This read-write enum disables MAC address learning on this port in one of several directions, working alongside a configured allow-source or allow-destination MAC filter: normal(1) means no port security is active, secure-src-addrs(2) stops learning new source addresses, secure-dest-addrs(3) stops learning destination addresses, and secure-src-and-dest-addrs(4) stops both. An admin locking down a port to a known, fixed set of devices would combine this with a MAC filter table entry so the port never learns and forwards to any unauthorized station. Setting secure-src-and-dest-addrs(4) on a port meant for a single dedicated server prevents the switch from ever learning a rogue device plugged into that same port.