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Difficulty: MediumSwitching Concepts and VLANs

A workstation attached to switch Port 4 (assigned to VLAN 15) transmits a unicast frame intended for another host on the network. Upon receiving the frame, the switch inspects its MAC address table (CAM table) and finds no matching entry for the destination MAC address. Which action will the switch take to forward this frame?

  1. Flood the frame out all operational ports assigned to VLAN 15, except Port 4.Answer
  2. B
    Flood the frame out all active ports across all VLANs configured on the switch.
  3. C
    Drop the frame immediately and send an ICMP Destination Unreachable message to the source host.
  4. D
    Forward the frame directly to the switch designated as the Spanning Tree Protocol (STP) root bridge for processing.

Answer

The switch will flood the frame out all operational ports assigned to VLAN 15, excluding Port 4.
When a Layer 2 switch encounters a unicast frame with a destination MAC address not present in its CAM table, it treats the frame as an unknown unicast. The switch forwards a copy of the frame out every active port configured for that specific VLAN, excluding the port that received the frame, ensuring VLAN isolation is preserved.

Step-by-Step Solution

1
Examine ingress port and source MAC address
The switch notes the ingress port (Port 4) and its assigned VLAN (VLAN 15), then updates its MAC address table with the source MAC address.
MAC address learning happens on ingress based on the source address of received frames.
2
Perform destination MAC address lookup in the CAM table
The destination MAC address is not found (unknown unicast).
Layer 2 switches rely on exact MAC table matches to forward frames out specific single ports.
3
Execute unknown unicast forwarding logic within the VLAN boundary
The frame is flooded to all active ports assigned to VLAN 15, except the port on which it arrived (Port 4).
Flooding ensures the frame reaches its intended recipient while strictly maintaining VLAN broadcast domain isolation.

Key Concept

Switch Unknown Unicast Flooding and VLAN Broadcast Domain Isolation
Estimated Time:1m 15s
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Topics

Switching Concepts and VLANs

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