How does a switch learn which port to reach a destination MAC address?

• A. By flooding all frames on all ports always.
• B. By inspecting source MAC addresses on frames and updating its CAM table with the port for that MAC.
• C. By performing ARP queries for the destination IP.
• D. By using the destination MAC to look up a routing table.

Answer: (B)

Switches learn where devices are by listening to the source MAC addresses on frames as they enter. Each time a frame arrives on a particular port, the switch records that the frame’s source MAC is reachable via that port in its MAC address table (often stored in CAM). With this mapping, when a frame destined for that MAC arrives, the switch can forward it only to the correct port. If the destination MAC isn’t in the table yet, the switch floods the frame to all ports in the same collision domain to locate the device and, in the process, learns which port that device is on. This learning uses the source MAC, not the destination, to build the table. ARP and IP routing aren’t used to determine which port to reach a MAC address; ARP resolves IP to MAC at Layer 3, and routing tables operate on IP addresses, not MACs.