N10-009 Network Troubleshooting Practice Question
Users in a department report that the network is extremely slow. A technician checks the access switch and notices that a single port shows a high number of CRC errors and runts. The link LED is solid green. Which of the following is the most likely cause of the issue?
⚠ Common exam trap
CompTIA often tests duplex mismatch by showing a solid green link LED alongside CRC errors and runts, tricking candidates into thinking the cable is faulty because they assume a solid LED means perfect physical connectivity.
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 between the switch port and the connected device
A duplex mismatch occurs when one device operates at full duplex and the other at half duplex. The full-duplex side does not perform Carrier Sense Multiple Access with Collision Detection (CSMA/CD), so it transmits without checking for collisions. The half-duplex side detects collisions, causing late collisions that manifest as CRC errors and runts on the switch port. The solid green link LED indicates Layer 1 connectivity is intact, ruling out a physical cable fault.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Duplex mismatch between the switch port and the connected device
Why this is correct
A duplex mismatch occurs when one device operates in full-duplex mode while the connected device operates in half-duplex mode, or vice-versa. This fundamental communication conflict leads to severe performance degradation because one device attempts to transmit while the other is simultaneously trying to receive, resulting in a high number of collisions, CRC errors, and runt frames. These errors necessitate frequent retransmissions, drastically reducing effective throughput and causing the network to feel extremely slow for users.
- ✗
Faulty network cable
Why it's wrong here
While a faulty network cable can indeed cause network slowness and errors, the presence of a solid link LED typically indicates a stable physical connection. A cable fault severe enough to consistently generate high CRC errors and runts, leading to an 'extremely slow' experience, would more commonly manifest as an intermittent link, no link at all, or significant packet loss without necessarily producing these specific error types on a stable link. Duplex mismatch is a more precise cause for CRC errors when the link is up.
- ✗
Broadcast storm
Why it's wrong here
A broadcast storm involves an overwhelming amount of broadcast traffic flooding the network, consuming significant bandwidth and processing power on connected devices. While this would certainly cause the network to become extremely slow due to congestion and resource exhaustion, its primary symptom is high network utilization across multiple ports and devices, not specifically an increase in CRC errors or runt frames on a single switch port, which are indicative of lower-layer physical or data link issues.
When this WOULD be correct
A broadcast storm would be the correct answer if the question described symptoms like all ports showing high utilization, the switch CPU spiking, and the network becoming unresponsive due to excessive broadcast frames, without physical-layer errors on a single port.
- ✗
VLAN misconfiguration
Why it's wrong here
VLAN misconfiguration primarily affects logical network segmentation and traffic forwarding rules, leading to connectivity issues rather than physical layer errors. For example, a device might be unable to reach resources in a different subnet, or inter-VLAN routing might fail entirely. These issues manifest as a complete lack of access or inability to communicate with specific services, not as general network slowness characterized by an increase in CRC errors or runt frames on a switch port.
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 mismatch between the switch port and the connected deviceCorrect answer▾
Why this is correct
A duplex mismatch occurs when one device operates in full-duplex mode while the connected device operates in half-duplex mode, or vice-versa. This fundamental communication conflict leads to severe performance degradation because one device attempts to transmit while the other is simultaneously trying to receive, resulting in a high number of collisions, CRC errors, and runt frames. These errors necessitate frequent retransmissions, drastically reducing effective throughput and causing the network to feel extremely slow for users.
✗Broadcast stormWrong answer — click to see why▾
Why this is wrong here
A broadcast storm causes excessive broadcast traffic, leading to high CPU utilization and general network congestion, but it does not specifically cause CRC errors or runts on a single port. CRC errors and runts are typically physical-layer issues, not broadcast-related.
★ When this WOULD be the correct answer
A broadcast storm would be the correct answer if the question described symptoms like all ports showing high utilization, the switch CPU spiking, and the network becoming unresponsive due to excessive broadcast frames, without physical-layer errors on a single port.
Why candidates choose this
Candidates may associate any network slowdown with broadcast storms, overlooking that the specific symptoms (CRC errors, runts on one port) point to a physical-layer problem like duplex mismatch rather than a broadcast issue.
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: Cable and Physical Layer
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
CSMA/CD
Carrier Sense Multiple Access with Collision Detection is a network protocol used in Ethernet to manage data transmission and handle collisions when two devices send data at the same time.
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.