Not really a good design. If was site already has two WLC’s in SSO or N+1, why would you want to add the other site controller for redundancy? You are better off centralizing your WLC’s and running FlexConnect, depending on how far the sites are away from the controller. The issue of having two or more sites as a backup is the fact that sites are usually layer 3 adjacent, meaning that the interfaces on the WLC will be different, so devices connected on site 1 SSID A to site 2 SSID A will break. A good design is for redundancy, but not overly complex as that might cause you more problems. You also have to understand that when you setup N+1, you now have to create mobility between the controllers, this means the ap’s now know of all controllers in the same mobility group, thus ap’s can move to another controller, because it might of lost connecting to the primary. So now, you will have users that associate to an ap on site 1 and roam to an ap that is in site 1 but joined to site 2 and that will cause auto anchoring to happen. That means the device is tunneled to site 2 and then anchors back to site 1 controller. Now if AP1 in site 1 joins site 2 controller, then users whom initially associate to this ap will get an IP address from site 2 and then you have the cut anchoring happening again int he reverse direction I mentioned previously.
So again, this goes back to you requirements, why do you need this redundancy between sites? What happens when the link between the sites goes down? Are your subnets sized to handle all devices in each location in case the controller(s) goes down? What is your decision for local vs flex connect based on? How many ap’s on each site, how far are the sites, what is the bandwidth between sites, how many users per site on wireless, where is your radius servers, DHCP servers, etc. You need to look at all these and make a good decision on the best design.
-Scott
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