N10-009 Network Troubleshooting Practice Question
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?
⚠ Common exam trap
The trap here is that candidates see 'acceptable optical power levels' and assume the physical layer is fine, overlooking that intermittent physical contamination can cause flapping without dropping the average power below threshold.
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
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.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Dirty fiber connectors
Why this is correct
Contamination on fiber end faces, such as dust or oil, can cause significant and intermittent signal degradation. These microscopic particles can partially block or scatter the light signal, leading to increased attenuation or back reflection that disrupts communication. The intermittent nature arises because the contamination might shift, vibrate, or only partially obscure the core, causing periods of good signal followed by periods of poor or lost signal, resulting in link flaps. Even if average power readings appear acceptable, the transient nature of the obstruction can cause connectivity to drop.
- ✗
Duplex mismatch
Why it's wrong here
A duplex mismatch occurs when one side of a link operates in full-duplex mode while the other operates in half-duplex. This typically leads to a high number of late collisions and frame errors, as the half-duplex device defers transmission while the full-duplex device transmits freely, causing data corruption. However, the physical link itself remains established and stable because the electrical or optical signaling parameters are compatible. The issue manifests as severe performance degradation and packet loss, not as the link intermittently going up and down.
When this WOULD be correct
A technician reports that a link is up but has a high number of CRC errors and slow performance. After checking, both switches are set to different duplex modes (one half, one full). In that scenario, duplex mismatch is the most likely cause.
- ✗
Speed mismatch
Why it's wrong here
A speed mismatch generally prevents a network link from establishing at all, or it forces both devices to link at the highest common speed if auto-negotiation is successful. If auto-negotiation fails or is disabled and the speeds are manually set differently (e.g., one port at 100 Mbps, the other at 1 Gbps), the physical layer signaling protocols are incompatible, and the link will simply not come up. It does not cause the link to intermittently flap, as the fundamental signaling required for link establishment is either present and stable at a negotiated speed, or entirely absent.
When this WOULD be correct
A technician connects two switches and the link fails to establish. The switches have different fixed speed capabilities (e.g., one is 100 Mbps and the other is 1 Gbps) and auto-negotiation is disabled. In this scenario, speed mismatch is the most likely cause.
- ✗
VLAN mismatch
Why it's wrong here
A VLAN mismatch is a Layer 2 (data link layer) logical configuration problem, typically involving incorrect native VLAN settings or allowed VLAN lists on trunk ports. While this will certainly cause traffic for specific VLANs to fail to pass or be misdirected, it has no impact on the underlying physical layer (Layer 1) link state. The physical connection between the two devices will remain stable and operational, as VLAN tagging and processing occur at a higher layer of the OSI model, independent of the physical link's establishment and maintenance.
When this WOULD be correct
A VLAN mismatch would be correct in a scenario where two switches are connected and the link is up/up, but devices on the same VLAN cannot communicate across the link, or there is no traffic passing despite a stable 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.
✓Dirty fiber connectorsCorrect answer▾
Why this is correct
Contamination on fiber end faces, such as dust or oil, can cause significant and intermittent signal degradation. These microscopic particles can partially block or scatter the light signal, leading to increased attenuation or back reflection that disrupts communication. The intermittent nature arises because the contamination might shift, vibrate, or only partially obscure the core, causing periods of good signal followed by periods of poor or lost signal, resulting in link flaps. Even if average power readings appear acceptable, the transient nature of the obstruction can cause connectivity to drop.
✗Duplex mismatchWrong answer — click to see why▾
Why this is wrong here
Duplex mismatch typically causes constant errors or complete link failure, not intermittent flapping. The symptoms here (up/down flapping with acceptable optical power) point to a physical layer issue like dirty connectors, not a duplex configuration problem.
★ When this WOULD be the correct answer
A technician reports that a link is up but has a high number of CRC errors and slow performance. After checking, both switches are set to different duplex modes (one half, one full). In that scenario, duplex mismatch is the most likely cause.
Why candidates choose this
Candidates may confuse intermittent flapping with duplex mismatch because both can cause connectivity issues, but they overlook that duplex mismatch usually results in persistent errors rather than link state changes.
✗Speed mismatchWrong answer — click to see why▾
Why this is wrong here
Speed mismatch typically causes a link to not come up at all or to flap continuously, but the question states optical power levels are within range, and the issue is intermittent. Speed mismatch would usually be constant, not intermittent, and is less common on fiber links with auto-negotiation.
★ When this WOULD be the correct answer
A technician connects two switches and the link fails to establish. The switches have different fixed speed capabilities (e.g., one is 100 Mbps and the other is 1 Gbps) and auto-negotiation is disabled. In this scenario, speed mismatch is the most likely cause.
Why candidates choose this
Candidates may confuse symptoms of speed mismatch (link flapping) with those of dirty connectors, or they may overgeneralize that any flapping is due to speed/duplex issues without considering the intermittent nature and acceptable power levels.
✗VLAN mismatchWrong answer — click to see why▾
Why this is wrong here
A VLAN mismatch would cause connectivity issues but typically results in a stable link state (up/up) with no communication, not intermittent flapping of the link status. The link flapping indicates a physical layer problem, not a layer 2 configuration issue.
★ When this WOULD be the correct answer
A VLAN mismatch would be correct in a scenario where two switches are connected and the link is up/up, but devices on the same VLAN cannot communicate across the link, or there is no traffic passing despite a stable link.
Why candidates choose this
Candidates may confuse VLAN mismatch with other misconfigurations that cause intermittent issues, or they might think that VLAN mismatch can cause link flapping due to spanning tree or other protocols reacting to the misconfiguration.
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?”
Go deeper
Related to this question
Learn chapter
Troubleshooting Methodology
Key term
RSSI
RSSI is a measurement of the power level in a received radio signal, used by wireless devices to gauge signal strength.
Key term
Received Signal Strength Indicator
Received Signal Strength Indicator (RSSI) is a measurement of how strong a wireless signal is when it reaches a receiving device, like a laptop connecting to Wi-Fi.
About these practice questions
This N10-009 question is part of Courseiva's 464-question bank — original exam-style content with full explanations and wrong-answer analysis, never real exam questions or exam dumps. Learn why practice questions differ from exam dumps →
JA
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.