- "Pure" mode, where the only Authentication Key Management (AKM) method listed in the Robust Security Network (RSN) Information Element is a FT method. Common FT methods are 802.1X FT or PSK FT. Clients that don't support 802.11r will not be able to connect to this type of WLAN. They may not even see it.
- "Mixed" mode, where both FT and non-FT AKM methods are included in the AKM suite. This mode allows both clients that do and don't support FT to connect. There will still be clients that get confused by the presence of a FT AKM. Notably, if you change an existing WLAN to mixed mode FT, macOS clients may not be able to connect until you delete the WLAN profile and re-connect.
- Adaptive 11r. In this mode, the beacon does not advertise the FT AKM at all, but will use FT when supported clients connect.
Let's look at beacons for different types of FT networks. First, here are the relevant IEs for a non-FT network.
|RSN IE For non-FT WLAN|
|RSN and Mobility Domain IEs|
|Adaptive 11r RSN and MD IEs|
With Adaptive 11r enabled on a WLAN, the RSN IE does not have any FT methods, but the Mobility Domain IE is present. Beacons will also contain the below IE, even if Aironet IE is disabled on the WLAN.
|Adaptive 11r Aironet IE|
|Apple Vendor IE|
|Association Request From Android Phone|
This frame looks normal, and is what you would expect when a client is connecting to a non-FT WLAN. There is no Mobility Domain IE, which implies that the client saw that there was no FT AKM method in the RSN IE. The client determined that the network did not support FT, and did not include the Mobility Domain IE. The expanded RSN IE shows that the client will use PSK as the Authentication Key Management. What happens when a client that supports Adaptive 11r connects?
|Association Request from an iPad|
The RSN IE shows that the AKM chosen was FT using PSK, which is not advertised in the beacons! This is the secret sauce of Adaptive 11r. You can also see the magic happen if you run "debug client" and "debug ft events enable" while an Adaptive 11r clients connects. Below is the output of debug commands while my iPad connected. Interesting lines are highlighted in red.
The most interesting part of the output is "Setting adaptive AKM 4 into RSN Data at 19". AKM 4 is short for FT using PSK, and "Data at 19" specifies position in the RSN IE that defines the AKM method. If you issue a "show client detail" command for an Adaptive 11r client, you will see that AKM method listed is an FT one.
Roaming with an Adaptive 11r compatible client is the same as roaming with regular old FT. When the client sends authentication and reassociation requests to a new AP, it includes Mobility Domain and Fast BSS Transition IEs. Roam time with the iPad I tested with was less than 10 ms. (That's how long it took to go from Authentication to the first data packet sent by the iPad. Getting the iPad to roam in the first place was a challenge, given the environment I was testing with).
I like this feature from Cisco and Apple. There appears to be no risk in breaking connectivity for non-iOS devices if you enable it, and the upside for supported devices is really good. Hopefully this blog gave readers some insight into how this feature works.