- A
Electromagnetic interference (EMI) from nearby power cables
Why wrong: Fiber optic cables are immune to EMI because they use light, not electrical signals. EMI is a concern for copper cabling.
- B
Dirty or contaminated fiber connectors
Contamination on fiber end-faces can attenuate the optical signal, causing intermittent connections. Cleaning the connectors often resolves such issues.
- C
Mismatched VLAN configurations on the switches
Why wrong: VLAN mismatches affect Layer 2 traffic but do not cause the physical link to flap. The interface would remain up, but traffic might not pass correctly.
- D
Duplex mismatch between the two switch ports
Why wrong: Duplex mismatch usually results in high error rates, collisions, and performance degradation, but the link itself stays up. It rarely causes link flaps.
Quick Answer
The answer is dirty or contaminated fiber connectors. Even microscopic dust, oil, or debris on the end face of a single-mode fiber connector scatters the laser light, causing enough signal loss to make the optical receiver lose synchronization, which triggers the frequent 'link up / link down' events known as intermittent link flapping. On the CompTIA Network+ N10-009 exam, this is a classic trap: technicians often suspect SFP incompatibility or distance issues first, but the exam tests that physical contamination is the most likely culprit when those factors are already verified. Remember the memory tip "Dirty ends flap friends"—if the SFPs match and the cable length is fine, always inspect and clean the connector end faces before replacing hardware.
N10-009 Network Troubleshooting Practice Question
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 an intermittent link between two switches connected via single-mode fiber. The interface logs show frequent 'link up / link down' events, sometimes several times per hour. The technician has verified that the SFPs are compatible and the fiber cable is within distance specifications. Which of the following is the most likely cause of the 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.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Dirty or contaminated fiber connectors
Intermittent link flaps on single-mode fiber, despite compatible SFPs and correct distance, are most often caused by dirty or contaminated fiber connectors. Even microscopic dust or oil on the end face can scatter light, causing signal loss that triggers the switch's optical receiver to lose sync and flap the link. This matches the symptom of frequent 'link up / link down' events without any configuration mismatch.
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.
- ✗
Electromagnetic interference (EMI) from nearby power cables
Why it's wrong here
Fiber optic cables are immune to EMI because they use light, not electrical signals. EMI is a concern for copper cabling.
- ✓
Dirty or contaminated fiber connectors
Why this is correct
Contamination on fiber end-faces can attenuate the optical signal, causing intermittent connections. Cleaning the connectors often resolves such issues.
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.
- ✗
Mismatched VLAN configurations on the switches
Why it's wrong here
VLAN mismatches affect Layer 2 traffic but do not cause the physical link to flap. The interface would remain up, but traffic might not pass correctly.
- ✗
Duplex mismatch between the two switch ports
Why it's wrong here
Duplex mismatch usually results in high error rates, collisions, and performance degradation, but the link itself stays up. It rarely causes link flaps.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume fiber is immune to physical-layer issues and jump to configuration problems like duplex mismatch or EMI, but CompTIA often tests the fact that fiber connectors are the most common source of intermittent link flaps due to contamination.
Detailed technical explanation
How to think about this question
Fiber connector contamination is a leading cause of intermittent link issues because even a 0.5 dB increase in insertion loss from dirt can push the optical power below the receiver's sensitivity threshold, causing the link to drop. Cleaning with a proper one-click cleaner or lint-free wipe and inspecting with a fiber microscope (per IEC 61300-3-35) is standard practice. In real-world scenarios, technicians often overlook this because the cable appears clean to the naked eye, but microscopic contamination is sufficient to cause flapping.
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.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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Network Troubleshooting — study guide chapter
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FAQ
Questions learners often ask
What does this N10-009 question test?
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: Dirty or contaminated fiber connectors — Intermittent link flaps on single-mode fiber, despite compatible SFPs and correct distance, are most often caused by dirty or contaminated fiber connectors. Even microscopic dust or oil on the end face can scatter light, causing signal loss that triggers the switch's optical receiver to lose sync and flap the link. This matches the symptom of frequent 'link up / link down' events without any configuration mismatch.
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.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Same concept, more angles
2 more ways this is tested on N10-009
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A technician is troubleshooting an intermittent connectivity issue between two switches connected by a fiber optic cable. The link status shows up/down flapping. The technician checks the optical power levels and finds they are within acceptable range. Which of the following is the most likely cause?
medium- ✓ A.Dirty fiber connectors
- B.Duplex mismatch
- C.Speed mismatch
- D.VLAN mismatch
Why A: Intermittent link flapping with acceptable optical power levels strongly indicates a physical-layer issue that is not related to signal strength. Dirty fiber connectors cause intermittent signal degradation due to scattering and absorption of light, leading to CRC errors and link flaps even when average power appears normal. Cleaning the connectors is the standard first step in such scenarios.
Variation 2. A network technician is troubleshooting an intermittent connectivity issue between two switches connected via fiber optic cable. The link status shows up/down flapping. The technician checks the optical power levels and finds they are within acceptable range. Which of the following is the most likely cause?
hard- ✓ A.Dirty fiber connectors
- B.Electromagnetic interference
- C.Incorrect VLAN configuration
- D.Duplex mismatch
Why A: Dirty fiber connectors cause intermittent connectivity by scattering or absorbing light, leading to bit errors and link flaps even when average optical power levels appear within acceptable range. The flapping occurs because transient contaminants (e.g., dust or oil) momentarily disrupt the optical signal, triggering link down events that recover when the connector is jostled or the contaminant shifts. Since the power meter measures average power, it may not detect brief attenuation spikes caused by dirt.
Last reviewed: Jun 30, 2026
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