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 is troubleshooting a switch port that shows a link light but has a high number of CRC errors in the interface statistics. The port is connected to a workstation's network interface card (NIC). Both devices are set to autonegotiate. What is the MOST likely cause of the CRC 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.
A: Duplex mismatch between the switch port and the workstation NIC
Duplex mismatch causes frame collisions and CRC errors on the half-duplex side.
B
B: VLAN mismatch between the switch and the workstation
Why wrong: VLAN mismatch would prevent communication altogether or cause connectivity issues, not specifically CRC errors.
C
C: Incorrect MTU setting on the switch port
Why wrong: MTU mismatch causes fragmentation or dropped packets, but CRC errors are not typical; they indicate physical layer issues.
D
D: Broadcast storm from a loop in the network
Why wrong: Broadcast storms cause high CPU usage and may flood the network, but CRC errors are not a direct symptom; they would more likely cause high bandwidth utilization and dropped packets.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
A: Duplex mismatch between the switch port and the workstation NIC
CRC errors indicate that frames received by the switch have failed the Ethernet frame check sequence (FCS) validation, meaning the data was corrupted during transmission. When both devices are set to autonegotiate but one fails to correctly negotiate the duplex setting, a duplex mismatch occurs: one side operates at full duplex while the other operates at half duplex. This causes collisions on the half-duplex side, which corrupts frames and generates CRC errors, while the full-duplex side does not detect collisions and retransmits, leading to a high error count.
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: Duplex mismatch between the switch port and the workstation NIC
Why this is correct
Duplex mismatch causes frame collisions and CRC errors on the half-duplex side.
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.
✗
B: VLAN mismatch between the switch and the workstation
Why it's wrong here
VLAN mismatch would prevent communication altogether or cause connectivity issues, not specifically CRC errors.
✗
C: Incorrect MTU setting on the switch port
Why it's wrong here
MTU mismatch causes fragmentation or dropped packets, but CRC errors are not typical; they indicate physical layer issues.
✗
D: Broadcast storm from a loop in the network
Why it's wrong here
Broadcast storms cause high CPU usage and may flood the network, but CRC errors are not a direct symptom; they would more likely cause high bandwidth utilization and dropped packets.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume CRC errors always indicate a bad cable or physical-layer issue, but Cisco often tests the fact that a duplex mismatch is a common logical-layer cause of CRC errors when autonegotiation is involved.
Detailed technical explanation
How to think about this question
When autonegotiation fails, the IEEE 802.3 standard specifies that the device defaults to half-duplex if the link partner does not advertise its capabilities. In a duplex mismatch, the full-duplex side transmits without listening for collisions, while the half-duplex side detects collisions and invokes the backoff algorithm, causing frame fragments and FCS errors. This is often seen when a switch port is manually set to full duplex but the workstation NIC remains in autonegotiation and falls back to half duplex, a classic scenario in Cisco troubleshooting.
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: A: Duplex mismatch between the switch port and the workstation NIC — CRC errors indicate that frames received by the switch have failed the Ethernet frame check sequence (FCS) validation, meaning the data was corrupted during transmission. When both devices are set to autonegotiate but one fails to correctly negotiate the duplex setting, a duplex mismatch occurs: one side operates at full duplex while the other operates at half duplex. This causes collisions on the half-duplex side, which corrupts frames and generates CRC errors, while the full-duplex side does not detect collisions and retransmits, leading to a high error count.
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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