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Difficulty: EasyL2 Switching Concepts and MAC Table Operation

Host A sends an Ethernet frame addressed to Host B into port FastEthernet0/1 of a Layer 2 switch. The switch MAC address table already contains an entry for Host B on port FastEthernet0/2, but has no prior entry for Host A. What action does the switch take upon receiving this frame?

  1. Learns Host A's MAC address on port FastEthernet0/1 and forwards the frame directly out port FastEthernet0/2.Answer
  2. B
    Floods the frame out all active switch ports except FastEthernet0/1 because the source MAC address was missing from the table.
  3. C
    Drops the frame immediately until an ARP broadcast is sent to verify Host A's identity.
  4. D
    Forwards the frame out port FastEthernet0/2 and sends an ICMP redirect out port FastEthernet0/1.

Answer

The switch records Host A's MAC address on FastEthernet0/1 and forwards the frame exclusively out FastEthernet0/2.
When a switch receives a frame, it always inspects the source MAC address to update its MAC table (learning Host A on FastEthernet0/1). It then checks the destination MAC address; since Host B is already recorded on FastEthernet0/2, the switch forwards the frame exclusively out that destination port.

Step-by-Step Solution

1
Examine the ingress frame's source MAC address
Host A's MAC address is learned and added to the MAC address table associated with port FastEthernet0/1.
Layer 2 switches dynamically populate their MAC address tables by inspecting incoming frame source MAC addresses and ingress port numbers.
2
Examine the ingress frame's destination MAC address
Host B's MAC address is found in the table associated with port FastEthernet0/2.
If the destination MAC address is already present in the MAC address table, the switch performs point-to-point known unicast forwarding.
3
Forward the frame
The frame is transmitted solely out port FastEthernet0/2.
Known unicast destination entries prevent unnecessary frame flooding across other switch ports.

Key Concept

Layer 2 MAC Address Table Learning and Forwarding Logic
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Topics

L2 Switching Concepts and MAC Table Operation

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