I was recently tasked with deploying two Cisco 5508 WLCs in a fault tolerant configuration for our enterprise WLAN. I searched CCO for documentation that clearly explained configuring mobility groups and/or HA to provide fault tolerance, and a means by which to test the config, but found none (feel free to post a link to any doc with which you might be familiar). I understood the concept behind mobility groups, but did not know how they would provide fault tolerance should a controller fail. I could see that the mobility group was configured correctly, but none of the access points in our environment were registered to the secondary WLC. My question, specifically, was “what happens to the access points when one of the WLCs in a mobility group fails?”
The answer turned out to be easy enough. When a secondary WLC is configured on the HA tab of the wireless access point configuration section, an access point will register with the secondary WLC, but only when the primary fails. That’s all there is to it. To test this, all that is required is to swap the order of the primary and secondary controllers on the HA tab of the wireless access point configuration page. As soon as that is done, the AP disappears from one WLC and appears on the other. It takes about 60-90 seconds for the registration process to complete. Provided that the configuration of the secondary matches the first (with the exception of the IP addresses of the interfaces), connectivity is restored to wireless users in less than two minutes. Not the fastest failover in the world, but certainly within acceptable tolerances.
From all that I’ve read and seen so far, mobility groups are not necessary to provide protection from hardware failures. The mobility groups provide a different type of high availability; the ability to roam through a wireless network without losing connectivity. In other words, HA and mobility groups are complementary technologies necessary to provide true high availability in a wireless environment. There is no overlap of functionality between the two. I intend to keep testing to learn more about HA and mobility groups, and would be happy to learn from others out there working on similar projects.