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Difficulty: MediumInterface and Cable Troubleshooting Issues

An administrator observes intermittent data corruption and dropped frames on interface `GigabitEthernet0/1` of an enterprise switch. Executing the `show interfaces GigabitEthernet0/1` command yields the following output:

text
GigabitEthernet0/1 is up, line protocol is up (connected)
Hardware is GigabitEthernet, address is 0012.7f3e.a101
Full-duplex, 1000Mb/s, media type is 1000BaseTX
Input queue: 0/75/0/0; Total output drops: 0
5 minute input rate 12000 bits/sec, 15 packets/sec
5 minute output rate 11000 bits/sec, 14 packets/sec
124501 packets input, 15890123 bytes
0 runts, 0 giants, 0 throttles
4812 input errors, 4812 CRC, 0 frame, 0 overrun, 0 ignored
0 output errors, 0 collisions, 0 late collision

Based on the CLI output, what is the most likely root cause of the reported interface errors?

  1. Physical cable damage or electromagnetic interference causing signal corruption along the Ethernet runAnswer
  2. B
    A duplex mismatch operating between the local switch interface and the connected device
  3. C
    A native VLAN configuration mismatch across the interconnected switch trunk link
  4. D
    A neighbor discovery protocol timer mismatch between CDP and LLDP frames on the link

Answer

Physical cable damage or electromagnetic interference causing signal corruption along the Ethernet run
The correct answer identifies physical cable damage or electromagnetic interference as the root cause. In Cisco IOS, Cyclic Redundancy Check (CRC) errors increment when a received frame fails its checksum verification. When CRC errors increase while collision and late collision counters remain at zero, the cause is physical layer corruption—such as defective copper wiring, faulty RJ-45 crimping, damaged cable shielding, or external electrical noise.

Step-by-Step Solution

1
Analyze the interface status and error counters in the CLI snippet.
The interface is operating in full-duplex at 1000 Mbps with `line protocol is up`. Input errors precisely match 4,812 Cyclic Redundancy Check (CRC) errors, while collision and late collision counters remain at 0.
CRC errors occur when the frame check sequence (FCS) calculated by the receiving interface does not match the FCS field attached by the sender.
2
Evaluate potential Layer 1 vs. Layer 2 failure modes.
Because late collisions are 0 and collisions are 0, duplex mismatch and excessive cable length issues are ruled out.
Duplex mismatches cause late collisions on full-duplex interfaces and collisions/runts on half-duplex interfaces.
3
Determine the physical cause of checksum failures.
Isolated CRC errors on an Ethernet interface are directly tied to physical media defects, bad shielding, loose RJ-45 connectors, or electrical noise (EMI).
Corrupted bits during transmission alter the mathematical CRC value without triggering collision detection circuits.

Key Concept

CRC errors in Cisco IOS indicate corrupt frames received over the physical link, most commonly caused by damaged cabling, improper shielding, EMI, or loose pin connections.
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Topics

Interface and Cable Troubleshooting Issues

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