What condition triggers a switch to learn a new MAC address on a port?

• A. When the switch experiences a power loss.
• B. When the switch is rebooted.
• C. When the switch receives a frame from a device with a MAC not in its table.
• D. When the switch forwards a frame to a router.

Answer: (C)

Switches learn which devices live on which ports by listening to the source MAC address on every frame. When a frame arrives on a port and its source MAC isn’t yet in the MAC address table, the switch adds a new entry that maps that MAC to that port. This dynamic learning lets future frames be forwarded directly to the correct port instead of flooding. So the trigger for learning a new MAC address on a port is receiving a frame from a device whose MAC address isn’t already in the switch’s table. Power loss or reboot would clear the table, not create a new entry, and forwarding a frame to a router doesn’t by itself create a new MAC entry unless the frame’s source MAC is new.