Consider a scenario where there are two Wireless LAN Controllers (WLCs) named WLC1 and WLC2. These WLCs are configured in the same subnet in one WLAN. In order to achieve high availability, this is how the WLAN is configured:
WLC1 and WLC2 are configured within the same mobility group.
Half of the access points are configured to use WLC1 as the primary WLC and use WLC2 as the secondary WLC.
The other half of the access points are configured to use WLC2 as the primary WLC and use WLC1 as the secondary WLC.
The fallback feature is enabled on both WLC1 and WLC2.
If any of the WLCs go down, the access point that is joined to the failed WLC recognizes this (keep alive (heartbeat) between access point and WLC). Therefore, the access point begins to join the good WLC, which still runs. This is not stateful failover, which means that the access point has to join the new WLC and therefore the wireless clients.
Also, if either of the WLCs do not work and the affected access points re-register to the other WLC, then the wireless clients have to re-associate and therefore lose wireless connection during failover as it is not stateful failover. The failover is not transparent to the WLAN client. That is, the WLAN clients lose their WLAN connectivity during access point failover.
Access points and clients are not effected on the WLC that runs. This means that the fallback of the access point is not transparent to the clients. Only access points and clients on the failed WLC are effected.
In order to configure the WLAN Controller failover for Lightweight Access points, the Access Point must be configured correctly in a mobility group for the AP failover and each Wireless LAN Controller (WLC) must have the AP failover feature enabled.
Configure the Fallback Feature on WLC
The last step is to configure the Fallback feature on the controller. This feature ensures that the AP switches return to the first WLC when the WLC that comes back on line. Complete these steps:
From the GUI, choose Controller > General.A list of options appears on the General screen.
For the AP Fallback option, choose Enabled from the drop-down menu.
Click Apply.Note: It is sufficient to enable the Fallback feature on the secondary controller alone. But it is recommended to configure it on the primary WLC as well because it can be configured as a secondary controller for other access points
After you complete these steps, the setup is configured for WLC failover. When the primary controller (WLC-1, in this case) goes down, the APs automatically get registered with the secondary controller (WLC-2). The APs register back to the primary controller when the primary controller comes back on line. AP switching between the primary and secondary controllers also affects the wireless clients associated with these APs.
In controller software release 220.127.116.11, you can configure the wireless network so that the backup controller recognizes a join request from a higher-priority access point and, if necessary, disassociates a lower-priority access point as a means to provide an available port. In order to configure this feature, failover priority must be enabled on the network and assign priorities to the individual access points. By default, all access points are set to priority level 1, which is the lowest priority level.
Note: Be aware that Failover priority takes effect only if there are more association requests after a controller failure than there are available backup controller ports.
Wireless LAN Controller Failover Priority
During installation, Cisco recommends you connect all lightweight access points to a dedicated controller, and configure each lightweight access point for final operation. This step configures each lightweight access point for a primary, secondary, and tertiary controller and allows it to store the configured mobility group information. When sufficient controllers are deployed, if one controller fails, active access point client sessions are momentarily dropped while the dropped access point associates with another controller, which allows the client device to immediately reassociate and reauthenticate.