ACS is not mandatory.
Just to be sure we are talking about the same thing "Infrastructure authentication" is the authentication of WDS Client AP to WDS master AP.
You can use any "standard" LEAP RADIUS Server (There is no LEAP standard ;-) ). ACS is of course the best ;-), but you also have the integrated LEAP RADIUS Server in the IOS AP (free). Other LEAP RADIUS Server should also work.
Hope this helps,
Vincent
(Please rate the post if it solve the question)
