The default behavior of a switch is to broadcast all traffic for which the destination MAC address is unknown.
However in certain circumstances, you wouldn’t want a port to receive unknown unicast or multicast packet floods.
This can be accomplished by turning on port blocking on the switch interface.
Switch# show interfaces fastEthernet 1/0/20 switchport | in Unknown
Unknown Unicast Traffic: Not Allowed
Unknown Multicast Traffic: Not Allowed