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      • 15. Re: DR / BDR concept
        Sudharsan Nammalwar

        Alkuin Melvin wrote:

         

        Thx eddie for the quick reply. Yes, i'm aware that BDR is used for backup. But what i don't understand is why DROther should exchange database with BDR too? Isn't the FULL state from DR is enough? i mean, DROthercan exchange database with BDR when DR is down, right?

         

        =====================================================================

        Answer:

         

        FACT: The DR and BDR listens in the multicast address 224.0.0.6 (All DRouters)

                   The DR will flood the updates in the multicast address 224.0.0.5 (AllSPF Routers)

         

        So the DROther will send the update in 224.0.0.6. Since the DR and BDR are member of that, both gets updated.

         

        Your question:

        "Why it also exchange database with BDR for BDR is just used as a backup?"

         

        Answer:

        Say, consider if there is a change happened and been updated only to DR and not to BDR. In that situation if something happens to the DR and it is down (even before updating the BDR), then how the latest update will be refelected in the BDR. The latest change will not be updated to BDR.

         

        To avoid this situation both DR and BDR been updated simultaneously so that all are in SYNC. However only DR will react and BDR will remain silent for any changes. If the DR is down, the BDR will promote itself as DR and take necessary action.

         

        Hope that Helps.

         

        Cheers,

        Sudharsan

        • 16. Re: DR / BDR concept
          Alkuin Melvin

          Hi, so i think DROther will maintain the same database table from DR and BDR. In case DR down, the database table still exist but will forward the LSA type 1 from DR that was BDR. Cheers

          • 17. Re: DR / BDR concept
            Sudharsan Nammalwar

            Hi Alkuin,

            Alkuin Melvin wrote:

             

            Hi, so i think DROther will maintain the same database table from DR and BDR. In case DR down, the database table still exist but will forward the LSA type 1 from DR that was BDR. Cheers

             

            No it does not work that way.

            The DROther will send all the LSA updates( Irrespective of the LSA types ) it has, only to the multicast address 224.0.0.6. Meaning only the DR and BDR get updated, since they are the member of that group.

             

            However the DR and BDR will only listen from DROther in 224.0.0.6 but will not speak in that multicast address, so its only one way.The DR to speak, to all DROthers uses multicast address 224.0.0.5. NOTE: here the BDR will never speak when the DR is alive. If the DR wants to talk to single DROther it will unicast specific to the DROther.

             

            The database will be the same for DR and BDR at any given point. So when the DR is down, then the BDR will promote itself as DR and will function actively as DR.

             

            Cheers,

            Sudharsan

            • 18. Re: DR / BDR concept
              Vijay Bhandari

              Hi Alkuin Melvin ,

               

              DRother maintains a full adjacency with both DR and BDR because if in case DR goes down then no new adjacency formation is required with the BDR. As the adjacency formation takes some time so it won't be feasible at the time of outage (when DR goes down) so a DROther maintains a FULL neighborship with both DR and BDR. HTH.

               

              Regards.

              • 19. Re: DR / BDR concept
                Alkuin Melvin

                Hi, please let me explain what i'm going through. I have this lab scenario of OSPF over NBMA, i have intentionally misconfigured the DR position as DR should be in R1 and R2/R3 have ip ospf priority 0.

                 

                 

                From scenario above as i mentioned, i have put R2 with priority 255, R1 with priority 100 and R3 with priority of 200.

                 

                The show ip ospf neighbor is below:

                 

                R1#sh ip ospf neighbor

                 

                Neighbor ID     Pri   State           Dead Time   Address         Interface

                2.2.2.2         255   FULL/BDR        00:01:52    10.12.3.2       Serial0/0.1

                3.3.3.3         200   FULL/DROTHER    00:01:54    10.12.3.3       Serial0/0.1

                 

                 

                R2#sh ip ospf neighbor

                 

                Neighbor ID     Pri   State           Dead Time   Address         Interface

                1.1.1.1         100   FULL/DR         00:01:50    10.12.3.1       Serial0/0.1

                 

                R3#sh ip ospf neighbor

                 

                Neighbor ID     Pri   State           Dead Time   Address         Interface

                1.1.1.1         100   FULL/BDR         00:01:54    10.12.3.1       Serial0/0.1

                 

                 

                Notice the relation between R1 and R3 as the relation is DROther <--> BDR (Full State).

                 

                My only question is : Do R3 will received the route from R1 as they established FULL state? Remember the DR is on R2 which R3 doesn't have direct connection (Frame Relay VC) with R2.

                 

                 

                Regards

                 

                Alkuin Melvin

                • 20. Re: DR / BDR concept
                  Vijay Bhandari

                  Hi Alkuin,

                   

                  I am not able to see the image but what can i understand from the show command is that it is quite confusing. Like in from the first output it is clearly visible that R1 is the DR and R2 is the BDR and R3 is DRother so for first case R3-->R1 will exchange routes.

                   

                  But from your last output states that R1 is the BDR, So in this case also R1-->R3 will exchange databases and updates.

                   

                  Please make sure that the router you want make DR should come up first in the network then the BDR or restart the ospf process.

                   

                  HTH.

                   

                  Regards.

                  • 21. Re: DR / BDR concept
                    Alkuin Melvin

                    Hi, sorry for the confusing output.

                     

                    Here is the new output (i have put the R2 to come up first, R1 and then R3)

                     

                    R1#sh ip ospf neighbor

                     

                    Neighbor ID     Pri   State           Dead Time   Address         Interface

                    2.2.2.2         255   FULL/DR              00:01:52    10.12.3.2       Serial0/0.1

                    3.3.3.3         200   FULL/DROTHER    00:01:54    10.12.3.3       Serial0/0.1

                     

                     

                    R2#sh ip ospf neighbor

                     

                    Neighbor ID     Pri   State           Dead Time   Address         Interface

                    1.1.1.1         100   FULL/BDR         00:01:50    10.12.3.1       Serial0/0.1

                     

                    R3#sh ip ospf neighbor

                     

                    Neighbor ID     Pri   State           Dead Time   Address         Interface

                    1.1.1.1         100   FULL/BDR         00:01:54    10.12.3.1       Serial0/0.1

                     

                    Thanks.

                     

                     

                    Regards

                     

                    Alkuin Melvin

                    • 22. Re: DR / BDR concept
                      Vijay Bhandari

                      Hi,

                       

                      From this output its clear that R1 and R3 will share there databases and also R3 will update the R1 of any N/W change but R3 will not be updated about changes from any other router because in a DR/BDR environment all the DRothers inform DR and BDR of the changes in the N/W and after that DR injects the information throughout the network.

                       

                      But in the above scenario R3 does not have any adjancency with the DR so it won't get any update from DR. This is a design issue, In a hub and spoke situation we generally make the Hub as a DR but it totally depends on your network needs.

                       

                      HTH.

                       

                      Regards.

                      • 23. Re: DR / BDR concept
                        Alkuin Melvin

                        Hi Vijay, that's exactly what i'm looking for. Thank you so much for helping me out. Thanks guys. Cheers

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