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 network technician notices that a switch port connected to a user's computer is showing a high number of CRC errors and late collisions. The link is operating at 100 Mbps, full duplex according to the switch. Which of the following is the most likely cause of these errors?
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: Excessive cable length can cause signal degradation and errors, but late collisions are a strong indicator of duplex mismatch, not cable length.
B
Duplex mismatch
If the computer is running at half duplex while the switch is at full duplex, collisions occur when both try to send simultaneously, leading to late collisions and CRC errors.
C
Faulty switch port
Why wrong: A faulty port would likely show link flapping or high error rates on all traffic, but late collisions specifically point to duplex mismatch.
D
Electromagnetic interference
Why wrong: EMI could cause CRC errors but is less likely to cause late collisions, which are timing-related and characteristic of duplex problems.
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 and late collisions in a full-duplex link are classic symptoms of a duplex mismatch. When one side is set to full duplex and the other to half duplex, the half-duplex side does not sense the carrier before transmitting, leading to collisions that are detected late in the frame. The switch reports full duplex, so the user's NIC is likely stuck at half duplex, causing these errors.
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.
✗
Cable length exceeds 100 meters
Why it's wrong here
Excessive cable length can cause signal degradation and errors, but late collisions are a strong indicator of duplex mismatch, not cable length.
✓
Duplex mismatch
Why this is correct
If the computer is running at half duplex while the switch is at full duplex, collisions occur when both try to send simultaneously, leading to late collisions and CRC errors.
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.
✗
Faulty switch port
Why it's wrong here
A faulty port would likely show link flapping or high error rates on all traffic, but late collisions specifically point to duplex mismatch.
✗
Electromagnetic interference
Why it's wrong here
EMI could cause CRC errors but is less likely to cause late collisions, which are timing-related and characteristic of duplex problems.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that CRC errors alone indicate a cabling issue, but the presence of late collisions alongside CRC errors is the key indicator of a duplex mismatch, not a cable length problem.
Trap categories for this question
Command / output trap
A faulty port would likely show link flapping or high error rates on all traffic, but late collisions specifically point to duplex mismatch.
Detailed technical explanation
How to think about this question
In Ethernet, late collisions occur when a collision happens after the first 64 bytes of a frame have been transmitted, which is impossible in a properly functioning full-duplex link because both devices can transmit simultaneously without collision detection. The half-duplex side's NIC uses CSMA/CD and may transmit while the full-duplex switch is already sending, causing the half-duplex side to detect a collision only after its preamble has passed, resulting in late collisions. A show interface command on the switch would reveal the duplex setting, and a similar command on the PC (e.g., ethtool on Linux) would confirm the mismatch.
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 practitioner preparing for the N10-009 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
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 and late collisions in a full-duplex link are classic symptoms of a duplex mismatch. When one side is set to full duplex and the other to half duplex, the half-duplex side does not sense the carrier before transmitting, leading to collisions that are detected late in the frame. The switch reports full duplex, so the user's NIC is likely stuck at half duplex, causing these errors.
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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