What happens when a switch receives a frame with an unknown destination MAC address?

• A. It drops the frame.
• B. It floods the frame to all ports in the VLAN except the source.
• C. It floods the frame only to ports that might have the destination, based on partial CAM knowledge.
• D. It forwards to a default gateway.

Answer: (C)

When a switch sees a frame with a destination MAC it hasn’t learned yet, it can’t forward it to a single port because the destination’s location isn’t known. The switch uses the CAM table to map MAC addresses to ports, and if the destination MAC isn’t present, there’s no unique egress port to choose. The correct behavior is to flood the frame out all ports in the same VLAN except the port it arrived on. This ensures the frame reaches the destination if it’s on the network and helps the switch learn the correct port when the destination responds, so future frames can be sent directly. Dropping the frame would break connectivity, and sending to a gateway isn’t part of MAC-level forwarding. Flooding to only a subset of ports based on partial CAM knowledge isn’t reliable because there’s no guaranteed location for the destination without a CAM entry.