In a switch network, what happens when a new source MAC address is observed on a port?

• A. It clears the MAC address table.
• B. It updates the MAC address table with the new source MAC and port.
• C. It forwards the frame to the router.
• D. It floods the frame to all ports.

Answer: (B)

Learning a source MAC address and the port it arrived on is how a switch builds its MAC address table. When a frame comes in, the switch notes the source MAC and records which ingress port it came from. If that MAC has not been seen before, an entry is created; if it has been seen on another port, the entry is updated to reflect the current port. This dynamic learning allows the switch to forward future frames directly to the correct port and reduces unnecessary flooding. Entries can age out if the device becomes silent for a while, after which the learning would occur again as new frames arrive.

So, the behavior described is that the switch updates the MAC address table with the new source MAC and the port it was received on.