DHCP Conflict

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DHCP Conflict
A coworker recently clued me in on a fix for the following problem. When I started a computer in the network, I would receive an IP conflict message. The network clients are also set to be DHCP clients.

Let's call the computer showing the conflict error Computer1, and the computer identified by the media access control (MAC) address in the error message Computer2. Computer1 showed the error message IP conflict xxx.xxx.xxx.148 with hardware having MAC address xx-xx-xx-xx-xx-xx, and DHCP Manager showed that Computer1 had a lease for the .148 address. I used SolarWinds.Net's IP Network Browser to show that Computer2 had a lease for the .148 address and Computer1 had a lease for a .110 address. When I ran Winipcfg on Computer2, just as I suspected, its MAC address matched the MAC address in the error message.

To solve the conflict, I tried deleting both entries from the DHCP Manager, but both computers still tried to grab the .148 IP address. I released and renewed both computers, but they again tried to grab the .148 IP address. Looking up the error message on TechNet left me empty-handed, and hard-coding wasn't a desirable solution.

Discussing this situation with a coworker, I discovered that he had experienced the same problem. He discovered an obscure registry entry that you can modify to fix the problem. At the computer identified by the MAC address in the error message (Computer2 in the previous scenario), clear the HKEY_LOCAL_MACHINE\SYSTEM\Cur-rentControlSet\Services\VXD\DHCP and DHCP Option subkeys, leaving only the PopupFlag and Version values. Then reboot this system, run Winipcfg, select the NIC, and perform a Renew All, as the system is now missing the default gateway information. This process causes the system to grab a valid IP address and DHCP information. Reboot the system. Next, reboot the system on which the error message appeared (Computer1 in the previous scenario), and it will grab a valid IP address. You might also need to go into DHCP Manager and delete the IP address that was in contention.

—Melissa Wise, MCSE
mwise@the-lair.com

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