When 802.11r, also known as Fast BSS Transition (FT) or fast secure roaming, was released and ratified, there was no doubt that it was going to improve the roaming process for wireless devices. Unfortunately it only worked in a BSS for those client devices that actually support that feature, those who didn't just couldn't connect to the FT enabled SSID.
I'm not going into the details of FT since there's a lot of it, but I'd like to mention that can happen over-the-air and over-the-DS (Distribution System), for further information I recommend to read the CWSP book, you can find a very detailed explanation on how it works, which is very similar in the way an 802.1x/EAP exchange creates a Master Session Key (MSK) which is used to create the Pairwise Master Key (PMK) for non-FT raoming, all that but with a more much complex key management solution.
So, the good news for those of you who do Cisco based WLANs, in AirOS Version 8 Cisco introduced a feature called "802.11r Mixed Mode" which basically removes the restriction of creating a separate SSID for 802.11r support. Non-802.11r clients with updated drivers can join 802.11r-enabled SSID.
The trick is easy, under the SSID configuration you need to enable 802.1x for both options, if you pay attention you will notice that there's two 802.1x options, the old option and FT 802.1x., that allows non-802.11r client to join even if FT is enabled.
How do I know if it works? Cisco has been working with Apple lately, so if you have an apple device an a old computer, may be not that old, but a computer or device with no support for 802.11r, have them both connected to your 802.11r SSID and this is what you will see:
This is an IPhone 6s
This is an IPad Mini
HP Laptop with Intel Advanced-N 6235 (Driver 22.214.171.124)
All devices were connected to the same SSID, so now you know how to check if your devices support 802.11r.