We have a lot of iOS clients on our wlans and I'm wanting to know if there is any reason NOT to enable 802.11r and k on these wlans? (we have ZD1100 running 9.9.1) Is there any downside or negative effect I'm not aware of? I'm always wary of settings that are defaulted to off... makes me wonder if there's a reason you wouldn't want to enable them.
From personal experience we have a large ruckus deployment and 802.11r isn't ready for prime time. We run a 802.1x network and with 802.11r IoS devices couldn't connect after the radius authentication hand shake. When we turned off 802.11r the devices could connect again. If your network is working fine then leave it off.
I am using the zero IT activation and have r/k enabled seems to work fine for most devices, however we have had a few 7-8 year old laptops that will not connect once the r/k was turned on. We where having issues with devices moving from AP to AP when mobile, with turning on the r/k that has improved that issue.
Certain extremely old pieces of hardware (especially 802.11b/g devices) refuse to connect or even recognize networks that have 802.11r enabled. I got burned by this after digging up some PC relics to salvage some data off of them.
But they are few and far in between. If you are concerned about this, you may want to make a separate "legacy" SSID for such devices.