- A
Incorrect VLAN configuration on the switch ports
Why wrong: VLAN mismatches typically result in connectivity issues or traffic errors, but they do not usually cause the physical link to flap (repeatedly lose carrier).
- B
Crossed fiber pairs
Why wrong: Crossed pairs (transmit/receive swapped) would prevent the link from ever establishing, not cause intermittent flapping.
- C
Dirty fiber connectors
Dirty connectors can cause intermittent signal loss as the light is partially blocked. This is a common cause of flapping fiber links. Cleaning the connectors often resolves the issue.
- D
Duplex mismatch between the switches
Why wrong: Duplex mismatch usually results in high error rates, collisions, and poor performance, but the link state remains up. It does not typically cause link flapping.
Quick Answer
The answer is dirty fiber connectors. Dirty connectors cause intermittent link flapping by scattering or absorbing light, which degrades the optical signal and forces the transceiver to repeatedly lose and re-establish synchronization, producing the classic “Link up / Link down” log pattern. On single-mode fiber, where the core is only 9 microns, even microscopic dust or oil can critically misalign the light path, making this the most likely cause over issues like faulty SFP modules or cable length. For the CompTIA Network+ N10-009 exam, this question tests your ability to distinguish physical-layer problems from configuration errors; a common trap is assuming a bad transceiver, but dirty connectors are far more frequent in real-world troubleshooting. Remember the mnemonic: “Dirty ends, link bends” — a clean connector ensures a straight, stable light path.
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 administrator is troubleshooting an intermittent link between two switches connected by single-mode fiber. The interface log shows "Link up / Link down" events multiple times per hour. Which of the following 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.
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 fiber connectors
Dirty fiber connectors cause intermittent signal loss by scattering or absorbing light, which leads to CRC errors and repeated link flaps as the optical transceiver struggles to maintain synchronization. This matches the 'Link up / Link down' pattern seen in the logs, especially on single-mode fiber where precise alignment is critical.
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.
- ✗
Incorrect VLAN configuration on the switch ports
Why it's wrong here
VLAN mismatches typically result in connectivity issues or traffic errors, but they do not usually cause the physical link to flap (repeatedly lose carrier).
- ✗
Crossed fiber pairs
Why it's wrong here
Crossed pairs (transmit/receive swapped) would prevent the link from ever establishing, not cause intermittent flapping.
- ✓
Dirty fiber connectors
Why this is correct
Dirty connectors can cause intermittent signal loss as the light is partially blocked. This is a common cause of flapping fiber links. Cleaning the connectors often resolves the issue.
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.
- ✗
Duplex mismatch between the switches
Why it's wrong here
Duplex mismatch usually results in high error rates, collisions, and poor performance, but the link state remains up. It does not typically cause link flapping.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often jump to duplex mismatch or VLAN misconfiguration as common causes of link issues, but the intermittent 'Link up / Link down' pattern specifically points to a physical-layer problem like dirty connectors, not a Layer 2 configuration error.
Detailed technical explanation
How to think about this question
Single-mode fiber uses a narrow core (9 µm) that requires precise end-face cleanliness; even microscopic dust or oil can cause back-reflection or attenuation exceeding the transceiver's receive sensitivity (typically -20 dBm to -30 dBm). The link flap occurs because the optical power fluctuates near the threshold, causing the transceiver to repeatedly lose and regain signal detect (SD) status. Cleaning with a lint-free swab and isopropyl alcohol is the standard remediation, and inspecting with a fiber microscope (per IEC 61300-3-35) is recommended before re-termination.
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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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 fiber connectors — Dirty fiber connectors cause intermittent signal loss by scattering or absorbing light, which leads to CRC errors and repeated link flaps as the optical transceiver struggles to maintain synchronization. This matches the 'Link up / Link down' pattern seen in the logs, especially on single-mode fiber where precise alignment is critical.
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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Last reviewed: Jun 11, 2026
This N10-009 practice question is part of Courseiva's free CompTIA certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the N10-009 exam.
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