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Difficulty: EasyL2 Switching Concepts and MAC Table Operation

An Ethernet frame with a destination MAC address of FFFF.FFFF.FFFF arrives on switch port FastEthernet0/5. The switch MAC address table currently contains no entry for the source MAC address of the frame. Which dual action will the switch perform upon receiving this frame?

  1. Record the source MAC address mapped to FastEthernet0/5 in the MAC table, and flood the frame out all active switch ports except FastEthernet0/5.Answer
  2. B
    Drop the frame immediately because FFFF.FFFF.FFFF is a reserved broadcast address that cannot be processed by Layer 2 switches.
  3. C
    Flood the frame out all active switch ports, including FastEthernet0/5, without updating the MAC address table.
  4. D
    Forward the frame directly to the default gateway router to resolve the destination broadcast address.

Answer

Record the source MAC address mapped to FastEthernet0/5 in the MAC table, and flood the frame out all active switch ports except FastEthernet0/5.
When a Layer 2 switch receives an Ethernet frame, it first inspects the source MAC address. If the source MAC is not present in the MAC table, the switch creates a new entry mapping that source MAC address to the receiving port (FastEthernet0/5). Next, it inspects the destination MAC address. Because FFFF.FFFF.FFFF is the Layer 2 broadcast address, the switch floods the frame out every active interface in the VLAN except the interface on which the frame arrived.

Step-by-Step Solution

1
Examine the incoming frame's source MAC address and ingress port.
The switch notes that the source MAC address is not currently in the MAC address table.
Layer 2 switches perform MAC learning by recording the incoming source MAC address associated with the ingress interface (FastEthernet0/5).
2
Examine the incoming frame's destination MAC address.
The destination MAC address is identified as FFFF.FFFF.FFFF (Layer 2 broadcast).
Broadcast frames are intended for all devices within the broadcast domain.
3
Execute frame forwarding.
The frame is flooded out all operational ports belonging to the same VLAN, excluding the receiving port (FastEthernet0/5).
Flooding ensures all hosts receive the broadcast, while excluding the ingress port prevents frame loops.

Key Concept

Layer 2 MAC Address Table Learning and Broadcast Frame Flooding Logic
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Topics

L2 Switching Concepts and MAC Table Operation

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