Cisco Virtual Wireless Controller (vWLC) - Release 7.3 supports HA. The latest 7.3 release enables a second controller to be configured in “Hot Standby” mode to a designated primary controller.
The redundancy ports of these two controller appliances are connected with an Ethernet cable. In case of WiSM2, you can have a redundant blade in the same or across chassis with VSS. This connection is used to exchange the configurations, the CAPWAP states of APs, and regular keep-alives. This is how a sub-second failover can be achieved for hundreds of access points to the standby controller in case of a hardware failure or network loss for the primary Controller. This means that there is NO SSID Outage because of Access Points Stateful Switch Over (AP-SSO) from primary to the standby controller.
In addition, the standby controller also syncs the Pairwise Master Key (PMK) key cache from the active primary controller, so when the client re-associates to the access point, there is no need for the controller to re-authenticate with the RADIUS server, resulting in downtime of only a handful of seconds.
