We have a single Windows Server 2008 R2 AD/DHCP server with 4 DHCP scopes on it. Two for data and two for voice. We use Cisco 3750X switches with voice VLAN enabled on every port and Cisco SPA504G phones with CDP enabled.
Every now and then I notice that a new phone won't pull an IP. During investigation, I discover that the voice VLAN scope on the server has assigned a lease to the MAC of the new phone and that the switch shows the phone's MAC as assigned to that IP in the ARP table. So all seems fine, until I unplug the new phone, ping the assigned IP, and get a response.
It turns out that even though some phones are on and responding, they are being dropped from the DHCP address lease table which causes the server to think those IPs are available for assignment. The only way I've found to make the leases show back up is to reboot the phone. I tried to reconcile the scope, but that made no difference at all.
I'm using an 8-hour lease and the rest of the options are default on the server. And the switch is using standard DHCP helper commands to forward the DHCP traffic to the server.
Should I be setting some special DHCP option(s) on the 504G's (I can't find any to set)? Should I change the lease time on the server? Is there a way to force the server to confirm all leases or force the phones to remind the server, so to say, that they still have their IPs? I thought that was the purpose of the reconcile option, but it reports that all is ok.
Thanks for your help!