Allied Telesis AT-S39 - Manual (Page 68)

Section II: Local and Telnet Management
68
❑ To stop the switch from learning new dynamic MAC addresses
and have it forward frames based only on static MAC addresses
and on those dynamic addresses that it has already learned, type
4 to select Lock all the ports now.
Note
Only one security level can be active on a switch at a time.
A change to the security level is immediately activated on the
switch.