When a switch receives a frame and the source MAC address is already present in the MAC address table, which action occurs?

When a switch receives a frame and the source MAC address is already present in the MAC address table, which action occurs?

• A. The switch shares the MAC address table entry with any connected switches.
• B. The switch refreshes the timer on that entry.
• C. The switch does not forward the frame.
• D. The switch adds it to its MAC address table associated with the port number.

Answer: (B)

When a switch sees a frame, it learns the source MAC and maps it to the ingress port in its MAC address table. If that source MAC is already present, the switch doesn’t repopulate the table; instead it refreshes the age timer for that entry. This reset keeps the mapping from aging out, reflecting ongoing activity from that device. The switch will still forward the frame based on the destination MAC, and the entry update itself doesn’t add a new mapping or force a different port unless the device’s location actually changed.

When a switch sees a frame, it learns the source MAC and maps it to the ingress port in its MAC address table. If that source MAC is already present, the switch doesn’t repopulate the table; instead it refreshes the age timer for that entry. This reset keeps the mapping from aging out, reflecting ongoing activity from that device. The switch will still forward the frame based on the destination MAC, and the entry update itself doesn’t add a new mapping or force a different port unless the device’s location actually changed.