N10-009 Network Troubleshooting Practice Question
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?
⚠ Common exam trap
The N10-009 exam often tests the misconception that CRC errors are always caused by bad cabling, but the most common cause in exam scenarios is a duplex mismatch, especially when one device is set to auto-negotiate and the other is hard-coded.
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
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.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
A faulty cable
Why it's wrong here
A faulty cable, particularly one with damaged insulation, improper termination, or excessive length, can introduce significant signal integrity issues. These issues lead to corrupted electrical signals, which are then interpreted as data errors and result in Cyclic Redundancy Check (CRC) errors. While a faulty cable can cause slow performance due to retransmissions of corrupted frames, it often manifests as intermittent connectivity or complete link failure rather than consistent slow performance primarily characterized by late collisions, which are more indicative of a duplex mismatch.
- ✓
Duplex mismatch
Why this is correct
A duplex mismatch occurs when two connected network devices are configured with different duplex settings; typically, one operates in full-duplex mode while the other operates in half-duplex. This configuration causes the full-duplex device to transmit data simultaneously with the half-duplex device, leading to late collisions that the half-duplex device detects but the full-duplex device does not. These undetected collisions result in corrupted frames, triggering a high number of CRC errors and significantly degrading network throughput due to extensive retransmissions and dropped packets.
- ✗
VLAN mismatch
Why it's wrong here
A VLAN mismatch occurs when a network device is configured to operate within a specific Virtual Local Area Network (VLAN), but the switch port it connects to is assigned to a different VLAN or is not configured to carry traffic for that VLAN. This configuration typically results in a complete lack of network connectivity for the device, as its frames are not forwarded to the correct broadcast domain. While it prevents communication, a VLAN mismatch does not cause data corruption during transmission and therefore does not directly generate CRC errors.
When this WOULD be correct
A VLAN mismatch would be correct if the question described a scenario where a workstation cannot communicate with devices on a different subnet, or when a switch port is configured with an incorrect VLAN assignment, leading to no connectivity or traffic being dropped at Layer 2.
- ✗
Incorrect MTU setting
Why it's wrong here
An incorrect Maximum Transmission Unit (MTU) setting primarily affects the handling of packet sizes at the network layer. If the MTU is set too large for a segment of the network path, packets may be fragmented, or dropped entirely if the 'Don't Fragment' (DF) bit is set. While this certainly impacts network performance by requiring retransmissions or causing data loss, it does not directly cause CRC errors, which are indicative of data corruption during physical or data link layer transmission, not packet size handling.
When this WOULD be correct
A question describing symptoms like 'large packets are being dropped' or 'connectivity fails with certain servers but not others' and asking for the cause would make incorrect MTU setting the correct answer, especially if jumbo frames are involved.
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 mismatchCorrect answer▾
Why this is correct
A duplex mismatch occurs when two connected network devices are configured with different duplex settings; typically, one operates in full-duplex mode while the other operates in half-duplex. This configuration causes the full-duplex device to transmit data simultaneously with the half-duplex device, leading to late collisions that the half-duplex device detects but the full-duplex device does not. These undetected collisions result in corrupted frames, triggering a high number of CRC errors and significantly degrading network throughput due to extensive retransmissions and dropped packets.
✗VLAN mismatchWrong answer — click to see why▾
Why this is wrong here
CRC errors indicate physical layer issues like cabling or duplex mismatch, not VLAN mismatch. VLAN mismatch typically causes connectivity issues or inability to communicate across VLANs, not CRC errors.
★ When this WOULD be the correct answer
A VLAN mismatch would be correct if the question described a scenario where a workstation cannot communicate with devices on a different subnet, or when a switch port is configured with an incorrect VLAN assignment, leading to no connectivity or traffic being dropped at Layer 2.
Why candidates choose this
Candidates may confuse VLAN mismatch with duplex mismatch because both involve configuration mismatches on switch ports, and they might think any mismatch can cause errors, but CRC errors are specifically linked to physical and duplex issues.
✗Incorrect MTU settingWrong answer — click to see why▾
Why this is wrong here
CRC errors are typically caused by physical layer issues like faulty cabling or duplex mismatches, not by MTU settings. An incorrect MTU would cause fragmentation or packet drops, not CRC errors.
★ When this WOULD be the correct answer
A question describing symptoms like 'large packets are being dropped' or 'connectivity fails with certain servers but not others' and asking for the cause would make incorrect MTU setting the correct answer, especially if jumbo frames are involved.
Why candidates choose this
Candidates may confuse CRC errors with packet fragmentation issues, or incorrectly assume that MTU mismatches can cause bit-level errors at the physical layer.
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?”
Quick reference
OSI Model Reference
| Layer | Name | PDU | Key Protocols / Devices |
|---|---|---|---|
| 7 | Application | Data | HTTP, HTTPS, DNS, SMTP, FTP, SSH |
| 6 | Presentation | Data | TLS / SSL, JPEG, ASCII encoding |
| 5 | Session | Data | NetBIOS, RPC, SIP |
| 4 | Transport | Segment / Datagram | TCP, UDP |
| 3 | Network | Packet | IP, ICMP, OSPF — Routers |
| 2 | Data Link | Frame | Ethernet, Wi-Fi, PPP — Switches, Bridges |
| 1 | Physical | Bits | Cables, NICs, Hubs, Repeaters |
Go deeper
Related to this question
Learn chapter
Troubleshoot: Cable and Physical Layer
Key term
Full duplex
Full duplex is a communication mode where data can be sent and received simultaneously between two parties, like a two-way conversation without interruptions.
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.
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.