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Difficulty: MediumInterface and Cable Troubleshooting Issues

A network administrator executes the `show interfaces GigabitEthernet0/2` command on a Cisco IOS switch to investigate frame loss on a point-to-point connection. The command output displays the following status:

text
GigabitEthernet0/2 is up, line protocol is up
Full-duplex, 1000Mb/s, media type is 10/100/1000BaseTX
14205 input errors, 14205 CRC, 0 frame, 0 overrun, 0 ignored
0 output errors, 0 collisions, 0 interface resets

Based on the CLI output, which condition is the most likely cause of the observed interface statistics?

  1. Damaged copper cabling or high levels of electromagnetic interference along the cable pathAnswer
  2. B
    An incorrect Ethernet pinout selection between legacy network devices with Auto-MDIX disabled
  3. C
    A native VLAN configuration mismatch between trunk interfaces at opposite ends of the link
  4. D
    A misinterpretation of incoming CDP or LLDP discovery protocol Type-Length-Value fields

Answer

Damaged copper cabling or high levels of electromagnetic interference along the cable path is the most likely cause of the CRC errors.
CRC errors occur when the receiving switch interface calculates a Frame Check Sequence (FCS) that does not match the FCS attached to the incoming frame. On a link with zero collisions and correctly operating full-duplex negotiation, high CRC error counts are almost exclusively caused by physical layer media degradation, such as a damaged twisted-pair cable, loose RJ-45 connectors, bad patch panel terminations, or electromagnetic interference (EMI).

Step-by-Step Solution

1
Analyze the interface status and duplex settings in the CLI output.
The interface status is 'up/up' and operating in Full-duplex at 1000Mb/s.
Verifies that Layer 1 physical link and Layer 2 line protocol are active, ruling out complete link loss caused by cabling mismatch.
2
Examine the error counters in the interface statistics.
Input errors equal 14,205, with all 14,205 registered specifically as CRC errors, while collisions remain 0.
A CRC error occurs when a frame's calculated Frame Check Sequence (FCS) does not match the checksum value sent by the transmitting node.
3
Correlate CRC errors with potential root causes.
Since duplex is matched and no collisions exist, frame corruption during transit points directly to physical Layer 1 media degradation (faulty cables, bad punch-downs, or external EMI noise).
Late collisions would indicate a duplex mismatch, whereas CRC errors with zero collisions indicate corruption of bits while crossing the physical medium.

Key Concept

Interface CRC and Input Errors Troubleshooting
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Topics

Interface and Cable Troubleshooting Issues

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