N10-009 Network Troubleshooting Practice Question
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?
⚠ Common exam trap
The N10-009 exam 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.
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 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.
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 is a Layer 2 configuration issue that logically segments network traffic. While placing a workstation in the wrong VLAN would prevent it from communicating with its intended subnet or other devices in different VLANs, it does not prevent the physical link from being established or the data link layer from coming up. The switch port would still detect the connected device, negotiate link parameters, and illuminate its LED green, signifying a valid physical and data link connection, even if the workstation is logically isolated.
When this WOULD be correct
A technician connects a workstation to a switch port. The cable passes a physical test, the switch port LED is green, but the workstation cannot communicate with any other device on the network, even though it can see its own MAC address in the ARP cache. What is the most likely cause?
- ✓
Duplex mismatch between the workstation and the switch
Why this is correct
A duplex mismatch occurs when one device operates in full-duplex mode while the other operates in half-duplex mode, or vice-versa, on the same link. This configuration error causes severe frame errors, excessive late collisions, and retransmissions, as devices attempt to transmit simultaneously without proper coordination or wait for a clear channel when the other device is transmitting. While the physical link may appear active with a green LED, indicating Layer 1 connectivity, successful Layer 2 communication is severely hampered, leading to extremely poor network performance or complete communication failure. This condition directly prevents successful data exchange despite a seemingly established physical connection.
- ✗
The workstation has a static IP address in the wrong subnet
Why it's wrong here
An incorrect static IP address or subnet mask is a Layer 3 configuration issue that prevents the workstation from communicating with other hosts outside its perceived local subnet. However, this misconfiguration does not affect the establishment of the physical link (Layer 1) or the data link connection (Layer 2) between the workstation and the switch port. The network interface card would still successfully negotiate link speed and duplex, and the switch port LED would illuminate green, indicating a functional physical connection, even if IP-level communication is impossible.
When this WOULD be correct
In a scenario where a workstation has a static IP address and can communicate with some devices but not others, or can ping the gateway but not reach the internet, the most likely cause is an incorrect subnet mask or default gateway. This would be correct when the link is up and basic connectivity exists but routing fails.
- ✗
A faulty cable
Why it's wrong here
A faulty cable would typically manifest as a complete lack of link establishment, intermittent connectivity, or a link that constantly flaps up and down. Since the question states the cable passed a physical test and the switch port LED is green, it explicitly indicates that the physical layer connection is stable and functional. A green LED confirms that the cable is successfully transmitting power and data signals, ruling out a physical cable defect as the cause of communication failure.
When this WOULD be correct
A technician connects a workstation to a switch port, but the link LED is off or blinking, and a cable tester indicates a fault. In that scenario, a faulty cable is the most 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 mismatch between the workstation and the switchCorrect answer▾
Why this is correct
A duplex mismatch occurs when one device operates in full-duplex mode while the other operates in half-duplex mode, or vice-versa, on the same link. This configuration error causes severe frame errors, excessive late collisions, and retransmissions, as devices attempt to transmit simultaneously without proper coordination or wait for a clear channel when the other device is transmitting. While the physical link may appear active with a green LED, indicating Layer 1 connectivity, successful Layer 2 communication is severely hampered, leading to extremely poor network performance or complete communication failure. This condition directly prevents successful data exchange despite a seemingly established physical connection.
✗Incorrect VLAN assignment on the switch portWrong answer — click to see why▾
Why this is wrong here
In this scenario, the switch port LED is green, indicating Layer 1 connectivity is established. Incorrect VLAN assignment would not prevent the link from coming up; the workstation would still get a link light but might not receive DHCP or communicate beyond the VLAN, but the question states 'cannot establish a network connection' which is more consistent with a duplex mismatch causing excessive errors.
★ When this WOULD be the correct answer
A technician connects a workstation to a switch port. The cable passes a physical test, the switch port LED is green, but the workstation cannot communicate with any other device on the network, even though it can see its own MAC address in the ARP cache. What is the most likely cause?
Why candidates choose this
Candidates often associate VLAN misconfiguration with connectivity issues, and the green LED might mislead them into thinking Layer 1 is fine, so they look for Layer 2 problems like VLAN mismatch, overlooking duplex mismatch which also manifests at Layer 2 with a green LED.
✗The workstation has a static IP address in the wrong subnetWrong answer — click to see why▾
Why this is wrong here
The workstation cannot establish a network connection despite the switch port LED being green and the cable passing a physical test, indicating Layer 1 is functional. A static IP in the wrong subnet would still allow link-local communication and ARP, but the symptom here is no connection at all, which points to a Layer 2 issue like duplex mismatch.
★ When this WOULD be the correct answer
In a scenario where a workstation has a static IP address and can communicate with some devices but not others, or can ping the gateway but not reach the internet, the most likely cause is an incorrect subnet mask or default gateway. This would be correct when the link is up and basic connectivity exists but routing fails.
Why candidates choose this
Candidates often jump to IP configuration issues because network connectivity problems are frequently caused by incorrect IP settings. The green LED and cable test pass, so they assume Layer 1 is fine and look to Layer 3, overlooking that duplex mismatch can cause intermittent or no connectivity despite a link light.
✗A faulty cableWrong answer — click to see why▾
Why this is wrong here
The cable passed a physical test and the switch port LED is green, indicating the cable is functional and the link is established. A faulty cable would likely cause a physical layer issue, not allow the link to come up.
★ When this WOULD be the correct answer
A technician connects a workstation to a switch port, but the link LED is off or blinking, and a cable tester indicates a fault. In that scenario, a faulty cable is the most likely cause.
Why candidates choose this
Candidates often default to cable issues as the first troubleshooting step, overlooking that a green LED confirms the physical layer is working.
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
Frame
A frame is a unit of data at the Data Link layer of networking, containing header, payload, and trailer information for local network delivery.
Key term
Duplex mismatch
A duplex mismatch occurs when two connected network devices are configured with different duplex settings (one half-duplex, one full-duplex), leading to poor performance and data errors.
About these practice questions
Courseiva writes every N10-009 question from scratch — 464 in total, each with an explanation and a wrong-answer breakdown. None are copied from real exams or dumps. Learn why practice questions differ from exam dumps →
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.