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Difficulty: HardL2 Switching Concepts and MAC Table Operation

A Cisco Catalyst switch receives an Ethernet frame on ingress port GigabitEthernet0/1 with Source MAC address 0010.ab12.3456 and Destination MAC address 0020.cd78.90ab. The switch's current MAC address table records 0010.ab12.3456 as learned on GigabitEthernet0/3, but has no entry for 0020.cd78.90ab. Which TWO actions will the switch perform upon processing this ingress frame? (Select two.)

  1. Update the MAC address table entry for 0010.ab12.3456 to associate it with port GigabitEthernet0/1.Answer
  2. Flood the frame out all operational ports within the same VLAN, excluding ingress port GigabitEthernet0/1.Answer
  3. C
    Forward the frame directly out GigabitEthernet0/3 based on the previous MAC table entry for the host.
  4. D
    Drop the frame immediately because a MAC address security violation has occurred due to a station move.
  5. E
    Send an ICMP Redirect message back out GigabitEthernet0/1 to notify the sender of the topology update.

Answer

The switch updates the MAC address table entry for 0010.ab12.3456 to GigabitEthernet0/1 and floods the unknown unicast frame out all operational ports on the VLAN except GigabitEthernet0/1.
When an Ethernet frame arrives at a switch port, the switch first performs MAC address learning by checking the frame's source MAC address. If the source MAC address exists in the MAC table but is associated with a different interface, the switch updates the table entry with the new ingress port (station move update). Next, the switch looks up the frame's destination MAC address. Because the destination MAC address is not present in the MAC table, the switch treats the frame as unknown unicast and floods it out all active ports in the same VLAN except the receiving port.

Step-by-Step Solution

1
Examine the source MAC address handling (MAC Learning & Station Move).
The source MAC address 0010.ab12.3456 was previously listed on GigabitEthernet0/3. Since it arrived on GigabitEthernet0/1, the switch rewrites the table entry to associate 0010.ab12.3456 with GigabitEthernet0/1.
Layer 2 switches dynamically update dynamic MAC table entries whenever a known MAC address arrives on a different port.
2
Examine the destination MAC address handling (Forwarding Decision).
The destination MAC address 0020.cd78.90ab is missing from the MAC address table.
When a destination MAC address is not found in the Layer 2 forwarding table, the frame is classified as an unknown unicast frame.
3
Execute unknown unicast forwarding logic.
The frame is flooded to all active ports assigned to the same VLAN, except the receiving interface (GigabitEthernet0/1).
Unknown unicast traffic must be flooded across the broadcast domain so the intended destination host can receive it and respond.

Key Concept

Layer 2 MAC Table Station Learning and Unknown Unicast Flooding Mechanics
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Topics

L2 Switching Concepts and MAC Table Operation

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