Having an IP Address conflict?

I just wanted to provide a quick “what to do” when having an IP address conflict. I’ve had a few of these cases this year. I’m not sure that one needs to open a support case with us to resolve this. In some instances, critical applications/resources are down due to IP address conflicts. I’ll provide a few steps one should take in troubleshooting this situation.

IP address conflicts happen within a subnet. I had a call recently in which a cluster resource wouldn’t come online. When the cluster resource attempted to come online, it issued an ARP request for its IP address. In a working scenario with no conflict, there would be no response. In this instance, a VMWare server responded. The cluster resource reported the conflict and failed to initiate. They were unable to locate the VMWare server, so the customer changed the IP address on the cluster resource.

Basic steps:

  1. Run IPConfig at a Command Prompt — is the IP address in conflict assigned to the machine you’re on?
  2. Ping the IP address in conflict (use “ping -a” as the name may resolve). Pinging with the –a switch causes a reverse DNS lookup. There is a chance the conflicting host registered the address in DNS.
  3. Run the ARP -a” command and locate the address in conflict (even if ping doesn’t work)
  4. Go to http://standards.ieee.org/regauth/oui/index.shtml and enter the MAC address. This will
    help narrow down the MAC address and locate the conflicting host.

It will help if there is a record of MAC addresses. Use Excel to keep a list of MAC addresses in your environment. Easily use SQL to query for the MAC returned by “ARP -a” command. Other monitoring software such as Systems Center Operations Manager will record and store MAC addresses.

Hopefully this saves you a support call down the road. Archive it if you must. Inevitably, you’ll have an IP address conflict. This will save you some time.

– Rich Chambers