This N10-009 practice question tests your understanding of network troubleshooting. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A user reports slow network performance on their workstation. The technician checks the switch port and sees a high number of CRC errors. Which of the following is the MOST likely cause of this issue?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Why wrong: A faulty cable can cause CRC errors, but it is less common than duplex mismatch in modern networks with auto-MDIX.
B
Duplex mismatch
A duplex mismatch occurs when one device is set to full duplex and the other to half duplex. This causes late collisions and CRC errors, leading to poor performance.
C
VLAN mismatch
Why wrong: VLAN mismatch prevents connectivity and causes the switch port to be in a non-forwarding state, not CRC errors.
D
Incorrect MTU setting
Why wrong: Incorrect MTU can cause packet fragmentation or drops, but CRC errors are usually not the direct result.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Duplex mismatch
CRC errors indicate data corruption at the data link layer, typically caused by collisions or electrical interference. A duplex mismatch occurs when one device operates at full duplex and the other at half duplex, leading to collisions on a full-duplex link that corrupt frames and generate CRC errors. This is the most common cause of CRC errors on a switch port.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
A faulty cable
Why it's wrong here
A faulty cable can cause CRC errors, but it is less common than duplex mismatch in modern networks with auto-MDIX.
✓
Duplex mismatch
Why this is correct
A duplex mismatch occurs when one device is set to full duplex and the other to half duplex. This causes late collisions and CRC errors, leading to poor performance.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
VLAN mismatch
Why it's wrong here
VLAN mismatch prevents connectivity and causes the switch port to be in a non-forwarding state, not CRC errors.
✗
Incorrect MTU setting
Why it's wrong here
Incorrect MTU can cause packet fragmentation or drops, but CRC errors are usually not the direct result.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that CRC errors are always caused by bad cabling, but the most common cause in exam scenarios is a duplex mismatch, especially when one device is set to auto-negotiate and the other is hard-coded.
Detailed technical explanation
How to think about this question
CRC (Cyclic Redundancy Check) errors occur when the frame check sequence (FCS) computed by the receiver does not match the sender's FCS. In a duplex mismatch, the half-duplex device detects collisions and backs off, while the full-duplex device continues transmitting, causing frame fragments that fail CRC. This is often seen with auto-negotiation failures where one side hard-codes speed/duplex and the other uses auto-negotiation, resulting in a mismatch (e.g., one side full, the other half).
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this N10-009 question in full detail.
Network Troubleshooting — This question tests Network Troubleshooting — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Duplex mismatch — CRC errors indicate data corruption at the data link layer, typically caused by collisions or electrical interference. A duplex mismatch occurs when one device operates at full duplex and the other at half duplex, leading to collisions on a full-duplex link that corrupt frames and generate CRC errors. This is the most common cause of CRC errors on a switch port.
What should I do if I get this N10-009 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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