The answer is to check the transmit and receive optical power levels. This is correct because persistent CRC errors on a fiber optic link, even after cleaning connectors and replacing patch cables, point to a physical-layer problem like excessive attenuation or dispersion, which directly degrades the signal-to-noise ratio. Measuring optical power with a meter confirms whether the received signal falls within the acceptable range, such as -3 dBm to -20 dBm for 10GBASE-LR, and can reveal a failing transceiver or splice loss that cleaning cannot fix. On the CompTIA Network+ N10-009 exam, this scenario tests your understanding that link lights alone do not guarantee signal integrity; a common trap is assuming the issue is always dirty connectors or bad cables. Remember the memory tip: “Lights are on, but CRC is wrong—power meter is where you belong.”
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 engineer is troubleshooting intermittent packet loss on a 10 km single-mode fiber link between two buildings. The link lights are on, but the interface shows a high number of CRC errors. The engineer has cleaned the fiber connectors and replaced the patch cables. What should the engineer check NEXT?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Check the transmit/receive optical power levels
CRC errors on a single-mode fiber link typically indicate physical-layer issues such as excessive attenuation or dispersion. Since cleaning connectors and replacing patch cables did not resolve the problem, the next logical step is to measure the optical power levels at both the transmitter and receiver using an optical power meter. This will confirm whether the received signal is within the acceptable range (e.g., -3 dBm to -20 dBm for 10GBASE-LR) and identify if a damaged transceiver or a splice loss is causing the 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.
✓
Check the transmit/receive optical power levels
Why this is correct
Insufficient optical power (signal too weak) can cause CRC errors. Checking power levels verifies the link budget is adequate.
Related concept
Read the scenario before looking for a memorised answer.
✗
Check the duplex settings on both ends
Why it's wrong here
Fiber optic links typically operate at full duplex; duplex mismatch is rare and would cause late collisions, not CRC errors.
✗
Verify the cable length is within specifications
Why it's wrong here
Single-mode fiber can support distances well beyond 10 km; length is unlikely to be the issue if it was previously working.
✗
Adjust the Spanning Tree Protocol priority
Why it's wrong here
STP priority affects path selection, not physical layer errors like CRC.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often jump to duplex mismatch or cable length issues because those are common in copper troubleshooting, but on long-haul single-mode fiber, optical power levels are the primary suspect when CRC errors persist after cleaning and patching.
Detailed technical explanation
How to think about this question
CRC errors on fiber are often caused by optical signal degradation due to factors like modal dispersion, chromatic dispersion, or excessive loss. A power meter measures the received optical power in dBm; if it is below the receiver's sensitivity threshold (e.g., -21 dBm for 10GBASE-LR), the signal-to-noise ratio drops, leading to bit errors that the CRC detects. Real-world scenarios include dirty or damaged transceiver ports, a bend in the fiber exceeding the minimum bend radius, or a faulty splice that attenuates the signal without breaking the link.
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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
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: Check the transmit/receive optical power levels — CRC errors on a single-mode fiber link typically indicate physical-layer issues such as excessive attenuation or dispersion. Since cleaning connectors and replacing patch cables did not resolve the problem, the next logical step is to measure the optical power levels at both the transmitter and receiver using an optical power meter. This will confirm whether the received signal is within the acceptable range (e.g., -3 dBm to -20 dBm for 10GBASE-LR) and identify if a damaged transceiver or a splice loss is causing the 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.
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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