Oh god. That took far too long. A while back the kids laptop internet access stopped working. I had a quick look at it and found it was doing something weird and rejecting DHCP leases from my Cisco 877W over wifi.

Figuring it would be some exotic driver/software incompatibility I left it alone. Until today I went to use my shiney new Windows 7 work laptop. And hit the exact same issue.

Some googling later, I find I’m not alone. Many people are having similar issues, though with no consistent “fix”. Spending some more time on this, I found the cause of my specific issue. God damn proxy-arp. I hadn’t set “no ip proxy-arp” under the dot11radio interface on my 877W, so of course when Windows 7 clients performed a gratuitous ARP on their assigned lease, they would ALWAYS get a response. In turn they’d reject the address and request another. The router would flag the address as bad and hand the laptop the next address. Rinse and repeat.

Lesson learnt: turn proxy-arp the hell off on EVERY layer 3 interface. I knew this already, but seeing as I didn’t have it set I got bit.