N10-009 Network Troubleshooting Practice Question
A network technician notices that a switch port connected to a user's computer is showing a high number of CRC errors and late collisions. The link is operating at 100 Mbps, full duplex according to the switch. Which of the following is the most likely cause of these errors?
⚠ Common exam trap
The N10-009 exam often tests the misconception that CRC errors alone indicate a cabling issue, but the presence of late collisions alongside CRC errors is the key indicator of a duplex mismatch, not a cable length problem.
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
✓
Duplex mismatch
CRC errors and late collisions in a full-duplex link are classic symptoms of a duplex mismatch. When one side is set to full duplex and the other to half duplex, the half-duplex side does not sense the carrier before transmitting, leading to collisions that are detected late in the frame. The switch reports full duplex, so the user's NIC is likely stuck at half duplex, causing these 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.
- ✗
Cable length exceeds 100 meters
Why it's wrong here
Exceeding the maximum recommended cable length of 100 meters for Ethernet can lead to significant signal attenuation and dispersion, resulting in various transmission errors like corrupted frames or dropped packets. However, excessive cable length primarily causes general signal degradation and increased bit error rates due to weakened signals, not the specific timing-related issue of late collisions, which are characteristic of simultaneous transmission attempts.
When this WOULD be correct
A question describing symptoms like intermittent connectivity, high error rates, or link loss on a 100BASE-TX link operating at half-duplex or with no duplex specified would make cable length the likely cause, as exceeding 100 meters degrades signal integrity.
- ✓
Duplex mismatch
Why this is correct
A duplex mismatch occurs when two connected network devices operate at different duplex settings, such as one at half-duplex and the other at full-duplex. This configuration error causes the full-duplex device to transmit without listening for collisions, while the half-duplex device attempts to transmit only when the line is clear. When both transmit simultaneously, the half-duplex device detects a collision late in its transmission, leading to late collisions and an increase in CRC errors, which are hallmark symptoms.
- ✗
Faulty switch port
Why it's wrong here
A genuinely faulty switch port would typically exhibit more generalized and severe symptoms, such as intermittent link connectivity (link flapping), complete loss of link, or consistently high error rates across all types of traffic. While a hardware fault could contribute to various network issues, the specific pattern of late collisions is a diagnostic signature for a duplex mismatch, rather than a generic hardware malfunction.
- ✗
Electromagnetic interference
Why it's wrong here
Electromagnetic interference (EMI) introduces external noise into network cables, which can corrupt data signals and lead to an increase in Cyclic Redundancy Check (CRC) errors as frames fail integrity checks. While EMI can certainly cause data corruption and errors, it does not directly cause late collisions. Late collisions are a specific symptom of simultaneous transmission attempts on a shared medium, indicating a timing and protocol issue, not external signal disruption.
When this WOULD be correct
A question where a network cable runs near heavy machinery or power lines, and the symptoms are CRC errors without late collisions, and the link is operating at 100 Mbps half-duplex or auto-negotiation is enabled. EMI would be the likely cause.
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.
✓Duplex mismatchCorrect answer▾
Why this is correct
A duplex mismatch occurs when two connected network devices operate at different duplex settings, such as one at half-duplex and the other at full-duplex. This configuration error causes the full-duplex device to transmit without listening for collisions, while the half-duplex device attempts to transmit only when the line is clear. When both transmit simultaneously, the half-duplex device detects a collision late in its transmission, leading to late collisions and an increase in CRC errors, which are hallmark symptoms.
✗Cable length exceeds 100 metersWrong answer — click to see why▾
Why this is wrong here
CRC errors and late collisions are not typically caused by cable length exceeding 100 meters; that would more likely cause attenuation and signal loss, not late collisions, especially in full-duplex mode where collisions should not occur.
★ When this WOULD be the correct answer
A question describing symptoms like intermittent connectivity, high error rates, or link loss on a 100BASE-TX link operating at half-duplex or with no duplex specified would make cable length the likely cause, as exceeding 100 meters degrades signal integrity.
Why candidates choose this
Candidates often associate CRC errors with physical layer issues like cable problems, and the 100-meter limit is a well-known Ethernet rule, leading them to overlook that late collisions and full-duplex operation point to a duplex mismatch instead.
✗Electromagnetic interferenceWrong answer — click to see why▾
Why this is wrong here
Electromagnetic interference (EMI) typically causes CRC errors but not late collisions, which are a hallmark of duplex mismatch. In full-duplex mode, collisions should not occur, so late collisions indicate a duplex mismatch rather than EMI.
★ When this WOULD be the correct answer
A question where a network cable runs near heavy machinery or power lines, and the symptoms are CRC errors without late collisions, and the link is operating at 100 Mbps half-duplex or auto-negotiation is enabled. EMI would be the likely cause.
Why candidates choose this
Candidates may associate CRC errors with signal degradation from interference, overlooking that late collisions specifically point to duplex mismatch. EMI is a common cause of CRC errors in general, making it a plausible but incorrect choice here.
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
Troubleshoot: Network Connectivity
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
NIC
A NIC is a hardware component that connects a computer to a network, enabling communication over wired or wireless media.
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.