AP ios version is always keeping same with wlc?

[wfqk]

Hello, Anyone can answer my question? We got wlc and APs. When we try to join APs to wlc, sometimes it failed due to different version. so we upgraded ios for APs then the version of AP is newer than wlc, and AP still cannot join wlc. So we think we might need to upgrade ios of wlc to match AP ios. Do you think if this is a good way to resolve this kind of issue? 

Second question is after the AP join to wlc, I can see the version of AP and wlc are same. I would like you to confirm if the version of both should always be same after AP join wlc no matter what version they are before the joining? Thank you

[Scott Fella]

First off, the WLC will provide the image to the access point after the join.  Now the requirement is that the wlc, wlc image, ap model is in alignment with being supported in order for the ap to join.  So make sure that you look at the wlc matrix and verify that the wlc and ap are supported.

Cisco Wireless Solutions Software Compatibility Matrix - Cisco

[wfqk]

Thank you for your reply.

So you agree that AP ios version is always keeping same with wlc after the joining? 

[Scott Fella]

That is how it’s always been for 20+ years:)

[Rich R]

Like Scott says that is the general rule: AP software must = WLC software.

But there are some exceptions - some by accident (bug) and some by design:

https://bst.cloudapps.cisco.com/bugsearch/bug/CSCve14291 meant that after upgrading from affected releases APs did not reload and kept on runnning old code connected to WLC which was on new code.  AP has to be manually reloaded to fix this and the bug is fixed in latest code.

During ISSU upgrade on 9800 series APs are upgraded in groups (AP rolling upgrade) to minimise client impact.  As a result some APs temporarily run old code while the upgrades are in progress. https://www.cisco.com/c/en/us/td/docs/wireless/controller/9800/17-3/config-guide/b_wl_17_3_cg/m_issu.html

In relation to your original question - an AP should still be able to join the WLC when running older or newer version of code and then immediately download the correct version from the WLC but that doesn't always work if the versions are incompatible due to a variety of bugs and changes between versions.  If there is a big difference between versions you're more likely to see problems.  Reset the AP to factory default then try again.  If that doesn't work then manually load the correct version onto the AP.  The matrix lists the version numbers needed for each release.

[wfqk]

If the two version of ios in ap and wlc are compatible and if the version of AP is newer than wlc, usually the AP version would get back to wlc version after the joining? 

[Rich R]

Correct