When trying to configure a Cisco 5508 WLAN Controller to use an external DHCP server running on an older PIX, clients were unable to obtain IP addresses. After spending some time troubleshooting, it turns out that the DHCP Proxy option must be disabled for this-- it's enabled by default.  But because other WLANs require this to be enabled, a different solution was needed.

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So we configured a DHCP scope on the &quot;Internal DHCP Server&quot; on the WLAN Controller for the appropriate subnet, and reconfigured the logical interface used for the WLAN to point to the MANAGEMENT IP address of the WLC for its DHCP services. Once the scope was enabled, users were able to immediately and consistently pull IP addresses--

For networks that are not available &quot;from a wire&quot;, like Guest networks, I highly recommend using the Internal DHCP Server on the WLCs for quick and easy IP address assignment!