This durable TruthValue, when set to true, switches the bridge port into a locked-down forwarding mode: it purges its learned MAC forwarding table entries, stops learning new ones, and forwards only frames whose source and destination addresses have been explicitly configured by management. Admins enable this to lock down a port to a known, fixed set of MAC addresses, a common security measure against unauthorized devices communicating through that port. Setting etsysESwitchAddressFilter to true(1) on a port serving a fixed server would block traffic from any newly learned, unrecognized MAC address plugged into that same port.