N10-009 Network Troubleshooting Practice Question
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?
⚠ Common exam trap
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.
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
✓
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.
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
Intermittent packet loss on fiber links, especially over distance, often points to physical layer issues. Checking transmit (Tx) and receive (Rx) optical power levels with an optical power meter is crucial. If Rx power is too low (below receiver sensitivity) or too high (saturating the receiver), it can lead to bit errors, which manifest as CRC errors and subsequent packet retransmissions or drops, causing intermittent loss. This verifies the optical link budget and ensures signal integrity.
- ✗
Check the duplex settings on both ends
Why it's wrong here
Fiber optic links, especially modern 10 km single-mode connections, almost universally operate in full-duplex mode, allowing simultaneous transmission and reception. A duplex mismatch, where one end is full-duplex and the other is half-duplex, is rare on fiber and primarily causes late collisions and significant performance degradation due to retransmissions, not typically intermittent packet loss stemming from physical layer bit errors or CRC issues.
When this WOULD be correct
When troubleshooting intermittent connectivity or performance issues on a copper Ethernet link (e.g., Cat5e) where link lights are on but there are errors, checking duplex settings is the next step after verifying cables, as mismatched duplex can cause high error rates.
- ✗
Verify the cable length is within specifications
Why it's wrong here
Single-mode fiber (SMF) is engineered for long-distance data transmission, typically supporting distances far exceeding 10 kilometers without significant signal degradation. While excessive cable length can cause attenuation, it usually results in consistent, not intermittent, packet loss if the link was never operational. If the 10 km link was previously working, its length is almost certainly within specifications and not the cause of new, intermittent packet loss.
When this WOULD be correct
If the question described a link exceeding the maximum distance for the fiber type (e.g., 100 km on single-mode without repeaters) or if the cable was installed with excessive bends or splices causing attenuation, then verifying cable length or loss budget would be the next step.
- ✗
Adjust the Spanning Tree Protocol priority
Why it's wrong here
Spanning Tree Protocol (STP) operates at Layer 2 (Data Link Layer) and is designed to prevent switching loops by blocking redundant paths. Adjusting STP priority influences which switch becomes the root bridge or which ports are designated/blocked, thereby affecting network topology and path selection. It has no direct impact on physical layer signal integrity, optical power levels, or the occurrence of intermittent packet loss due to bit errors on a specific fiber link.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The N10-009 exam frequently reuses these exact scenarios with slightly different constraints.
✓Check the transmit/receive optical power levelsCorrect answer▾
Why this is correct
Intermittent packet loss on fiber links, especially over distance, often points to physical layer issues. Checking transmit (Tx) and receive (Rx) optical power levels with an optical power meter is crucial. If Rx power is too low (below receiver sensitivity) or too high (saturating the receiver), it can lead to bit errors, which manifest as CRC errors and subsequent packet retransmissions or drops, causing intermittent loss. This verifies the optical link budget and ensures signal integrity.
✗Check the duplex settings on both endsWrong answer — click to see why▾
Why this is wrong here
CRC errors on a fiber link with link lights on typically indicate signal degradation or physical layer issues, not duplex mismatches, which usually cause frame check sequence errors or late collisions on copper links, not on single-mode fiber.
★ When this WOULD be the correct answer
When troubleshooting intermittent connectivity or performance issues on a copper Ethernet link (e.g., Cat5e) where link lights are on but there are errors, checking duplex settings is the next step after verifying cables, as mismatched duplex can cause high error rates.
Why candidates choose this
Candidates often associate CRC errors with duplex mismatches from common Ethernet troubleshooting scenarios, but they overlook that fiber links are almost always full-duplex and duplex mismatch is rare on modern fiber interfaces.
✗Verify the cable length is within specificationsWrong answer — click to see why▾
Why this is wrong here
The question states the link is 10 km, which is within the typical range for single-mode fiber (up to 40 km or more). CRC errors are more likely due to signal degradation or dirty connectors, not cable length.
★ When this WOULD be the correct answer
If the question described a link exceeding the maximum distance for the fiber type (e.g., 100 km on single-mode without repeaters) or if the cable was installed with excessive bends or splices causing attenuation, then verifying cable length or loss budget would be the next step.
Why candidates choose this
Candidates may confuse CRC errors with distance-related issues like signal attenuation, assuming that exceeding the cable length specification causes bit errors, but in this scenario the length is standard and the problem points to optical power issues.
Analysis generated from the official N10-009blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Visual reference
Go deeper
Related to this question
Learn chapter
Troubleshooting Methodology
Key term
Cyclic Redundancy Check
A Cyclic Redundancy Check (CRC) is an error-detecting code used to verify that data transmitted over a network or stored on a device has not been corrupted.
Key term
RX
RX (Receive / Receiver) is the signal path or hardware component that accepts incoming data from a network medium.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.