Switch forwarding behavior: When a switch receives a frame and the destination MAC address is unknown (not present in the MAC table), what action does it take?

Introduction / Context:Ethernet switches learn MAC addresses by inspecting the source MAC of inbound frames and associating them with ingress ports. When the destination is unknown, the switch must still attempt delivery without prior knowledge, using a defined fallback behavior.

Given Data / Assumptions:

• The switch operates in store-and-forward or cut-through but still consults its MAC table.
• The destination MAC does not exist in the current table.
• The switch is not using higher-layer protocols for decision-making.

Concept / Approach:The IEEE bridging behavior (learning bridge) dictates that unknown unicast frames are flooded out all ports in the same VLAN except the port on which the frame arrived. The first return traffic from the true destination allows the switch to learn the proper MAC-to-port mapping.

Step-by-Step Solution:

Verification / Alternative check:Packet captures on multiple switch ports show replication of unknown unicast frames during the learning phase. After learning, only the correct egress port sees the traffic.

Why Other Options Are Wrong:

• Forwarding to the “first available link” is not defined behavior.
• Dropping would prevent initial communication.
• Switches do not perform name resolution; that is a host/DNS task.
• Sending only to a default gateway is incorrect within the same VLAN.

Common Pitfalls:Confusing broadcast with unknown unicast; forgetting VLAN separation (flooding occurs within VLAN boundaries).

Final Answer:Floods the frame out all ports except the ingress port