If you have statically configured your BVI1 interface with an IP address, you may come across this issue while attempting to transfer the IOS image.  During the the extraction process the connection breaks and displays the following error message:

“Could not discover WLC using static IP. Forcing AP to use DHCP”

This issue occurs when trying to convert from Lightweight to Autonomous only.

Why does the transfer process break? 

As AP is already running the lightweight IOS – by default the AP will look for the controller, either by broadcast, DHCP or DNS. in your environment you will most likely not have a WLC which is why you are converting in the first place right? so the AP will never find one – this forces the AP to change its interface mode from static to DHCP to look for the WLC. As the change is forcefully applied the transfer process breaks causing the connection to time-out.

Resolution

The AP will keep changing the interface mode to DHCP unless it can locate a WLC no matter how many times you keep statically assigning an IP. The time between the static assignment and the forceful change is no where near enough time to transfer the autonomous image across. So the only solution to this problem is to allow DHCP to assign the AP an IP address on the same subnet as the TFTP server, this way the interface mode is already set to DHCP the AP can keep looking for the WLC but it wont break the transfer process.