The answer is a duplex mismatch between the workstation and the switch. This is the most likely cause because even when a cable passes a physical test and the switch port LED is green—indicating good Layer 1 connectivity—a mismatch in duplex settings prevents successful Layer 2 communication. One side operating at full-duplex while the other runs at half-duplex leads to excessive collisions and frame errors, effectively killing the connection despite the link light appearing healthy. On the CompTIA Network+ N10-009 exam, this scenario is a classic trap: students see a green LED and assume the link is fully functional, but the question tests your understanding that Layer 1 success does not guarantee Layer 2 reliability. A duplex mismatch often results from failed autonegotiation, especially when one device is manually set. Remember the memory tip: “Green light, no bytes—check the duplex fights.”
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 technician connects a user's workstation to a switch port. The cable passes a physical test, and the switch port LED is green, but the workstation cannot establish a network connection. What 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.
Why wrong: Incorrect VLAN assignment would allow link establishment but might isolate the workstation from its intended subnet; it would not prevent a connection from being established.
B
Duplex mismatch between the workstation and the switch
A duplex mismatch can cause severe frame errors and prevent successful communication even though the physical link is up.
C
The workstation has a static IP address in the wrong subnet
Why wrong: A wrong IP address would prevent communication with other hosts but would still allow the workstation to sense the link as active.
D
A faulty cable
Why wrong: The cable passed a physical test and the LED is green, indicating the cable is functional.
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 workstation and the switch
A duplex mismatch occurs when one side of a link is set to full-duplex and the other to half-duplex, often due to a failed autonegotiation. Even though the cable passes a physical test and the switch port LED is green (indicating link at Layer 1), the mismatch causes excessive collisions and frame errors at Layer 2, preventing successful network communication. The workstation cannot establish a connection because the switch and NIC are not coordinating their transmission/reception timing correctly.
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.
✗
Incorrect VLAN assignment on the switch port
Why it's wrong here
Incorrect VLAN assignment would allow link establishment but might isolate the workstation from its intended subnet; it would not prevent a connection from being established.
✓
Duplex mismatch between the workstation and the switch
Why this is correct
A duplex mismatch can cause severe frame errors and prevent successful communication even though the physical link is up.
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.
✗
The workstation has a static IP address in the wrong subnet
Why it's wrong here
A wrong IP address would prevent communication with other hosts but would still allow the workstation to sense the link as active.
✗
A faulty cable
Why it's wrong here
The cable passed a physical test and the LED is green, indicating the cable is functional.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests duplex mismatch by pairing a green link LED (Layer 1 OK) with a complete lack of connectivity, tricking candidates into thinking the issue must be at Layer 3 (IP addressing) or Layer 2 (VLAN), when the real problem is a Layer 2 duplex negotiation failure that corrupts frames without breaking the physical link.
Detailed technical explanation
How to think about this question
Duplex mismatch is a classic Layer 2 issue where autonegotiation fails, often because one side is hardcoded to full-duplex while the other remains at half-duplex. This results in late collisions and CRC errors on the half-duplex side, which can be observed with 'show interface' counters showing excessive collisions or 'input errors'. In real-world scenarios, this commonly occurs when a technician manually sets speed/duplex on a switch port but forgets to match the NIC settings, or when using older hubs that only support half-duplex.
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 workstation and the switch — A duplex mismatch occurs when one side of a link is set to full-duplex and the other to half-duplex, often due to a failed autonegotiation. Even though the cable passes a physical test and the switch port LED is green (indicating link at Layer 1), the mismatch causes excessive collisions and frame errors at Layer 2, preventing successful network communication. The workstation cannot establish a connection because the switch and NIC are not coordinating their transmission/reception timing correctly.
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.
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