On an Ethernet switch, a frame arrives on a port and the destination MAC address is already in the MAC address table, but the source MAC address is not yet learned. What action does the switch take with this frame and its MAC address table?

Introduction / Context:
Ethernet switches use a learning process to build a MAC address table that maps MAC addresses to switch ports. Every time a frame arrives, the switch uses the source MAC address to update its table. When the destination MAC address is known, the frame can be forwarded efficiently out a single port instead of being flooded. This question tests whether you understand the basic learning and forwarding behavior of a switch when the destination is known but the source is a new device that has just joined the network.

Given Data / Assumptions:

• A switch receives a frame on one of its ports.
• The source MAC address has not previously been seen and is not in the MAC address table.
• The destination MAC address already exists in the MAC address table with an associated port.
• The switch is operating in normal learning and forwarding mode without any security features such as port security that would alter behavior.

Concept / Approach:
Switches operate as transparent bridges that learn source addresses and use destination addresses for forwarding decisions. The learning step is: when a frame arrives, the switch maps the source MAC address to the incoming port and adds or updates this entry in the MAC table. The forwarding step is: the switch looks up the destination MAC address in the table. If found, it forwards the frame only out that corresponding port. If not found, it floods the frame. In this scenario, the destination is known, so no flooding is needed. Instead, the switch both learns the new source MAC and forwards the frame unicast to the known destination port.

Step-by-Step Solution:
1. The switch receives the frame on a specific interface. 2. It examines the source MAC address and sees that it is not yet in the MAC address table. 3. The switch creates or updates an entry in the MAC address table mapping the new source MAC address to the incoming port. 4. The switch then looks up the destination MAC address in the MAC table. 5. Since the destination MAC address is in the table, the switch determines the correct egress port. 6. The switch forwards the frame only out that destination port, providing efficient unicast forwarding.

Verification / Alternative check:
You can verify this by recalling the difference between unknown unicast and known unicast behavior. When the destination is unknown, flooding occurs. When the destination is known, the switch forwards the frame only to the correct port. Learning the source address is independent of whether the destination is known. The switch always learns the source from the incoming frame, which ensures that subsequent frames sent to the new host can be forwarded with unicast efficiency.

Why Other Options Are Wrong:
Option a: The switch does not discard frames simply because the source MAC address is not known; learning new MAC addresses is normal behavior. Option b: Flooding is required only when the destination MAC address is unknown, not when the source is new. Option d: The destination MAC address is already in the MAC address table, so there is no need to add it again; the important action is learning the source. Option e: Queuing the frame until manual updates is not standard Ethernet switch behavior and would break basic connectivity.

Common Pitfalls:
Students sometimes think that any unknown address triggers flooding, but it is specifically an unknown destination address that causes flooding. The source address being unknown is exactly how a switch learns. Another pitfall is forgetting that learning and forwarding happen independently: even if the destination is known, the switch still updates the source mapping. By remembering that the MAC table is built using source addresses and used for forwarding based on destination addresses, you can easily deduce the correct behavior in scenarios like this.

Final Answer:
The switch will add the new source MAC address and its incoming port to the MAC address table and forward the frame only out the destination port.