What action occurs when a switch receives a frame and the source MAC address is already in the MAC address table?

• A. Refreshes the timer on that entry.
• B. Adds it to the MAC address table.
• C. Forwards the frame to the port.
• D. Sends to a connected router.

Answer: (A)

The switch updates its MAC address table entry for that source. When frames arrive, the switch learns which port the frame came from and records the source MAC with that port. If that source MAC is already in the table, the switch simply refreshes the aging timer on that entry to show recent activity. This keeps the mapping valid longer and prevents it from aging out.

Other options don’t fit because the entry already exists, so no new entry is created; the action described isn’t about forwarding decisions or sending to a router—those depend on the destination MAC, not the source.