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Question

Difficulty: MediumSwitching Concepts and VLANs

A managed Layer 2 switch receives an Ethernet frame on access port FastEthernet 0/1, which is assigned to VLAN 20. The switch inspects its MAC address table and finds an entry for the source MAC address, but the destination MAC address is not present in the table. Which action will the switch take to forward this frame?

  1. Flood the frame out of all operational ports assigned to VLAN 20, excluding port FastEthernet 0/1.Answer
  2. B
    Forward the frame out of all operational ports on the switch regardless of their VLAN assignment.
  3. C
    Add an 802.1Q tag for VLAN 20 to the frame header and forward it out of the native VLAN port.
  4. D
    Drop the frame immediately and transmit an ICMP destination unreachable notification to the sender.

Answer

Flood the frame out of all operational ports assigned to VLAN 20, excluding port FastEthernet 0/1.
When a Layer 2 switch encounters an unknown destination MAC address, it handles the frame as unknown unicast by flooding it to all active interfaces assigned to the frame's ingress VLAN, excluding the port on which the frame entered.

Step-by-Step Solution

1
Examine the switch forwarding table state for the destination MAC address.
The destination MAC address is absent from the MAC address table, classifying the packet as an unknown unicast frame.
Layer 2 switches rely on MAC address table lookups to perform selective frame filtering and forwarding.
2
Apply Layer 2 unknown unicast forwarding logic within the ingress VLAN.
The switch floods the frame out of every active interface in VLAN 20 except the receiving interface FastEthernet 0/1.
Flooding allows the frame to reach the intended destination host while preserving VLAN broadcast domain isolation and preventing loops on the ingress interface.

Key Concept

Layer 2 Unknown Unicast Flooding and VLAN Isolation
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Topics

Switching Concepts and VLANs

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