A Cisco lightweight wireless AP needs to be paired with a WLC to function.
An AP must be very diligent to discover any controllers that it can join-all without any preconfiguration on your part. To accomplish this feat, several methods of discovery are used.
The goal of discovery is just to build a list of live candidate controllers that are available, using the following methods:
+ Prior knowledge of WLCs
+ DHCP and DNS information to suggest some controllers (DHCP Option 43)
+ Broadcast on the local subnet to solicit controllers
Reference: CCNP and CCIE Enterprise Core ENCOR 350-401 Official Cert Guide If you do not tell the LAP where the controller is via DHCP option 43, DNS resolution of "Cisco- capwap-controller.local_domain", or statically configure it, the LAP does not know where in the network to find the management interface of the controller.
In addition to these methods, the LAP does automatically look on the local subnet for controllers with a 255.255.255.255 local broadcast.
Reference: https://www.cisco.com/c/en/us/support/docs/wireless/5500-series-wireless- controllers/119286-lap-notjoin-wlc-tshoot.html