The method used to delete old unused entries refresh_flag: The high level sets a flag when it adds or updates an entry, the Switch clears the flag each time, when it learns the station once more. aging_flag: The high level clears a flag when it adds or updates an entry, the Switch sets the flag each time, when it learns the station once more. aging_time: The high level defines a timeout, the Switch sends a message to the high level each time when the station doesn't send messages during interval between timeout and 2*timeout. Boundaries: The Switch doesn't support any mechanism, and the high level uses two boundaries: red and yellow. When the number of entries exceeds the red boundary, the high level deletes the oldest entries.