The answer is a duplex mismatch, which is the most likely cause of intermittent connectivity when the link LED is blinking slowly and the cable tests fine. This occurs because one device operates at full duplex while the other is forced to half duplex, causing the half-duplex device to detect collisions whenever the full-duplex device transmits simultaneously, leading to frame retransmissions and CRC errors that manifest as sporadic drops. On the CompTIA Network+ N10-009 exam, this scenario tests your understanding of how physical-layer symptoms—like a slow-blinking LED—point to layer-1 or layer-2 issues, with duplex mismatch being a classic trap that technicians overlook when the cable appears good. Remember the mnemonic: “Slow blink, duplex stink”—if the link light pulses slowly and the cable passes inspection, always suspect mismatched duplex settings before blaming the hardware.
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.
A network technician is troubleshooting a workstation that intermittently loses network connectivity. The link LED on the switch port is blinking slowly. The technician checks the cable and it appears fine. Which of the following is the most likely cause?
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.
A duplex mismatch often causes a slow blinking link light, intermittent connectivity, and errors on the interface.
B
Incorrect VLAN assignment
Why wrong: Incorrect VLAN assignment would cause no connectivity at all or consistent connectivity to the wrong subnet, not intermittent with a slow blinking LED.
C
Bad cable
Why wrong: A bad cable typically results in a completely dead link or excessive errors, not a slow blinking LED; the cable is already checked.
D
Spanning Tree blocking
Why wrong: STP blocking would result in the interface being in a blocking state (no traffic) and the link LED typically solid or off, not slow blinking.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Duplex mismatch
A slow-blinking link LED on a switch port typically indicates a duplex mismatch, where one device is set to full duplex and the other to half duplex. This causes frame collisions and CRC errors, leading to intermittent connectivity even though the cable appears fine. The symptom of intermittent loss with a physically good cable is classic for duplex mismatch, as the port is still electrically connected but suffers from excessive retransmissions.
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
Why this is correct
A duplex mismatch often causes a slow blinking link light, intermittent connectivity, and errors on the interface.
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.
✗
Incorrect VLAN assignment
Why it's wrong here
Incorrect VLAN assignment would cause no connectivity at all or consistent connectivity to the wrong subnet, not intermittent with a slow blinking LED.
✗
Bad cable
Why it's wrong here
A bad cable typically results in a completely dead link or excessive errors, not a slow blinking LED; the cable is already checked.
✗
Spanning Tree blocking
Why it's wrong here
STP blocking would result in the interface being in a blocking state (no traffic) and the link LED typically solid or off, not slow blinking.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often associate a blinking link LED with a physical layer issue like a bad cable, but Cisco tests the specific pattern of a slow blink to indicate a duplex mismatch, not a cable fault.
Detailed technical explanation
How to think about this question
Duplex mismatch occurs when autonegotiation fails or is manually misconfigured, causing one side to use full-duplex (sending and receiving simultaneously) while the other uses half-duplex (only one direction at a time). This leads to late collisions on the half-duplex side, which triggers retransmissions and backoff algorithms, resulting in high error rates and intermittent connectivity. In real-world scenarios, this is often seen when a device is hardcoded to full-duplex while the switch port is set to autonegotiate, which defaults to half-duplex if negotiation fails.
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 — A slow-blinking link LED on a switch port typically indicates a duplex mismatch, where one device is set to full duplex and the other to half duplex. This causes frame collisions and CRC errors, leading to intermittent connectivity even though the cable appears fine. The symptom of intermittent loss with a physically good cable is classic for duplex mismatch, as the port is still electrically connected but suffers from excessive retransmissions.
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
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
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. A network technician is troubleshooting a switch port that shows a link light but has a high number of CRC errors in the interface statistics. The port is connected to a workstation's network interface card (NIC). Both devices are set to autonegotiate. What is the MOST likely cause of the CRC errors?
medium
✓ A.A: Duplex mismatch between the switch port and the workstation NIC
B.B: VLAN mismatch between the switch and the workstation
C.C: Incorrect MTU setting on the switch port
D.D: Broadcast storm from a loop in the network
Why A: CRC errors indicate that frames received by the switch have failed the Ethernet frame check sequence (FCS) validation, meaning the data was corrupted during transmission. When both devices are set to autonegotiate but one fails to correctly negotiate the duplex setting, a duplex mismatch occurs: one side operates at full duplex while the other operates at half duplex. This causes collisions on the half-duplex side, which corrupts frames and generates CRC errors, while the full-duplex side does not detect collisions and retransmits, leading to a high error count.
Variation 2. A user reports slow network performance on their workstation. The technician checks the switch port and sees a high number of CRC errors. Which of the following is the MOST likely cause of this issue?
medium
A.A faulty cable
✓ B.Duplex mismatch
C.VLAN mismatch
D.Incorrect MTU setting
Why B: CRC errors indicate data corruption at the data link layer, typically caused by collisions or electrical interference. A duplex mismatch occurs when one device operates at full duplex and the other at half duplex, leading to collisions on a full-duplex link that corrupt frames and generate CRC errors. This is the most common cause of CRC errors on a switch port.
Last reviewed: Jun 11, 2026
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