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Difficulty: EasyInterface and Cable Troubleshooting Issues

A network administrator executes the 'show interfaces gigabitethernet 0/1' command on a Cisco switch connected to a workstation. The output indicates that GigabitEthernet0/1 is up and the line protocol is up, but the error statistics show a high number of late collisions and FCS errors. Which two underlying physical or data-link conditions are the most probable causes of these error counters?

  1. A duplex mismatch between the switch port and the connected workstationAnswer
  2. A damaged or degraded Ethernet cable causing signal corruptionAnswer
  3. C
    A native VLAN ID mismatch configured between the switch and the endpoint
  4. D
    An incorrect Ethernet pinout selection on a legacy link without Auto-MDIX enabled

Answer

The most probable causes are a duplex mismatch between the connected devices and a damaged or degraded Ethernet cable.
Late collisions are the classic symptom of a speed/duplex mismatch where one end of the link is configured for half-duplex and the other for full-duplex. FCS errors indicate that frames arrived corrupted, which is commonly caused by poor physical cabling, damaged copper conductors, or bad connectors.

Step-by-Step Solution

1
Analyze the late collisions symptom.
Late collisions occur when a device detects a collision after transmitting the first 64 bytes of a frame. This typically occurs when one side operates in half-duplex while the opposing side operates in full-duplex.
Full-duplex transmits without checking for collisions, causing the half-duplex side to detect collisions mid-transmission.
2
Analyze the FCS errors symptom.
Frame Check Sequence (FCS) errors occur when the calculated checksum at the receiving end does not match the checksum in the trailer of the received frame.
This indicates physical Layer 1 corruption caused by faulty cabling, improper grounding, or interference.
3
Evaluate the incorrect choices.
Native VLAN mismatches affect VLAN tagging/log warnings, and cable pinout mismatches without Auto-MDIX cause the link state to remain completely down.
Neither native VLAN mismatch nor pinout mismatches produce late collisions or FCS errors on an operational up/up interface.

Key Concept

Identifying interface error symptoms and corresponding physical/data-link Layer 1 and Layer 2 root causes
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Topics

Interface and Cable Troubleshooting Issues

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