How does a switch learn MAC addresses and forward frames in a LAN?

• A. It builds a MAC address table from source MACs and forwards frames to destination ports, flooding unknown destinations.
• B. It uses an IP routing table to forward frames.
• C. It relies on ARP to forward frames.
• D. It uses DNS to resolve destinations.

Answer: (A)

Switches operate at the data link layer and learn how to forward traffic by building a MAC address table from the frames they receive. As each frame arrives, the switch notes the source MAC address and the port it came on, associating that MAC with that port. This creates a map of where each device can be reached at the switch level. When a frame with a destination MAC is received, the switch checks the table: if it knows which port that destination MAC is on, it forwards the frame only to that port. If the destination MAC isn’t in the table yet, the switch doesn’t know where to send it, so it floods the frame out all ports in the same VLAN except the one it came from. Over time, entries age out if they aren’t refreshed by new frames, keeping the table current. This is why forwarding is done using MAC addresses on a LAN, not IP routing. ARP is used to map IP addresses to MAC addresses, not to decide how frames are forwarded, and DNS has no role in frame forwarding.