A stale session means a bad session, which is generated when the router suddenly disconnects from the radius server. For example, if the router shuts down unexpectedly or somehow disconnects or unable to send back an interim update to the radius server, for this kind of reason stale session generates. Radius server thinks that user still online because Radius server did not receive any disconnect request from the router, that's why Radius show offline users online. In this situation, Zal Pro will automatically clear the stale sessions, so that both Zal Pro active users and router active users match. You don't need to do anything for the stale session, Zal Pro will do it all itself. You may see that sometimes users are auto disconnecting from the router, its because of the stale sessions. Those users will be auto connect again within a minute. So don't worry about it. If you want you can disable the auto clear stale session from settings.
1. Check both Zal Pro & Router time, both time should match 2. Set interim update 00:03:00 or 00:05:00 3. Set "Stale Session Clear Time" in Zal Pro setting higher than router interim update