I have 2 4400 controllers. Each is located at a different site and the access points at each site will have a primary and secondary controller. Each site is on a different subnet. When the link between the sites fails then each site works fine as a standalone. However, if one controler fails then I am trying to understand what will happen to the clients. The APs will come up on the failover controller which is now in a different subnet. How can I get the clients on the "primary" subnet to get the correct IP subnet address when they are connected to the secondary controller? I know H-REAP offers some solutions but if there is delay in response most users will reboot their clients and then not be able to authenticate. Am I correct in assuming that failover controllers only work if both controllers are on the same subnet?
The failover controllers do not have to be on the same subnet. We have tested this type of failover and have had good results. First of all make sure your WLANs match for both sites. If the controller at site "A" fails and all the AP's at that site attach to the controller at site "B" your DHCP clients at site "A" will issue a new DHCP request and take an IP address from the subnet at site "B". We have staged these failure tests with 100+ access points. It takes about 90 seconds for the APs to connect to the new controller and the clients to get a new IP address.
During the failover the APs at site A are offering up the same WLANs as before and will be tunneling the traffic from their connected clients to controller B. The interfaces/WLANs that are configured on controller B are using the subnets at site B so your clients must be using the IP addressing that is set up at site B. We have tested VoIP clients (Vocera, Spectralink) and data clients in this scenario and it works like a charm.