I have two WLCs 9800 in different sites, managing different APs and SSIDs. They are connected by a WAN link now and I want to add redundancy to this design. Can I configure one as primary for site A and backup for site B and the other one as the backup for site A and primary for site B? Is there a way to do it or I have to use one as primary for both sites and the other one as secondary for both sites?
This is 9800 so no AP Groups - it's tags but essentially achieve the same thing.
Few things to consider, but it is possible.
The WLCs will need pretty-much the same configuration so you can still advertise the correct SSIDs at each site regardless of which WLC the APs are joined to, but you can use different L3 segments at each WLC for the client VLANs if you're doing central switching. If you don't have L2 connectivity between the WLCs this will be required, but if you're doing FlexConnect don't worry about it. I would usually advise FlexConnect where APs need to talk to a remote WLC across a WAN.
This will work but you need to mimic the two WLCs configuration except of course the IPs and VLANs and any other differences as location specific. You don’t need L2 connectivity between the two WLCs in N+1 scenario that you have or trying to have here.
Also very important to configure AP Tags as below:
If you assign APs on one WLC with (Policy Tag, Site Tag and RF Tag) and if you have N+1 deployment then the other WLC need to be configured for those APs with the same (Policy Tag, Site Tag and RF Tag) even if the APs are not there and not connected to the second WLC and even if you already have the same (Policy Tag, Site Tag and RF Tag) configured on the other WLC.
This method will assign (Policy Tag, Site Tag and RF Tag) to the future connected APs.
Attaching Policy Tag, Site Tag and RF Tag to an AP:
WLC(config)# ap xxxx.xxxx.xxxx <-- The mac-address should be a wired mac address.
Or you can use CSV file from Configuration > Tags & Profiles > Tags > AP > Static
From the same above location, you can see all those Static MACs and corresponding Tags and even you can change their tags.
to show all Tags for the MAC Addresses that have been added to the WLC
WLC#show ap static tag join status