The LWAPP APs ( 120 x Access Point 1020, configure with static IP address ) and WLCs ( totally 3 WLC 4402 switches ) are in different networks. I guess the DHCP option 43 and DNS resolution method not work. The only one method is configure the primary / secondary / tertiary controller name in each APs. My question is how the LWAPP APs resolve the controller name to correct controller IP address if LWAPP APs and WLCs are in different IP networks?
Using DNS, simply create the CISCO-LWAPP-CONTROLLER entry three times, and map it to each of the different WLC Mgmt Addresses. Nominate one of your WLCs as a Master Controller, and then regardless of which WLC the AP initially joins, the AP will re-home its self to the master controller.
Using DHCP, the hex string you specify always starts "f1", the second two characters denote the number of WLC Addresses are included in the rest of the string, multiplied by four; you have 3 WLCs, so the number is "0c" (3 x 4 = 12 = 0c) after that you just append the WLC Addresses, one after the other. So if you had WLCs with addresses 10.248.50.1, 10.249.50.1 & 10.250.50.1, the entire string would break down as follows (without the spaces).
f1 0c 0af83201 0af83301 0af83401
Trouble is, 1020 APs need that specifying in ASCII, not hex, which I've never done before. I guess it's a case of just presenting all of that as ascii, but I'm not 100% sure. If ASCII, then I guess it will look like this (without the spaces);
241 12 010248050001 010249050001 010250050001
Addressing your other questions about how do APs resolve controller names, you need to configure the 'Mobility Groups' on each controller, this is basically a case of telling each WLC the Mobility Group Name (should be the same throughout), MAC Address & IP Address of each of the other WLCs. Then, when an AP joins the WLC, it will automatically learn the names of the other WLCs in the mobility group, allowing you to use names in the primary / secondary / tertiary fields as opposed to IP Addresses.
Hope all that makes sense! Any questions, just ask.
So if I install new LWAPP APs, First I must connect it to the same network with WLCs, after LWAPP APs joinned to WLCs, configure AP's primary / secondary / tertiary controller and change it's IP address, after saved, then connect this AP to other routed network?