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Difficulty: MediumTroubleshooting VLAN, Trunking, and Switching Issues

A network technician is investigating performance degradation on a desktop host connected to switch port FastEthernet 0/12. The switch port speed and duplex settings were manually configured to 100 Mbps and Full Duplex, while the desktop network adapter was left on auto-negotiation. Running the `show interface fastEthernet 0/12` command reveals a rapidly incrementing counter for late collisions and Frame Check Sequence (FCS) errors.

Which of the following is the root cause of this connectivity issue?

  1. A duplex mismatch caused by the desktop adapter auto-negotiating to Half Duplex while the switch port is fixed at Full Duplex.Answer
  2. B
    A native VLAN mismatch causing untagged traffic from the desktop to be dropped by the switch port.
  3. C
    A VLAN isolation issue where the host and switch port belong to different broadcast domains without a Layer 3 router.
  4. D
    Spanning Tree Protocol continuously blocking port FastEthernet 0/12 due to an STP root bridge priority election.

Answer

The root cause is a duplex mismatch resulting from the host network adapter auto-negotiating to Half Duplex while the switch interface is manually hardcoded to Full Duplex.
The correct answer identifies a duplex mismatch caused by mismatched negotiation modes. Standard IEEE auto-negotiation rules state that if one endpoint is manually set to fixed speed and duplex (e.g. 100/Full) and the other is set to auto-negotiate, the auto-negotiating side can sense speed but cannot detect duplex mode, defaulting to Half Duplex. The resulting collision domain mismatch causes late collisions and FCS errors on the switch interface.

Step-by-Step Solution

1
Analyze the switch port interface configuration and error counters.
The switch interface is statically set to 100/Full, and the interface counter shows high numbers of late collisions and FCS errors.
Late collisions occur when a device detects a collision after transmitting the first 64 bytes (512 bits) of a frame, characteristic of half-duplex operation.
2
Evaluate auto-negotiation behavior on Ethernet links.
The host NIC uses auto-negotiation, but because the switch port does not send auto-negotiation pulses, the host falls back to the IEEE default of Half Duplex.
When speed/duplex auto-negotiation fails to receive response pulses, speed is detected via link pulses, but duplex defaults to half-duplex for backward compatibility.
3
Identify the cause of frame corruption and late collision errors.
The switch port sends frames at will (Full Duplex) while the host listens for quiet wire before sending (Half Duplex). When the switch transmits while the host is sending, the host detects a late collision.
This physical mismatch leads to frame aborts, retransmissions, FCS alignment errors, and significant throughput loss.

Key Concept

Duplex Mismatch Diagnosis and IEEE Auto-Negotiation Fallback Rules
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Topics

Troubleshooting VLAN, Trunking, and Switching Issues

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