N10-009 Network Troubleshooting Practice Question
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?
⚠ Common exam trap
Candidates often assume CRC errors always indicate a bad cable or physical-layer issue, but The N10-009 exam often tests the fact that a duplex mismatch is a common logical-layer cause of CRC errors when autonegotiation is involved.
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
✓
A: Duplex mismatch between the switch port and the workstation NIC
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.
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: Duplex mismatch between the switch port and the workstation NIC
Why this is correct
A duplex mismatch occurs when one side of a link operates in full-duplex mode while the other operates in half-duplex. For instance, if the switch port is full-duplex and the workstation NIC is half-duplex, the half-duplex NIC will attempt to detect collisions and back off, while the full-duplex switch port transmits without listening for collisions. This leads to late collisions, frame corruption, and subsequent retransmissions, which are precisely what CRC (Cyclic Redundancy Check) errors indicate—data integrity issues within the Ethernet frame.
- ✗
B: VLAN mismatch between the switch and the workstation
Why it's wrong here
A VLAN mismatch occurs when a workstation is assigned to a different Virtual Local Area Network than the switch port it connects to. This configuration error prevents the workstation from communicating with other devices within its intended logical network segment. The switch will either drop frames from the workstation because they are untagged on a tagged port, or it will forward them to the wrong broadcast domain, effectively isolating the device. This is a logical segmentation issue, not a physical layer problem causing frame corruption or CRC errors.
When this WOULD be correct
In a scenario where a workstation cannot communicate with the network despite having a link light, and the switch shows no CRC errors, a VLAN mismatch between the switch port and the workstation's configured VLAN (or lack thereof) would be the likely cause.
- ✗
C: Incorrect MTU setting on the switch port
Why it's wrong here
An incorrect Maximum Transmission Unit (MTU) setting primarily affects packet fragmentation and delivery at Layer 3, not Layer 2 frame integrity. If a packet exceeds the MTU of a link, it must either be fragmented into smaller pieces or, if the "Don't Fragment" bit is set, it will be dropped. This can cause connectivity issues for specific applications or slow performance, but it does not typically introduce corruption into individual frames that would manifest as CRC errors, which are indicators of physical layer transmission problems.
When this WOULD be correct
A question where a host cannot communicate with a remote server, and pings fail with 'Frag needed' messages, or where jumbo frames are enabled on one device but not the other, would make incorrect MTU the correct answer.
- ✗
D: Broadcast storm from a loop in the network
Why it's wrong here
A broadcast storm is caused by a Layer 2 loop in the network, where broadcast frames are endlessly retransmitted, consuming excessive bandwidth and switch processing power. This leads to severe network congestion, high CPU utilization on network devices, and dropped packets due to buffer overflows as switches struggle to process the overwhelming traffic. While it cripples network performance, a broadcast storm does not directly cause individual frames to become corrupted during transmission, thus it does not typically generate CRC errors.
When this WOULD be correct
In a question describing a switch port with high utilization, packet loss, and spanning-tree topology changes, a broadcast storm from a loop would be the correct answer.
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.
✓A: Duplex mismatch between the switch port and the workstation NICCorrect answer▾
Why this is correct
A duplex mismatch occurs when one side of a link operates in full-duplex mode while the other operates in half-duplex. For instance, if the switch port is full-duplex and the workstation NIC is half-duplex, the half-duplex NIC will attempt to detect collisions and back off, while the full-duplex switch port transmits without listening for collisions. This leads to late collisions, frame corruption, and subsequent retransmissions, which are precisely what CRC (Cyclic Redundancy Check) errors indicate—data integrity issues within the Ethernet frame.
✗B: VLAN mismatch between the switch and the workstationWrong answer — click to see why▾
Why this is wrong here
A VLAN mismatch would prevent communication entirely or cause connectivity issues, but it does not directly cause CRC errors. CRC errors are typically caused by physical layer issues like duplex mismatches or cabling problems.
★ When this WOULD be the correct answer
In a scenario where a workstation cannot communicate with the network despite having a link light, and the switch shows no CRC errors, a VLAN mismatch between the switch port and the workstation's configured VLAN (or lack thereof) would be the likely cause.
Why candidates choose this
Candidates may confuse VLAN mismatches with duplex mismatches because both involve mismatched configurations between devices, and both can cause connectivity problems, leading them to incorrectly associate VLAN issues with CRC errors.
✗C: Incorrect MTU setting on the switch portWrong answer — click to see why▾
Why this is wrong here
CRC errors are typically caused by physical layer issues like duplex mismatch or faulty cabling, not by MTU settings. An incorrect MTU would cause fragmentation or dropped packets, not CRC errors.
★ When this WOULD be the correct answer
A question where a host cannot communicate with a remote server, and pings fail with 'Frag needed' messages, or where jumbo frames are enabled on one device but not the other, would make incorrect MTU the correct answer.
Why candidates choose this
Candidates may confuse CRC errors with packet size issues, thinking that large frames cause bit errors, or they may recall that MTU mismatches can cause performance problems, but they misapply it to CRC errors.
✗D: Broadcast storm from a loop in the networkWrong answer — click to see why▾
Why this is wrong here
CRC errors are caused by physical layer issues like duplex mismatch or faulty cabling, not by broadcast storms. A broadcast storm would cause high utilization and packet loss, not CRC errors.
★ When this WOULD be the correct answer
In a question describing a switch port with high utilization, packet loss, and spanning-tree topology changes, a broadcast storm from a loop would be the correct answer.
Why candidates choose this
Candidates may confuse symptoms of broadcast storms (high utilization, packet loss) with CRC errors, or assume any network problem with errors could be caused by loops.
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
Troubleshooting Methodology
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
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.