So a migration from good old Cisco AirOS 5508 redundant wireless LAN controllers to the brand spanking new (yikes!) Catalyst 9800-40 chassis in an SSO design (HA basically).
In running Amsterdam 17.3.4, we were rapidly replacing our nearly EOL 2602s, 2702s, 3702s, 1572s and even 2802s as these will lose support on the 9800 controller with the next major release version of firmware.
However we noticed that some 9120s were unexpectedly not joining the WLC as we got new ones in. There were all different versions from the factory - this version (VID) can be confirmed on the label on the access point itself, will be small print in the upper right corner - V01, V03, V05, V07 and the like. You can also check with show inventory command if you have CLI access.
We had gotten a box of V07 and after hanging all of them was when we realized none of them would join which was totally unexpected, hence finding the field notice 72424.
It turned out there was a bug reported and in the Cisco tool where certain versions of APs and V07 APs would not join if they joined after a certain date in December 2022 or January 2023.
Of course, however, after doing an emergency code upgrade to the recommended 17.3.6 version, all of the 9120s then happily joined, and we lost all remaining 3702s and 1572s to stuck in Downloading and or failing to join due to a DTLS - another bug with a workaround.
Workaround (listed to be performed before upgrading to 17.3.6...lol) is as follows:
disable NTP on the controller > change date on WLC to recent enough date before Dec. 4, 2022 > wait for APs to finish downloading and able to join the controller > reboot any APs failing to join after upgrade completes > enable NTP again once all APs are joined again.
I am still in the process of testing if this workaround, listed to be done before you upgrade, on a notice for issues after upgrade, applies or not. Otherwise looks like in the second Field Notice there might be additional AP service packs available for 17.3.6 (or relevant/related others with similar problem) to resolve that issue if already installed.
You'll need a Cisco account to view in the bug tool links I believe, FYI.
Sources:
https://www.cisco.com/c/en/us/support/docs/field-notices/724/fn72424.html
https://bst.cloudapps.cisco.com/bugsearch/bug/CSCwd80290