One of the reasons for not putting all APs in the same network, is due to that fact that all roaming information, 11k/v, etc are shared between APs witihn the same network. If you have APs at different sites, but in the same network, they would essentially be sharing neighbor information, keys and sessions. Especially when using 11r Fast Transition. So you'd have APs at site A having information about clients at Site B, and reverse.
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