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Difficulty: EasyTroubleshooting VLAN, Trunking, and Switching Issues

A workstation connected to a FastEthernet port on a local switch experiences extremely slow speeds and dropped packets. When examining the switch interface statistics, a network administrator observes a high count of late collisions and frame check sequence (FCS) errors on the port. Which of the following configuration issues is the most likely cause of these symptoms?

  1. A duplex mismatch between the switch port and the connected host interfaceAnswer
  2. B
    A native VLAN mismatch across an 802.1Q trunk link connecting two switches
  3. C
    An incorrect Spanning Tree Protocol (STP) root bridge priority setting
  4. D
    The absence of a Layer 3 router to forward traffic between broadcast domains

Answer

A duplex mismatch between the switch port and the connected host interface is the cause of late collisions and FCS errors.
A duplex mismatch occurs when one end of an Ethernet connection is configured for full-duplex while the other operates in half-duplex. The full-duplex host transmits data continuously without carrier sensing, leading the half-duplex host to detect collisions late in its frame transmission, registering late collisions and FCS/CRC errors.

Step-by-Step Solution

1
Analyze the interface error counters.
Identified high numbers of late collisions and FCS errors on an access port.
Late collisions occur when a device detects a collision after transmitting the first 64 bytes of a frame.
2
Correlate error counters with known Layer 2 duplex operational characteristics.
Full-duplex interfaces transmit without checking for collisions, while half-duplex interfaces listen for collisions. When connected together, the half-duplex side receives incoming frames while transmitting, triggering late collision flags.
This behavior directly produces late collisions, alignment errors, and FCS errors on the interface.

Key Concept

Duplex Mismatch Symptoms
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Topics

Troubleshooting VLAN, Trunking, and Switching Issues

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