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Difficulty: MediumTroubleshooting VLAN, Trunking, and Switching Issues

A network administrator is investigating user reports of severe performance degradation and file transfer timeouts when accessing a local backup server connected to Switch-A. The administrator reviews the interface statistics on switch port GigabitEthernet0/1 and observes a rapidly increasing counter for late collisions, FCS errors, and runts, despite the physical link showing up/up at 100 Mbps. Which of the following is the primary cause of this connectivity issue?

  1. A duplex mismatch between switch port GigabitEthernet0/1 and the backup server network interface cardAnswer
  2. B
    An un-tagged frame leakage issue caused by a native VLAN mismatch on switch port GigabitEthernet0/1
  3. C
    Spanning Tree Protocol placing port GigabitEthernet0/1 into an alternate blocking state due to a root bridge priority misconfiguration
  4. D
    The backup server and client workstations residing on different VLANs without a configured Layer 3 routing gateway

Answer

A duplex mismatch between switch port GigabitEthernet0/1 and the backup server network interface card
The combination of high late collision counters, FCS errors, and runts on a connected link is a classic indicator of a duplex mismatch. This commonly occurs when one device is manually set to full-duplex while the connecting switch port is left on auto-negotiation, causing the switch port to fall back to half-duplex.

Step-by-Step Solution

1
Analyze the reported interface symptoms and error counters
Identified high late collisions, FCS errors, and runts on an operational link.
Late collisions occur when a device detects a collision after transmitting the first 64 bytes (512 bits) of a frame.
2
Correlate error counters with Layer 1/2 configuration mismatch types
Determined that half-duplex devices listen for collisions during transmission while full-duplex devices transmit continuously without checking for carrier sense.
When one end is full-duplex and the other is half-duplex, the full-duplex end sends data whenever ready, causing the half-duplex end to detect collisions mid-frame and log late collisions.
3
Select the root cause that matches the physical error profile
Confirmed duplex mismatch as the correct root cause.
Only a duplex mismatch produces late collisions; trunk mismatches or routing failures result in dropped packets or log warnings without collision counter increments.

Key Concept

Duplex Mismatch Symptom Identification
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Topics

Troubleshooting VLAN, Trunking, and Switching Issues

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