802.11r Fast Transition how to understand that FT works?

I have not even tried to measure any difference between them.

If you wish to try WPA3-SAE on your own, I'll give you some clues. The minimum config you need is to enable 802.11r, and make sure to DISABLE Generate PMK locally (ft_psk_generate_local). This option is currently not working with WPA3.

OpenWRT will provide default values for the keys and identifiers, so there's no need to set them: nas_identifier is taken from the BSSID; mobility_domain will be the first 4 hex digits of the md5sum of the SSID; FT key is the md5sum of mobility_domain/radius_scret, and then used to set r0kh and r1hk with wildcard MACs and NAS identifiers. Beware that up to OpenWrt 21.02.xx, in case of PSK/SAE, the input is limited to 4 bytes (radius_secret will be empty), so it is safer to set your own key, but it will work without it nonetheless. From 22.03 onward, a 128-bit key will be generated from the mobility_domain & psk, and hostpad will then transform it into a 256-bit key.
Also, the transition will occur with any AP that uses that same key (nas_id and MAC will not be checked), which is not such a bit deal. If you wish to set your key with the wildcard MAC/nas_id, then use
ff:ff:ff:ff:ff:ff,*,Use-Your-Own-256bit-hex-key-here for r0kh (128-bit keys are
00:00:00:00:00:00,00:00:00:00:00:00,Repeat-Your-256bit-hex-key-here for r1kh

Support for 128-bit keys was maintained in hostapd for backward compatibility.

You may check /var/run/hostapd-phy* just to be sure everything is right:

mobility_domain=0123 # must be the same for all APs & interfaces
ft_psk_generate_local=0 # it will not work with WPA3 or EAP if not 0
nas_identifier=e89fxxxxxxd1 # this should be unique for every interface, default is the bssid
r0kh=ff:ff:ff:ff:ff:ff * 00112233445566778899aabbccddeeff # If using wildcard MAC/nas_id, this should be the same across all APs & interfaces within the same mobility domain
r1kh=00:00:00:00:00:00 00:00:00:00:00:00 00112233445566778899aabbccddeeff # then this key should match the one in r0hk above
wpa_key_mgmt=SAE FT-SAE # It should have FT-SAE, FT-PSK or FT-EAP

Since this appears to be relevant 4 years after I wrote it, I edited the post in Feb/2024 to fix the example by using commas to separate keys from MACs, to point out the keys should be 256 bits long, and to update the status of default key-generation after OpenWrt 22.03.

TLDR: starting with OpenWrt 22.03, when not using WPA3, all you need to do to make FT work is to enable 802.11r from LuCI (option ieee80211r '1' in /etc/config/wireless). With WPA3, including mixed mode, you must disable "Generate PMK locally" in LuCI (option ft_psk_generate_local '0' in /etc/config/wireless).
With current snapshots, the Generate PMK locally option will be automatically disabled when using WPA3, and will not even show up in LuCI.