i have 5508 WLC and 1242 LAP .
i 5508 connects to core switch and LAP connects to access switch, and there is L3 link from core swtich to access switch , so i have to use HREAP to let my WLC to control my APs.
in my access switch i set ip helper-address to my WLC in the client vlam, then all the wireless clients cannot get ip address from the WLC.
but if i set ip helper-address to another DHCP server , the wireless clients can get ip address .
so i dont know why WLC cannot be the DHCP server of the wireless client?
The WLC's DHCP can only serve clinets connected to it. If the request come's from a relay agent (the switch configured with ip-helper in your case) the WLC does not respond becaue the requester is not is not one of it's clinets.
You always better to avoid DHCP on the WLC.
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You need to make sure DHCP Proxy is enabled on the WLC for the internal DHCP server to be used.
Controller > Advanced > DHCP >DHCP Proxy
Like Amjad mentioned, I wouldn't use the WLC if you do have another DHCP server, especially in large environments.
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Amjad's response is the correct answer, this will not work the way you have described. In the scenario you have described, using local switching, these wireless clients are not having their traffic processed centrally, therefore the WLC will not provide a DHCP offer to the clients. The DHCP request hits the local network segment and the gateway unicasts the DISCOVER to the WLC if using that as helper, but the WLC will ignore this because it's not coming "in" via traditional CAPWAP client traffic from an AP centrally switching traffic. Pointing to a different DHCP server should work just fine as you described.
If you "require" DHCP to be handled on the WLC, then these either need to be local mode APs, or "centrally" switching the traffic for the H-REAP/FlexConnect APs.