N10-009 Network Troubleshooting Practice Question
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?
⚠ Common exam trap
Watch out — candidates often assume acceptable average optical power levels rule out physical-layer issues, but The N10-009 exam often tests that intermittent faults like dirty connectors cause flapping despite passing a static power measurement.
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 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.
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, oil, or residue, can significantly attenuate the optical signal, leading to intermittent signal degradation. This occurs because the foreign material obstructs the light path, causing reflections and refractions that reduce the received power. The intermittent nature often arises from slight movements, vibrations, or temperature changes affecting the contact points, causing the signal to drop below a usable threshold and then recover, manifesting as link flapping.
- ✗
Electromagnetic interference
Why it's wrong here
Fiber optic cables transmit data using pulses of light, not electrical signals, rendering them inherently immune to electromagnetic interference (EMI). EMI affects electrical conductors by inducing unwanted currents or voltages, which can corrupt data or disrupt signal integrity on copper cabling. Since light transmission is unaffected by external electromagnetic fields, EMI cannot be the cause of intermittent signal degradation or link flapping on a fiber optic link.
When this WOULD be correct
A network technician is troubleshooting intermittent connectivity between two switches connected via copper Ethernet cable in a factory with heavy machinery. The link status shows up/down flapping. Optical power levels are not applicable. In this scenario, electromagnetic interference from nearby equipment is a likely cause.
- ✗
Incorrect VLAN configuration
Why it's wrong here
An incorrect VLAN configuration operates at Layer 2 (Data Link Layer) of the OSI model, affecting how logical network segments are defined and how traffic is forwarded. While such a misconfiguration would certainly cause specific traffic to be blocked, misrouted, or unable to reach its destination, it does not impact the physical integrity or stability of the underlying network link. The physical link itself would remain up and operational, even if the data traversing it is logically incorrect or unreachable.
When this WOULD be correct
A network technician is troubleshooting a connectivity issue where devices on the same switch can communicate, but devices on different switches cannot. The link lights are solid. Which of the following is the most likely cause?
- ✗
Duplex mismatch
Why it's wrong here
Duplex mismatch typically occurs on copper Ethernet links when one device operates in full-duplex mode while the other is in half-duplex. This leads to severe performance degradation, high collision rates, and constant errors, not intermittent link flapping. While fiber links generally operate in full-duplex using separate transmit and receive fibers, any configuration error would result in a persistent, non-intermittent communication failure rather than the link repeatedly dropping and re-establishing.
When this WOULD be correct
A question describing a connectivity issue between two switches where one is set to full duplex and the other to half duplex, resulting in packet loss, collisions, or poor performance but not necessarily link flapping. The correct answer would be duplex mismatch.
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, oil, or residue, can significantly attenuate the optical signal, leading to intermittent signal degradation. This occurs because the foreign material obstructs the light path, causing reflections and refractions that reduce the received power. The intermittent nature often arises from slight movements, vibrations, or temperature changes affecting the contact points, causing the signal to drop below a usable threshold and then recover, manifesting as link flapping.
✗Electromagnetic interferenceWrong answer — click to see why▾
Why this is wrong here
Fiber optic cables are immune to electromagnetic interference (EMI) because they transmit light, not electrical signals. Thus, EMI cannot cause flapping on a fiber link.
★ When this WOULD be the correct answer
A network technician is troubleshooting intermittent connectivity between two switches connected via copper Ethernet cable in a factory with heavy machinery. The link status shows up/down flapping. Optical power levels are not applicable. In this scenario, electromagnetic interference from nearby equipment is a likely cause.
Why candidates choose this
Candidates may confuse fiber with copper cabling, where EMI is a common issue, or mistakenly think that EMI can affect any type of cable.
✗Incorrect VLAN configurationWrong answer — click to see why▾
Why this is wrong here
Incorrect VLAN configuration typically causes consistent connectivity failures or inability to communicate across VLANs, not intermittent link flapping. The link status up/down issue is physical layer, not logical layer 2.
★ When this WOULD be the correct answer
A network technician is troubleshooting a connectivity issue where devices on the same switch can communicate, but devices on different switches cannot. The link lights are solid. Which of the following is the most likely cause?
Why candidates choose this
Candidates may confuse symptoms of VLAN misconfiguration (like no connectivity across switches) with physical layer issues, especially when the problem is intermittent.
✗Duplex mismatchWrong answer — click to see why▾
Why this is wrong here
Duplex mismatch typically causes symptoms like high error rates or slow performance, not link flapping. The question specifies that optical power levels are acceptable, and fiber links are generally immune to duplex issues because they use separate transmit and receive paths.
★ When this WOULD be the correct answer
A question describing a connectivity issue between two switches where one is set to full duplex and the other to half duplex, resulting in packet loss, collisions, or poor performance but not necessarily link flapping. The correct answer would be duplex mismatch.
Why candidates choose this
Candidates may confuse symptoms of duplex mismatch (e.g., errors, slow throughput) with link flapping, or they may recall that duplex mismatch is a common cause of connectivity problems on copper Ethernet links and incorrectly apply it to fiber.
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
Bit
A bit is the smallest unit of data in computing and digital communications, representing a single binary value of either 0 or 1.
Key term
Fiber optic cable
A fiber optic cable is a high-speed data transmission medium that uses pulses of light traveling through thin strands of glass or plastic to carry information over long distances with minimal signal loss.
About these practice questions
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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.