Here is how to configure Option 43 on an MX appliance for a Cisco Aironet or Catalyst AP to discover its Wireless LAN Controller (WLC).
My Catalyst 9800-CL controller IP address: 22.214.171.124
Meraki MX appliance DHCP server configuration
Open Dashboard and go to Security & SD-WAN > Configure > DHCP > scroll down to the right VLAN > DHCP options > Add a DHCP option:
Format of the hex string
In my example, the final string would be “f1:04:ad:26:db:21”
“f1:04” tells the AP that only one WLC IP address is used, followed by the actual address
“ad” is hex representation of 173
“26” is hex representation of 38
“db” is hex representation of 219
“21” is hex representation of 33
To convert the 4 decimal octets of the IP address to hexadecimal format, you can use this online tool, macOS Calculator or Windows Calculator.
Verification on the AP
If you provide the AP with IP addresses of 2 standalone controllers (think N+1 HA mode), then simply change “f1:04” to “f1:08” and append the second controller’s IP address in hex representation to the end of the hex string.
Primary controller IP address: 126.96.36.199
Secondary controller IP address: 188.8.131.52
Hex string: f1:08:ad:26:db:21:ad:26:db:22