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Difficulty: Very hardInterface and Cable Troubleshooting Issues

An engineer is troubleshooting severe frame corruption and packet drops on a GigabitEthernet connection between a Catalyst switch and an enterprise router. Both interfaces are explicitly configured for 1000Mb/s Full-duplex. The CLI output from the switch is shown below:

text
GigabitEthernet0/1 is up, line protocol is up
Hardware is Gigabit Ethernet, address is 5006.ab12.cd34 (bia 5006.ab12.cd34)
Full-duplex, 1000Mb/s, media type is 10/100/1000BaseTX
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
5 minute input rate 452000 bits/sec, 510 packets/sec
5 minute output rate 489000 bits/sec, 540 packets/sec
1245012 packets input, 1084201948 bytes, 0 no buffer
0 runts, 0 giants, 0 throttles
14205 input errors, 14205 CRC, 0 frame, 0 overrun, 0 ignored
1589022 packets output, 1482910244 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resets

Physical inspection reveals that a custom UTP patch cable was wired using non-standard pin pairings where pins 3 and 6 were split across different wire pairs instead of sharing a single twisted pair. Which physical or data link issue is directly responsible for the high number of CRC errors observed on this full-duplex interface?

  1. Severe near-end crosstalk (NEXT) and electromagnetic interference caused by the split-pair Ethernet cable pinout mismatch.Answer
  2. B
    An undetected duplex mismatch causing late collisions that corrupt the frame check sequence on the switch interface.
  3. C
    A native VLAN mismatch across the trunk link causing 802.1Q tag insertion errors that fail cyclic redundancy checks.
  4. D
    An invalid MTU size negotiation caused by misinterpreting CDP TLV neighbor parameters from the connected router.

Answer

Severe near-end crosstalk (NEXT) and electromagnetic interference caused by the split-pair Ethernet cable pinout mismatch.
The correct answer identifies signal degradation and crosstalk resulting from split-pair cabling as the root cause of the CRC errors. Twisted-pair Ethernet (such as 1000BASE-T) relies on differential signaling across twisted pairs to neutralize electromagnetic interference. Splitting pins 3 and 6 across separate pairs destroys signal cancellation, producing high rates of Near-End Crosstalk (NEXT). When frames arrive corrupted, the receiving NIC's Frame Check Sequence (FCS) calculation fails, incrementing the CRC input error counter while collision counters remain zero due to full-duplex operation.

Step-by-Step Solution

1
Analyze the interface statistics from the 'show interfaces' CLI snippet.
The interface is operating in Full-duplex at 1000Mb/s with line protocol up. There are 14,205 input errors, all of which are specifically CRC errors, while collision counts are 0.
Full-duplex interfaces disable collision detection mechanisms because transmission and reception occur over separate dedicated pairs. Therefore, collisions are not expected.
2
Evaluate the physical cabling scenario described in the stem.
The custom cable has split pairs across pins 3 and 6 (transmitting signals over wires from different twisted pairs).
Twisted pair cabling relies on balanced differential signaling to cancel out noise and crosstalk. When a pair is split, differential cancellation fails, creating massive Near-End Crosstalk (NEXT).
3
Correlate physical layer noise with interface error counters.
NEXT corrupts electrical signals during transit, causing the receiving interface to compute a Frame Check Sequence (FCS) that does not match the trailer, generating CRC errors.
CRC errors without collisions or late collisions on a full-duplex link are a classic sign of Layer 1 physical medium defects, such as cable damage or pinout/twist errors.

Key Concept

Twisted Pair Cable Pinout Mechanics & Interface Error Counter Diagnosis
Estimated Time:2m 0s
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Topics

Interface and Cable Troubleshooting Issues

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