Switch learning and forwarding behavior: If a switch receives a frame where the source MAC is not yet in the MAC table but the destination MAC is present, what does the switch do?

Introduction / Context:
Learning bridges and switches build a MAC table by observing source addresses on incoming frames. Correctly understanding this process helps explain forwarding, flooding, and convergence behaviors in Ethernet LANs.

Given Data / Assumptions:

• Source MAC is currently unknown to the switch.
• Destination MAC is known and mapped to a specific egress port.
• No security features (like port security) are preventing learning.

Concept / Approach:
Switches learn on source, forward based on destination. Upon receiving a frame, the switch adds the source MAC and ingress port to the table. If the destination is known, it forwards only out that destination port (unicast forwarding).

Step-by-Step Solution:

Verification / Alternative check:
show mac address-table after traffic shows the learned source entry; subsequent frames will be forwarded without flooding.

Why Other Options Are Wrong:
Flooding: Occurs only when destination is unknown or broadcast/multicast.
Adding the destination: Destination learning does not occur on lookup; learning is based on source addresses.

Common Pitfalls:
Assuming learning occurs on both source and destination; it does not. Only sources trigger learning.

Final Answer:
Add the source address and port to the MAC address table and forward the frame out the destination port