Detailed description of parameter:
https://blog.fem.tu-ilmenau.de/archives/1002-HowTo-enable-WiFi-roaming-with-hostapd-and-VLANs.html
OpenWRT:
https://www.reddit.com/r/openwrt/comments/7in2vq/problem_setting_up_80211r_ft
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Old version
https://www.reddit.com/r/openwrt/comments/515oea/finally_got_80211r_roaming_working