Procedure for replacing the primary unit in an 8540 WLC HA pair

[mat_rouch]

We have an HA failover pair of 8540 WLC controllers.  The primary unit in the pair has thrown an error (though it is still functional) and cisco has sent us a replacement unit.  I have been unable to find any official procedural documentation for replacing one unit of an HA pair.  The closest I have seen so far is a brief procedure for an 8510 HA pair, posted on reddit 5 years ago  (https://www.reddit.com/r/Cisco/comments/b3r6kg/primary_8510_wlc_needs_to_be_replaced/).  Can anyone point me in the right direction?  Our WLCs are running version 8.10.

Thanks,

-Mathew Rouch

[Leo Laohoo]

1.  Make sure the replacement unit has the same firmware (and AP Bundle) as the existing pair. 
2.  Make sure the replacement unit is factory defaulted. 
3.  Make sure the replacement unit have SSD in them. 
4.  Upgrade the CIMC of the replacement unit. 
5.  Configure the replacement unit with the most basic configuration such as, Management IP address and Redundancy IP address (Controller > Redundancy > Global Configuration).
6.  Fail over to the secondary unit. 
7.  Remove the faulty unit.  
8.  Power up and connect the replacement unit to the network.  

[Rich R]

And remember the HA SKU secondary WLC inherits the license from the primary so you will need to configure the license (usually RTU) on the new primary too.

For extra info see:
https://www.cisco.com/c/en/us/support/docs/interfaces-modules/wireless-services-module-2-wism2/117729-configure-wlan-00.html#anc7 and 
https://www.cisco.com/c/en/us/td/docs/wireless/controller/technotes/8-7/High_Availability_DG.html

[ammahend]

All great points, make sure you have a backup and if using any certificates, then they are unique to controller so install certs before adding to HA pair.

[Rich R]

Good point @ammahend which also reminds me that the network routes must also be configured separately on each WLC in the pair so you'll need to add those to the new one too.

Also remember to configure the CIMC before you join the new WLC to HA.

[mat_rouch]

Thanks for the replies, everyone.  Correction to my original post: the reddit link I found was actually referencing a cisco doc, though it is 9 years out of date and I can find nothing newer.  Here it is: https://www.cisco.com/c/en/us/support/docs/interfaces-modules/wireless-services-module-2-wism2/117729-configure-wlan-00.html .  When I opened a TAC case they gave me a link to that same 9-year-old doc. 

I have summarized your recommendations and forwarded a list of them back to TAC, asking them to comment on any that did not appear in their originally-proposed procedure, e.g. "are these steps also needed" and "if so, can I get details for them".  

-Mat