How does a switch learn MAC addresses?
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Correct answer: adds unknown source MAC addresses to the CAM table.
Why this is the answer
A switch learns MAC addresses by examining the source MAC address of incoming frames. When a frame arrives on a port, the switch records the source MAC address and the port it arrived on in its Content Addressable Memory (CAM) table. This process is called MAC address learning. If the source MAC address is already in the CAM table, the switch updates the entry's timestamp. This ensures the switch always has the most current location for a device. The other options are incorrect: A switch does not send frames back to the source to verify availability; it forwards based on destination MAC. A switch does not rewrite source or destination MAC addresses; that's typically a Layer 3 function (routing) or NAT. A switch does not drop unknown MAC addresses; it floods frames with unknown destination MACs out all ports except the ingress port.
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