Has your Cisco 8510 WLC failed in a High-Availaibility configuration and you just received your replacement from RMA? Did you already set it up and are now stuck with the output below due to a software mismatch?

(Cisco Controller) >show redundancy summary
Redundancy Mode = SSO ENABLED
Local State = MAINTENANCE
Peer State = UNKNOWN - Communication Down
Unit = Secondary
Unit ID = 00:00:00:00:00:00
Redundancy State = Non Redundant
Mobility MAC = 00:00:00:00:00:00
Redundancy Port = UP
Maintenance Mode = Enabled
Maintenance cause= Secondary unit has booted without pairing up with peer

Fear not! Follow these steps if you have already enabled HA during the initial setup while under the impression that the new box with a different version will download the code from the primary WLC.

Solution:

Step 1. Disconnect the RP port cable from the controller that you just replaced.

Step 2. Execute the following command: 

config redundancy mode disable

which will prompt you that the system will reboot.

Step 3. Once the controller boots up, you will have to change the management IP of the controller and make it a standalone WLC. To make those changes, execute the following: 

config wlan disable all
config interface address management <IP ADDRESS> <NETMASK> <GATEWAY>
config port adminmode all enable

Step 4. Login to the newly minted standalone controller and upgrade/downgrade the image either through CLI or GUI. This will take approximately 10-15 minutes to flash the controller.

Step 5. Once upgrade/downgrade has completed. Reconnect the RP port cable back to the controller and execute the following command:

config redundancy mode sso

You may be prompted “Mandatory parameters to make redundancy are not configured. Please configure Local and Peer Redundancy Management IP addresses before enabling redundancy.” You know what to do. That is self-explanatory!

Step 6. Profit!