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.
Users on a VLAN report intermittent network disconnections lasting a few seconds. The network technician checks the switch and notices a high number of CRC errors on the port connecting to the core switch. The cable test passes. What is the most likely cause?
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.
Correct. A duplex mismatch causes collisions and CRC errors, leading to intermittent connectivity.
B
STP reconvergence due to topology change
Why wrong: Incorrect. STP reconvergence causes temporary loss of connectivity but does not produce CRC errors.
C
Broadcast storm caused by a loop
Why wrong: Incorrect. A broadcast storm would cause high traffic and latency, but CRC errors are not characteristic.
D
Faulty SFP transceiver on the core switch
Why wrong: Incorrect. While a faulty SFP can cause errors, a cable test that passes suggests the physical layer is functional; duplex mismatch is a more likely cause of CRC errors.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Duplex mismatch between the two switches
A duplex mismatch causes one side to send frames while the other is still transmitting, leading to collisions that are interpreted as CRC errors on the receiving interface. Since the cable test passes, the physical layer is fine, and the intermittent nature (lasting seconds) matches the symptom of a duplex mismatch where the half-duplex side backs off after collisions, causing brief outages.
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.
✓
Duplex mismatch between the two switches
Why this is correct
Correct. A duplex mismatch causes collisions and CRC errors, leading to intermittent connectivity.
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.
✗
STP reconvergence due to topology change
Why it's wrong here
Incorrect. STP reconvergence causes temporary loss of connectivity but does not produce CRC errors.
✗
Broadcast storm caused by a loop
Why it's wrong here
Incorrect. A broadcast storm would cause high traffic and latency, but CRC errors are not characteristic.
✗
Faulty SFP transceiver on the core switch
Why it's wrong here
Incorrect. While a faulty SFP can cause errors, a cable test that passes suggests the physical layer is functional; duplex mismatch is a more likely cause of CRC errors.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that CRC errors always indicate a bad cable or physical layer issue, but the trap here is that a passing cable test points to a duplex mismatch as the root cause, especially when combined with intermittent disconnections.
Detailed technical explanation
How to think about this question
CRC errors occur when the frame check sequence (FCS) computed by the receiver does not match the sender's FCS, often due to electrical interference or collisions. In a duplex mismatch, the half-duplex side's collision detection causes it to abort transmission, resulting in runt or corrupted frames that fail CRC; the full-duplex side never sees collisions and simply discards these frames. A common real-world scenario is when one switch is hard-set to full duplex and the other is set to auto-negotiation, which defaults to half duplex if negotiation fails.
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 between the two switches — A duplex mismatch causes one side to send frames while the other is still transmitting, leading to collisions that are interpreted as CRC errors on the receiving interface. Since the cable test passes, the physical layer is fine, and the intermittent nature (lasting seconds) matches the symptom of a duplex mismatch where the half-duplex side backs off after collisions, causing brief outages.
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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