Consult about NAS

Hello everybody I'm implementing a freeradius server, with some NAS, that have pppoe server that validate their users in that Freeradius server, but I have a little problem!! IF some NAS turn off, the radius doesn't disconnect the users that was login in that NAS. The radius clean that client, when the NAS turn on again, and in that time the clients can't reconnect. Someone know how to have an attribute, like time-out, for the NAS, that check if NASs are working, and if not kill all the users connect in that dead NAS? Thanks Rafael Lore