When does a switch learn the MAC address of a connected device?

When does a switch learn the MAC address of a connected device?

• A. On power-up
• B. On receiving the first frame
• C. On periodic keep-alive
• D. On ARP request

Answer: (B)

Switches learn a device’s MAC address by watching the source MAC on frames that arrive on each port. When a connected device first transmits a frame, the switch notes the sender’s MAC address and the port it came in on, and it stores that mapping in its MAC (CAM) table. That way, future frames addressed to that MAC can be forwarded directly to the correct port. There’s no learning on power-up or from keep-alives; it happens only once a frame from the device is seen. ARP requests are just one type of frame; learning occurs for the source MAC whenever any frame from that device is received.

Switches learn a device’s MAC address by watching the source MAC on frames that arrive on each port. When a connected device first transmits a frame, the switch notes the sender’s MAC address and the port it came in on, and it stores that mapping in its MAC (CAM) table. That way, future frames addressed to that MAC can be forwarded directly to the correct port. There’s no learning on power-up or from keep-alives; it happens only once a frame from the device is seen. ARP requests are just one type of frame; learning occurs for the source MAC whenever any frame from that device is received.