The answer is a duplex mismatch between the switch port and the connected device. This is the most likely cause because when one side operates at full duplex and the other at half duplex, the full-duplex side ignores Carrier Sense Multiple Access with Collision Detection (CSMA/CD) and transmits freely, while the half-duplex side detects these unsolicited transmissions as late collisions. Those late collisions manifest as CRC errors and runts on the switch port, even though the solid link LED indicates Layer 1 connectivity is fine—ruling out a bad cable. On the CompTIA Network+ N10-009 exam, this scenario tests your ability to distinguish physical-layer issues from data-link-layer misconfigurations; a common trap is assuming a solid link LED means the link is fully healthy. Remember the memory tip: “Solid light, but wrong fight”—the link light is solid, but the duplex settings are fighting each other, producing errors without a physical break.
N10-009 Network Troubleshooting Practice Question
This N10-009 practice question tests your understanding of network troubleshooting. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Duplex mismatch between the switch port and the connected device
Duplex mismatch causes collisions and errors because one device transmits while the other cannot receive properly, leading to CRC and runt frames.
B
Faulty network cable
Why wrong: A faulty cable can cause errors, but the solid link LED indicates a physical connection. Duplex mismatch is a more common cause of CRC errors when the link is up.
C
Broadcast storm
Why wrong: A broadcast storm would cause high broadcast traffic on many ports, not necessarily CRC errors on a single port.
D
VLAN misconfiguration
Why wrong: VLAN misconfiguration typically causes connectivity issues (cannot reach resources) but does not directly result in CRC errors or runts.
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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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
Duplex mismatch causes collisions and errors because one device transmits while the other cannot receive properly, leading to CRC and runt frames.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Faulty network cable
Why it's wrong here
A faulty cable can cause errors, but the solid link LED indicates a physical connection. Duplex mismatch is a more common cause of CRC errors when the link is up.
✗
Broadcast storm
Why it's wrong here
A broadcast storm would cause high broadcast traffic on many ports, not necessarily CRC errors on a single port.
✗
VLAN misconfiguration
Why it's wrong here
VLAN misconfiguration typically causes connectivity issues (cannot reach resources) but does not directly result in CRC errors or runts.
Common exam traps
Common exam trap: answer the scenario, not the keyword
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.
Detailed technical explanation
How to think about this question
CRC errors occur when the Frame Check Sequence (FCS) computed by the receiver does not match the sender's FCS, indicating data corruption. Runts are frames smaller than the minimum 64 bytes (excluding preamble) and often result from collisions. In a duplex mismatch, the half-duplex device's late collisions cause it to abort frames, producing runts, while the full-duplex device's lack of collision detection leads to corrupted frames that fail CRC. The 'show interfaces' command on Cisco switches displays duplex mismatch indicators such as 'input errors' with CRC and runts, and 'output errors' with collisions.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this N10-009 question in full detail.
Network Troubleshooting — This question tests Network Troubleshooting — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this N10-009 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Users on a VLAN report intermittent network disconnections lasting a few seconds. The network technician checks the switch and notices a high number of CRC errors on the port connecting to the core switch. The cable test passes. What is the most likely cause?
hard
✓ A.Duplex mismatch between the two switches
B.STP reconvergence due to topology change
C.Broadcast storm caused by a loop
D.Faulty SFP transceiver on the core switch
Why A: A duplex mismatch causes one side to send frames while the other is still transmitting, leading to collisions that are interpreted as CRC errors on the receiving interface. Since the cable test passes, the physical layer is fine, and the intermittent nature (lasting seconds) matches the symptom of a duplex mismatch where the half-duplex side backs off after collisions, causing brief outages.
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Question Discussion
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