IOS7 DHCP IP address scopes being depleted

Question

I work for a large school district in Texas. We have a large deployment of iPads (10,000+) as well, we encourge BYOD.
Our district has recently updated to iOS7. We now are experiencing IP address scopes running out of IP's, 6000 per campus for students, 5 campuses. With some testing we found the iOS7 devices leave the network - IE: go to sleep mode or disassociate from wireless in some fashion, generally for just a short period, 1 hr or 2. Once the iOS7 devices rejoin the network they are not keeping their original IP address and allocated another IP thus in a days time depleting IP addresses.
Our DHCP lease times are set for 8 hours.
We have tested this with iOS6 devices and these devices leave and rejoin and retain their original IP address from the scope.
I was wondering if any other ISD or other enviroment with a large deployment of iOs devices have seen this problem?
Thanks
LB

Answer 1

Hi All,
Found this thread when searching for a solution to a similar dhcp problem on Friday.
I came across this issue when building a wireless env similar to LB_MISD. Although in my env I have a linux server for dhcp and have two pools (one for known and another for unknown clients). Ios 7 devices were actually making so many requests it caused the DHCP server to stall at times.
From the logs I noticed that I had problems with unknown clients but not with known clients ..... the difference - an assigned hostname in dhcp for the known clients.
IOS 7 seems to require a hostname be given to the client device with the rest of the dhcp offer. To get around this I added an extra line to the dhcp pool that send the clients mac address back as it's hostname. It does not appear to actually get used by anything but does seem to fix the problem.
"option host-name = concat ( binary-to-ascii (16, 8, ":", substring (hardware, 1, 6));"
I don't know what the equivalent config would be for a windows server or home router - although a workaround might be to grab the mac addresses of all the IOS 7 devices and reserve an address and create a hostname for each device --- a pain yes but at least the dhcp server may work for everyone else?