11 Replies Latest reply: Jun 13, 2011 2:51 AM by Diwakar Sharma -CCNA/CCNP/JNCIA-ER RSS

    EIGRP ACTIVE AND PASSIVE

    bins

      Respected Experts,

       

      Here i am litile confused about Active and Passive states in Eigrp,here i attached  one diagram.

       

      Router A have one successor or Best route to Router C  through Router B,and back up path through Router D.IF the path via router B to router C goes down

      Does the route via Router B come to Active State? if yes or no why? Let me hear your explanation over both Active and Passive State.

       

      Thanks&Regard

      Bins

        • 1. Re: EIGRP ACTIVE AND PASSIVE
          Martin

          I think if there is one successor and feasible successor, router will no go into Active state; it puts that "backup" route in place right away;

          if there is no feasible successor, router will start looking for alternative; so it goes into Active;

          • 2. Re: EIGRP ACTIVE AND PASSIVE
            bins

            Thanks for your reply but i need to know one thing if there is no feasible successor in the network, which router will be in active state destination router or source router.

             

            Thanks&Regard

            Bins

            • 3. Re: EIGRP ACTIVE AND PASSIVE
              Bogdan

              if the route through Router D is a feasible successor then the route to reach router C stays passive

              if the route through Router D is not a feasible successor then the route goes active and searches a path to reach router C

              do a show ip eigrp topology and you will see if the route is in passive or active mode and how many succesors it has

              if you have one successor to reach the router C network then the route trough D is not a feasible successor

               

              hope it helps

              cheers

              • 4. Re: EIGRP ACTIVE AND PASSIVE
                smsnaqvi

                Hi Bins

                If there is no feasible successor then its the original router i.e. RoutrA in your case that ill go active to actively look for another path towards the destination. It will send query messages to the neighbor routers who will quickly check if they have the path the destination and will send a unicast response back to the RouterA, which will then converge to the new path.

                 

                 

                Hth

                • 5. Re: EIGRP ACTIVE AND PASSIVE
                  Enrique

                  Bins, if there is a feasible successor it will not go into active state. It will choose this FD, as this is the reason why is there. If there is no FD it will go into active state and hopefully not get Stuck in Active, know as SIA.

                   

                  Enrique

                  • 6. Re: EIGRP ACTIVE AND PASSIVE
                    Francisco Garcia

                    I have a related question: If the succesor route is lost in a router and this route is replaced in the routing table with a feasible route, why the FD remains unchanged in the topology table? In the routing table the new route is added using its own metric. Please check:

                     

                    1. Before succesor lost:

                     

                    (topology table)

                    P 192.168.1.0/24, 1 successors, FD is 2812416

                            via 172.30.24.2 (2812416/2809856), FastEthernet0/0

                            via 10.1.134.3 (2835456/2323456), Serial2/0.3

                     

                    (routing table)

                    D    192.168.1.0/24 [90/2300416] via 172.30.13.1, 00:41:36, FastEthernet0/0

                     

                    2. After succesor lost:

                     

                    (topology table)

                    P 192.168.1.0/24, 1 successors, FD is 2812416

                            via 10.1.134.3 (2835456/2323456), Serial2/0.3

                     

                    (routing table)

                    D    192.168.1.0/24 [90/2835456] via 10.1.134.3, 00:00:38, Serial2/0.3

                    • 7. Re: EIGRP ACTIVE AND PASSIVE
                      Bogdan

                      something is rong with this output besides the unchanged topology table FD

                      1. topology table FD is 2812416 via 172.30.24.2

                          routing table FD is 2300416 via 172.30.13.1

                      2. FD is 2812416 but no successor with that FD exists

                       

                      what program did you use ?  

                      • 8. Re: EIGRP ACTIVE AND PASSIVE
                        Francisco Garcia

                        Bogdan, it is correct your comment. It was an error when copy-paste, the correct item 1 is:

                         

                        1. Before succesor lost:

                         

                        (topology table)

                        P 192.168.1.0/24, 1 successors, FD is 2812416

                                via 172.30.24.2 (2812416/2809856), FastEthernet0/0

                                via 10.1.134.3 (2835456/2323456), Serial2/0.3

                         

                        (routing table)

                        D    192.168.1.0/24 [90/2812416] via 172.30.24.2, 00:04:44, FastEthernet0/0

                         

                        About item 2, the FD value of 2812416 refers to the lost succesor as you can see on item 1. My question is why this value remains unchanged after succesor route is replaced by feasible succesor via FE0/0 because a link failure.

                         

                        I am using GNS3 and Lab 2-1 step 7 of ROUTE Lab Guide official course. Check this verification comment taken from Lab Guide:

                         

                        "Verify that secondary path through router R3 (using FE0/0) is put into the routing table of router R4 in case of a primary path failure without the FD being changed".

                         

                        Thanks.

                        • 9. Re: EIGRP ACTIVE AND PASSIVE
                          Adam - CCNP

                          Hi Francisco,

                           

                          The answer to your question requires an understanding of the underlying DUAL algorithm that EIGRP uses, more specifically how it uses the FD values in response to local computations (Passive routes) and also in response to going Active on a route.

