496), Serial3/0
                      via 172.16.124.4 (3370496/2858496), Serial3/0
                    P 10.1.3.0/24, 1 successors, FD is 128256
                      via Connected, Loopback3
                    P 10.3.0.0/22, 1 successors, FD is 2297856
                      via 172.16.124.3 (2297856/128256), Serial3/0
                    P 10.2.1.0/24, 1 successors, FD is 3321856
                      via 172.16.124.3 (3321856/2809856), Serial3/0
                      via 172.16.124.4 (3370496/2858496), Serial3/0
                    P 10.1.2.0/24, 1 successors, FD is 128256
                      via Connected, Loopback2
                    P 10.2.2.0/24, 1 successors, FD is 3321856
                      via 172.16.124.3 (3321856/2809856), Serial3/0
                      via 172.16.124.4 (3370496/2858496), Serial3/0
                    P 10.3.3.0/24, 2 successors, FD is 3370496
                      via 172.16.124.2 (3370496/2858496), Serial3/0
                      via 172.16.124.4 (3370496/2858496), Serial3/0
                    P 10.1.1.0/24, 1 successors, FD is 128256
                      via Connected, Loopback1
                    P 10.2.3.0/24, 1 successors, FD is 3321856
                      via 172.16.124.3 (3321856/2809856), Serial3/0
                      via 172.16.124.4 (3370496/2858496), Serial3/0
                    P 10.3.2.0/24, 2 successors, FD is 3370496
                      via 172.16.124.2 (3370496/2858496), Serial3/0
                      via 172.16.124.4 (3370496/2858496), Serial3/0
                    P 192.168.1.0/24, 1 successors, FD is 2730496
                      via 172.16.124.2 (2730496/2218496), Serial3/0
                      via 172.16.124.3 (3193856/2681856), Serial3/0
                    P 192.168.2.0/24, 1 successors, FD is 2681856
                      via 172.16.124.3 (2681856/2169856), Serial3/0
                    P 192.168.3.0/24, 1 successors, FD is 2730496
                      via 172.16.124.4 (2730496/2218496), Serial3/0
                      via 172.16.124.3 (3193856/2681856), Serial3/0
                    P 172.16.124.0/29, 1 successors, FD is 2169856
                      via Connected, Serial3/0

                    R1#show ip route
                    Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
                      D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
                      N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
                      E1 - OSPF external type 1, E2 - OSPF external type 2
                      i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
                      ia - IS-IS inter area, * - candidate default, U - per-user static route
                      o - ODR, P - periodic downloaded static route

                    Gateway of last resort is not set

                      172.16.0.0/29 is subnetted, 1 subnets
                    C 172.16.124.0 is directly connected, Serial3/0
                      10.0.0.0/8 is variably subnetted, 11 subnets, 2 masks
                    D 10.3.1.0/24 [90/3370496] via 172.16.124.4, 00:04:39, Serial3/0
                      [90/3370496] via 172.16.124.2, 00:04:39, Serial3/0
                    D 10.2.0.0/22 [90/2297856] via 172.16.124.2, 00:04:39, Serial3/0
                    C 10.1.3.0/24 is directly connected, Loopback3
                    D 10.3.0.0/22 [90/2297856] via 172.16.124.3, 00:04:40, Serial3/0
                    D 10.2.1.0/24 [90/3321856] via 172.16.124.3, 00:04:40, Serial3/0
                    C 10.1.2.0/24 is directly connected, Loopback2
                    D 10.3.3.0/24 [90/3370496] via 172.16.124.4, 00:04:40, Serial3/0
                      [90/3370496] via 172.16.124.2, 00:04:40, Serial3/0
                    D 10.2.2.0/24 [90/3321856] via 172.16.124.3, 00:04:40, Serial3/0
                    C 10.1.1.0/24 is directly connected, Loopback1
                    D 10.3.2.0/24 [90/3370496] via 172.16.124.4, 00:04:41, Serial3/0
                      [90/3370496] via 172.16.124.2, 00:04:41, Serial3/0
                    D 10.2.3.0/24 [90/3321856] via 172.16.124.3, 00:04:41, Serial3/0
                    D 192.168.1.0/24 [90/2730496] via 172.16.124.2, 00:04:41, Serial3/0
                    D 192.168.2.0/24 [90/2681856] via 172.16.124.3, 00:04:41, Serial3/0
                    D 192.168.3.0/24 [90/2730496] via 172.16.124.4, 00:04:41, Serial3/0

                    • 7. Re: EIGRP Successors and Feasible Successors?
                      shashank

                      sorry bro to disturb you....i just go through some more topic ...i hope this is because of load balancing ...please just confirm

                      • 8. Re: EIGRP Successors and Feasible Successors?
                        daniel

                        You should post a picture of your topology but basically from R1's perspective the route to 10.3.2.0/24, 10.3.3.0/24, and 10.3.1.0/24 are the same cost. EIGRP load balances automatically across same cost routes.