Inter-Release Controller Mobility support for locally switched WLAN

[g.pregnolato]

In order to provide hardware support, I have the necessity to register certain Flex Connect APs with two different Catalyst 9800 Wireless LAN Controllers (WLCs) running different software versions.

To guarantee uninterrupted mobility across the APs managed by these two WLCs, I am currently investigating whether the configuration of a mobility tunnel between them supports seamless roaming for locally switched Wireless LANs (WLANs) between two FlexConnect APs that are registered to the two different WLCs.

More generally, inter-controller roaming is supported in FlexConnect deployments?

[marce1000]

  - Ref : https://www.cisco.com/c/en/us/td/docs/wireless/controller/9800/config-guide/ConfiguringIRCM/b-configuring-inter-release-controller-mobility-in-wireless-deployments-supporting-aireos-and-catalyst-9800-controllers/m-restrictions-for-ircm-.html
                        >...IRCM is not supported in FlexConnect and FlexConnect+Bridge modes.

 M.

[Rich R]

This is not IRCM because it's not between AireOS and IOS-XE so not sure that statement applies.  However I can't see anything documenting it in the 9800 docs.  However on AireOS there's a neat table: 
https://www.cisco.com/c/en/us/support/docs/wireless/5500-series-wireless-controllers/112042-technote-wlc-00.html#anc13
which basically says the answer is NO.

But ignoring that for the moment let's go back to your intro ... "In order to provide hardware support, I have the necessity to register certain Flex Connect APs with two different Catalyst 9800 Wireless LAN Controllers (WLCs) running different software versions."
Can you be more specific?  Is this a case of one running 17.3.x for IOS APs and one running 17.6.x or 17.9.x for catalyst 91xx models?
If so then the answer to your problem is 17.9.3 which is due out in the next few weeks - that will support the IOS APs like 17.3 and that support will continue for all future 17.9.x releases (but almost certainly nothing after that despite rumours that it might be extended to 17.12 - I doubt it).  So when 17.9.3 is released you can put all your APs on the same WLC but plan to decommission the IOS APs before 17.9 end of life.

[Rich R]

Oh I did find this: https://www.cisco.com/c/en/us/td/docs/wireless/compatibility/matrix/compatibility-matrix.html#ircm-ios-xe-rls
So in theory there is inter release compatibility between 17.3 and 17.9 but I still don't think that will help you with flex local switched roaming.