What does a switch do with an unknown unicast frame?

A switch handles an unknown unicast frame by flooding it out all ports, except for the port on which the frame was received. When a switch receives a frame with a destination MAC address that is not in its MAC address table, it does not know which specific port to send the frame out. To ensure that the frame reaches its intended destination, the switch takes the precautionary measure of broadcasting the frame to all ports. This approach allows the switch to discover the appropriate port for the destination MAC address, as the intended device will respond, allowing the switch to learn and remember the MAC address association for future traffic.

This behavior is fundamental to how switches operate within a network, helping to dynamically build their MAC address tables through learning as devices communicate. As the switch learns the MAC addresses of devices that respond, it can update its table to send future traffic directly to the correct port, improving overall network efficiency.