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Duplicate ip on Cisco 888?

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  • Duplicate ip on Cisco 888?

    I have a Cisco 888. To that, I have one single Win7-machine connected. Static ip.

    If I reboot the machine, or if I disconnect the network cable, so there are no active connections to the Cisco, I then get an ip conflict/duplicate ip message in Windows once rebooted/cable connected.

    If I change the static ip to something else, it starts to work right away. I can then change the address back to the original ip and still have it working. I can also reboot the Cisco and that will solve the ip conflict problem.

    If I have two machines connected to the Cisco, both with static ip's, and disconnect one of them, and then reconnect it again, I have no problems at all. As long as the Cisco have at least one active connection, it doesn't matter whether the other machines are disconnected or not.

    In Windows event log, there is a message about duplicate ip for a totally different mac address than on the network card. Could this be related to some IPV6/ISATAP?

    The problem is that I have this issue on several sites, across the world... All my sites behave the same, and they have the same config in the 888's.

    Do you have any pointers what to do?

  • #2
    Re: Duplicate ip on Cisco 888?

    First thing is to make sure your ip addressing is unique for each host.


    Are you getting a message like this on the 888 console or buffer?

    Feb 18 23:37:53.098: %IP-4-DUPADDR: Duplicate address 10.237.66.3 on Vlan601, sourced by 001d.096b.4858
    Feb 18 23:38:58.092: %IP-4-DUPADDR: Duplicate address 10.237.66.3 on Vlan601, sourced by 001d.096b.47ed

    As you can see it will show you the "duplicate ip address", the vlan and the host mac address.

    You could then run the command below to match up the learned mac and see what port its coming from.


    show mac-address-table (to view the layer3/layer 2 mappings)


    You could also try disabling the ipv6 stack on the windows host to see if it goes away. Could also be a code bug. May want to see about upgrading the IOS.
    CCNA, CCNA-Security, CCNP
    CCIE Security (In Progress)

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    • #3
      Re: Duplicate ip on Cisco 888?

      I do not have console access to the 888, it is being owned by the ISP but they show very little interest in helping to fix this issue.

      I have disabled Ipv6 in network config and I have disabled the Isatap-interface and Toredo-tunnel in device manager. Still the same problem, when I reboot the single machine it still gets the duplicate ip conflict.

      All sites have their own subnet, e.g. 10.11.12.0/24 so there is no ip conflict between networks either.

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      • #4
        Re: Duplicate ip on Cisco 888?

        Well if the ISP wont help you troubleshoot then at this point your very limited on what you can do. If that where my ISP I would be on the phone with them or my rep to get this resolved.
        CCNA, CCNA-Security, CCNP
        CCIE Security (In Progress)

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        • #5
          Re: Duplicate ip on Cisco 888?

          Problem solved, it was the proxy-arp setting that messed up when the Cisco had no devices connected. It took all the addresses and wouldn't let go.

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          • #6
            Re: Duplicate ip on Cisco 888?

            Thanks for the update.
            CCNA, CCNA-Security, CCNP
            CCIE Security (In Progress)

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