Cisco WLC 9800 configured as N+1 and query on software upgrade

[palani2010]

Cisco WLC 9800 configured as N+1 with Primary and secondary WLC and query on software upgrade

Which WLC should upgrade first ?

Primary ?

Secondary ?

Both ?

What kind of impact which AP will face during software upgrade of N+1 WLCs

[MHM Cisco World]

Check this it can help you 

https://www.cisco.com/c/en/us/support/docs/wireless/catalyst-9800-series-wireless-controllers/215550-hitless-software-upgrade-on-catalyst-980.html

MHM

[Saikat Nandy]

As @MHM Cisco World said, one option is hitless upgrade. There is an another option seen people prefer sometime. 
# Before MW push image to both the controllers.
# Perform AP image predownload on Primary WLC - where the APs are. You can select the option during image push as well from GUI.
# While that is going on, perform activate (reload) on Secondary and Commit.
# Shutdown the interface of Secondary from switch side.
# Perform activate (reload) on Primary and Commit.
# Wait for all the APs to join back to Primary and once done, no-shut Secondary WLC switchports.

although we do have option of AP Fallback, but sometime APs get distributed across WLCs and if that happen, shutting down the Secondary WLC switchport during reload of Primary will help. In terms of impact, while your Primary will be reloading, you will be completely out of wireless, but if you perform the image push+ap image predownload before hitting the MW, then everything should get completed within 30mins.

[Leo Laohoo]

If both controllers are on N + 1, make sure Mobility Group is enabled because this is the lynchpin to make Hitless Upgrade work.  

We have done multiple 9800 controller firmware upgrade to a hospital campus in the middle of a working day and nobody reported experiencing any WiFi outage.  

Ironically, Hitless Upgrade is something Cisco does not like to talk about.