User disconnect

Hello,     Does anyone know how to solve following issue? In this case : 1. NAS (Chillispot based access point) 2. Freeradius If NAS is shutdowned, the freeradius keeps all user logins of that NAS. In this case, the users session is alive. In fact, freeradius doesn't know if NAS's status is shutdowned or running. Is there any configuration that freeradius disconnects the users automatically when NAS is shutdowned? On the other hand, I would like to terminate the user's session when NAS is shutdowned or unavailable.