                           

                          Here is my understanding;

                           

                          The Feasible Distance is an established lowest cost to a destination. When the EIGRP process boots it multicasts Hellos and if neighbors are found and adjacencies are formed it will receive full routing tables from these neighbors. If multiple paths to the same destination are received the router will set the FD and the lowest (best) of these values. This is now the baseline distance to reach this destination.

                           

                          Next comes to feasibility condition to be met if a route can become a feasible successor. The feasibility condition requires that a neighbors Advertised Distance to a destination is less than the Feasible Distance to that same destination. If the neighbors AD is less than the FD, it qualifies to be a successor.

                           

                          Now here’s where it gets tricky.

                           

                          Taking your scenario you have the following:

                           

                           

                          (Topology table)

                           

                          P 192.168.1.0/24, 1 successors, FD is 2812416

                           

                                  via 172.30.24.2 (2812416/2809856), FastEthernet0/0

                           

                                  via 10.1.134.3 (2835456/2323456), Serial2/0.3

                           

                           

                          So in this example the FD is 2812416 which is the route out FastEthernet0/0. The route out Serial2/0.3 has an AD of 2323456 which is less than 2812416 and so is a feasible successor.

                           

                          The route via FastEthernet0/0 goes down. Because the router has a feasible successor through Serial2/0.3 the route stays in a Passive state. This means that no queries are sent and a local computation is carried out. This local computation makes the route via Serial2/0.3 the new successor but does not update the FD. So in this sense, it is entirely possible for the FD of the successor to be higher than the FD in the topology table.

                           

                          If you perform the same test again, but make sure that there is no feasible successor to 192.168.1.0/24 and then break this network, the route will go active. The router will set the FD of this route to infinite will then send EIGRP Query packets out to this advertising these new details and asking ‘do you have a route to this destination that’s better than these conditions’ . Assuming there is another path to this destination; once the router has received all queries it will place this new route in the topology table (it has to meet the FC as the AD is less than infinite) and update the FD to be the FD of this new route.

                           

                          So the FD will change if:

                           

                          a)     The lowest cost to a network is reduced (This will trigger an update being sent)

                           

                          b)     The route becomes Active and a new route is found

                           

                          The FD won’t change if:

                           

                          a)     The cost to a network is increased

                           

                          b)     A successor is replaced with a feasible successor in the topology table

                           

                          I can only hope that all made sense!

                           

                          Cheers

                           

                          Adam

                          • 10. Re: EIGRP ACTIVE AND PASSIVE
                            Francisco Garcia

                            Hi Adam,

                             

                            Your answer is clear, solid and plenty of sense. It confirms what I think about the situation, but I needed a foundation. Thank you for share your knowledge and time.

                            • 11. Re: EIGRP ACTIVE AND PASSIVE
                              Diwakar Sharma -CCNA/CCNP/JNCIA-ER

                              Hi bins,

                               

                              Active routes are 'BAD ' as they are actively trying to find a backup path but not able to get .

                               

                              Passive routes are 'GOOD' everything is working fine.

                               

                              stuck in active :

                               

                              When EIGRP returns a stuck in active (SIA) message, it means that it has not received a reply to a query. EIGRP sends a query when a route is lost and another feasible route does not exist in the topology table. The SIA is caused by two sequential events:

                              • The route reported by the SIA has gone away.
                              • An EIGRP neighbor (or neighbors) have not replied to the query for that route

                               

                               

                              You must answer two questions to troubleshoot the EIGRP stuck in active problem:

                              • Why is the route active?
                              • Why is the route stuck?

                              Determining why the route is active is not a difficult task. Sometimes, the route that constantly is going active could be due to flapping link. Or, if the route is a host route (/32 route), it's possible that it is from a dial-in connection that gets disconnected. However, trying to deter-mine why the active route becomes stuck is a much harder task—and more important to learn. Usually, an active route gets stuck for one of the following reasons:

                              • Bad or congested links
                              • Low router resources, such as low memory or high CPU on the router

                                 


                              • Long query range


                              • Excessive redundancy


                              By default, the stuck in active timer is only three minutes. In other words, if the EIGRP neighbor doesn't hear a reply for the query in three minutes, neighbors are reset. This adds difficulty in troubleshooting EIGRP stuck in active because every time an active route is stuck, you have only three minutes to track down the active route query path and hopefully find the cause.

                              The tool that you need to troubleshoot the EIGRP stuck in active error is the show ip eigrp topology active command. This command shows what routes are currently active, how long the routes have been active, and which neighbors have and have not replied to the query. From the output, you can determine which neighbors have not replied to the query, and you can track the query path and find out the status of the query by hopping to the neighbors that have not replied.

                               

                               

                              Here is the sample output

                               

                              Router#show ip eigrp topology active

                              IP-EIGRP Topology Table for AS(1)/ID(10.1.4.2)

                              A 20.2.1.0/24, 1 successors, FD is Inaccessible, Q

                              1 replies,

                              active

                               

                              00:01:43

                              , query-origin: Successor Origin

                                via 10.1.3.1 (Infinity/Infinity), Serial1/0

                                 via 10.1.4.1 (Infinity/Infinity), Serial1/1, serno 146

                              Remaining r

                              eplies

                              :

                                 Via 10.1

                              .5.2,

                              r

                              , Serial

                              1/2