I installed a couple of Catalyst 9800-CL at my homelab. I decided to host the access points in a separate L3-Subnet so I had to have a way to deploy the WLC ip address to the access points. A commonly used mechanism to achieve that besides DNS is option 43 via DHCP.
With option 43 the DHCP server provides the ip address of the controller to the access point in a hexadecimal encoded fashion.
In my case the option looks like that:
f1 = Sub option
04 = payload length
0a to 0d = the controller ip address in hex
Another example would be – if you would have two controllers kind of HA setup:
Controller 1 IP: 192.168.1.10
Controller 2 IP: 192.168.1.20
The resulting hex string will be:
option 43 hex f108c0a8010ac0a80114
f1 = Sub option
08 = payload length
c1 to 14 = the controller ip addresses in hex concatenated.
There is a pretty handy litte tool on the web calculating the appropriate hex string:
https://shimi.net/services/opt43/
Another good source is Marco’s website – providing also hints for configuring other DHCP servers:
https://tcpip.wtf/en/unifi-l3-adoption-with-dhcp-option-43-on-pfsense-mikrotik-and-others.htm
Another option would be, to provide a dns domain and dns server via DHCP to the access point – then the AP would try to reach the controller via DNS.
A Cisco AP would try to resolve “CISCO-CAPWAP-CONTROLLER.your-provided-dns-domain.tld”