Why AP lost Joining from the WLC?

[eeebbunee]

Hello Professionals,

Some of 2802 Series APs are suddenly lost connection from WLC even they are up and running.
It happens for only some of APs, at the sametime with no reason.

I searched the solution from here, and they recommended to change date to be past.
It resolves the issue as a temporary, and it happens again few weeks later. 
I don't understand why date and time can resolve this issue, is there any permanent resolution that we can try?

Can it possibly be the certificates related issue?

I appreciate your comments.

[Haydn Andrews]

you are potentially hitting this field notice:

https://www.cisco.com/c/en/us/support/docs/field-notices/639/fn63942.html 

Particully this bug CSCvs22835

You need to be running 8.5.160 or later and apply then:

If you have upgraded to software with this fix, then:

* configure "config ap cert-expiry-ignore mic enable" on the WLC

* set your date back, so that unfixed APs can join

* wait till all AP-COS APs have downloaded the fixed code, and have rebooted and rejoined

* now you can set the WLC date back to the current date

[Leo Laohoo]

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@eeebbunee wrote:

is there any permanent resolution that we can try?

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Update the firmware of the controller.