6 Replies Latest reply: Jun 12, 2013 12:56 AM by satwant RSS

    BGP Router ID

    Deepak

      There are following two steps comes in BGP path selection process...

       

      Prefer the route with the lowest neighbor BGP router ID value.

       

      If the BGP router IDs are the same, prefer the route with the lowest neighbor IP address.

       

      Just wanted to know is there a possiblity of having BGP router IDs to be same, if yes then in please explain the scenario.

       

      Thanks in advance,

      Deepak

        • 1. Re: BGP Router ID
          Conwyn

          Hi Deepak

           

          I did not think you could have the same router-id

           

          Regards Conwyn

           

          router bgp 2
          no synchronization
          bgp router-id 1.1.1.1
          bgp log-neighbor-changes
          neighbor 10.0.0.1 remote-as 1
          no auto-summary

           

          *Mar  1 00:02:27.587: %BGP-3-NOTIFICATION: sent to neighbor 10.0.0.1 2/3 (BGP identifier wrong) 4 bytes 01010101

          FFFF FFFF FFFF FFFF FFFF FFFF FFFF FFFF 002D 0104 0001 00B4 0101 0101 1002 0601 0400 0100 0102 0280 0002 0202 00

          r1(config-router)#bgp router-id 2.2.2.2
          r1(config-router)#
          *Mar  1 00:05:25.219: %BGP-5-ADJCHANGE: neighbor 10.0.0.1 Up

          • 2. Re: BGP Router ID
            Deepak

            Hey thanks for reply...now its confusing..means what to look at...because i've taken these lines from BSCI Self-study guide by Catherine Paquet and Diane Teare, page no: 509-510-511, The Route selection decision process section.

             

            Hope there must be some sort of explanation to this

             

             

             

            Regards,

            Deepak

            • 3. Re: BGP Router ID
              Petr Lapukhov 4xCCIE/CCDE

              BGP Router-IDs are supposed to be unique values, and this is normally achieved by using global IPv4 addresses as the IDs. The purpose of BGP Router-ID is to recover from TCP session collision, i.e. when both speakers open TCP connections simultaneosuly. A local system will reject the OPEN message with the same Router-ID as the local one. However, it does not compare Router-IDs of two different peers as this is not needed for the purpose of collision recovery.   That is, consider the topology below:

               

              X----A----Y

               

              If the BGP speakers X and Y peer with A, they could use the same Router-ID value as long as it does not match A's Router-ID. Hence the best-path selection step - if the two speakers with the same IDs advertise the same path, break the tie based on the peering transport IP address.

               

              HTH,

               

              Petr

              • 4. Re: BGP Router ID
                Conwyn

                Hi Deepak

                 

                An example below

                 

                Regards Conwyn

                 

                r0#show run | section router

                router bgp 1
                no synchronization
                bgp router-id 1.1.1.1
                bgp log-neighbor-changes
                neighbor 10.0.0.2 remote-as 2
                no auto-summary
                r0#show ip bgp summary
                BGP router identifier 1.1.1.1, local AS number 1
                BGP table version is 1, main routing table version 1

                Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
                10.0.0.2        4     2       7       7        1    0    0 00:04:32        0

                 

                r1#show run | section router
                router bgp 2
                no synchronization
                bgp router-id 2.2.2.2
                bgp log-neighbor-changes
                neighbor 10.0.0.1 remote-as 1
                neighbor 10.0.0.6 remote-as 3
                no auto-summary
                r1#show ip bgp summ
                BGP router identifier 2.2.2.2, local AS number 2
                BGP table version is 1, main routing table version 1

                Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
                10.0.0.1        4     1       8       8        1    0    0 00:05:40        0
                10.0.0.6        4     3       6       6        1    0    0 00:02:14        0

                 

                r2#show runn | section router
                router bgp 3
                no synchronization
                bgp router-id 1.1.1.1
                bgp log-neighbor-changes
                neighbor 10.0.0.5 remote-as 2
                no auto-summary
                r2#show ip bgp summary
                BGP router identifier 1.1.1.1, local AS number 3
                BGP table version is 1, main routing table version 1

                Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
                10.0.0.5        4     2       7       7        1    0    0 00:03:39        0

                • 5. Re: BGP Router ID
                  Deepak

                  So its point view thats came into play...

                   

                  Anyways thanks for helping me understand...

                   

                  Regards,

                  Deepak

                  • 6. Re: BGP Router ID
                    satwant

                    great explanation....