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Difficulty: MediumSwitching Concepts and VLANs

An unmanaged switch receives an Ethernet frame on port FastEthernet 0/1. The frame has a source MAC address of 0011.2233.4455 and a destination MAC address of 6677.8899.AABB. The switch's MAC address table currently contains no entries for either address. Which of the following operations will the switch perform first upon receiving this frame?

  1. Record MAC address 0011.2233.4455 associated with port FastEthernet 0/1 in the MAC address table, and flood the frame out all operational ports except FastEthernet 0/1 within the frame's VLAN.Answer
  2. B
    Record destination MAC address 6677.8899.AABB associated with port FastEthernet 0/1 in the MAC address table, and forward the frame directly to the Layer 3 default gateway.
  3. C
    Record MAC address 0011.2233.4455 associated with port FastEthernet 0/1 in the MAC address table, and flood the frame out every active switch port regardless of configured VLAN boundaries.
  4. D
    Drop the frame immediately and generate an ICMP destination unreachable message because the destination MAC address is absent from the MAC table.

Answer

The switch updates its MAC address table by associating the source MAC address 0011.2233.4455 with FastEthernet 0/1, then floods the frame out all other active ports belonging to the same VLAN.
The correct answer accurately describes the standard two-step Layer 2 switch operational logic: learning and forwarding. First, the switch learns the source MAC address (0011.2233.4455) and maps it to ingress port FastEthernet 0/1. Second, because the destination MAC address (6677.8899.AABB) is unknown in the MAC table, the switch performs unknown unicast flooding by copying the frame to all operational ports within the same VLAN except the receiving port.

Step-by-Step Solution

1
Examine the frame's source MAC address
The switch reads source MAC address 0011.2233.4455 and ingress port FastEthernet 0/1.
Switches dynamically build their MAC address (CAM) table by inspecting the source MAC address of incoming frames on each interface.
2
Update the MAC address table (Learning Process)
An entry mapping 0011.2233.4455 to port FastEthernet 0/1 is added/refreshed in the table.
Learning source MAC addresses allows future frames destined for 0011.2233.4455 to be forwarded directly to FastEthernet 0/1.
3
Lookup the frame's destination MAC address (Forwarding Decision)
Destination MAC address 6677.8899.AABB is not found in the MAC address table.
When a destination MAC address is missing from the table, the frame is classified as an unknown unicast frame.
4
Execute unknown unicast flooding
The frame is forwarded out all active switch ports belonging to the ingress port's VLAN, excluding the port on which it arrived (FastEthernet 0/1).
Flooding ensures that the intended recipient host receives the frame and can reply, enabling the switch to learn the recipient's MAC address location.

Key Concept

Switch Learning and Forwarding Operations (Unknown Unicast Flooding)
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Topics

Switching Concepts and VLANs

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