N10-009 Network Troubleshooting Practice Question
Users on a VLAN report intermittent network disconnections lasting a few seconds. The network technician checks the switch and notices a high number of CRC errors on the port connecting to the core switch. The cable test passes. What is the most likely cause?
⚠ Common exam trap
The N10-009 exam often tests the misconception that CRC errors always indicate a bad cable or physical layer issue, but the trap here is that a passing cable test points to a duplex mismatch as the root cause, especially when combined with intermittent disconnections.
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 two switches
A duplex mismatch causes one side to send frames while the other is still transmitting, leading to collisions that are interpreted as CRC errors on the receiving interface. Since the cable test passes, the physical layer is fine, and the intermittent nature (lasting seconds) matches the symptom of a duplex mismatch where the half-duplex side backs off after collisions, causing brief outages.
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 between the two switches
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. This fundamental misconfiguration causes severe packet collisions, as the half-duplex device expects to wait for a clear channel before transmitting, while the full-duplex device transmits whenever it has data. These collisions lead to a high number of Cyclic Redundancy Check (CRC) errors and frame retransmissions, resulting in the reported intermittent network disconnections and poor performance for users.
- ✗
STP reconvergence due to topology change
Why it's wrong here
Spanning Tree Protocol (STP) reconvergence occurs when a network topology change is detected, causing STP to recalculate the loop-free path. During this process, which can last several seconds, ports transition through listening and learning states, temporarily blocking traffic. While this certainly causes a brief loss of connectivity, it is a controlled process that does not inherently generate CRC errors; instead, frames are simply dropped or buffered until the new topology is stable.
When this WOULD be correct
This would be correct if the question described intermittent disconnections without any CRC errors, and the cable test passed, suggesting a Layer 2 loop or topology change causing STP to reconverge.
- ✗
Broadcast storm caused by a loop
Why it's wrong here
A broadcast storm results from a Layer 2 loop in the network, where broadcast frames are endlessly forwarded, consuming all available bandwidth. This leads to severe network congestion, extremely high latency, and potential network collapse as devices become overwhelmed processing the excessive broadcast traffic. While a broadcast storm causes significant connectivity problems and performance degradation, it does not primarily generate CRC errors; rather, it causes packet loss due to buffer overflows and congestion.
When this WOULD be correct
This option would be correct if the question described symptoms like all users on the VLAN experiencing severe slowdowns, high CPU usage on switches, and the switch logs showing excessive broadcast traffic, with a loop detected in the topology.
- ✗
Faulty SFP transceiver on the core switch
Why it's wrong here
A faulty SFP (Small Form-Factor Pluggable) transceiver typically manifests as consistent physical layer issues, such as complete link failure, persistent link flapping, or a high volume of various physical errors like excessive runts or giants. While a failing SFP can certainly disrupt connectivity, the specific symptom of intermittent disconnections coupled with CRC errors points more directly to a data link layer protocol issue like a duplex mismatch. A faulty SFP often results in more severe and less intermittent physical layer corruption or loss of signal.
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 two switchesCorrect 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. This fundamental misconfiguration causes severe packet collisions, as the half-duplex device expects to wait for a clear channel before transmitting, while the full-duplex device transmits whenever it has data. These collisions lead to a high number of Cyclic Redundancy Check (CRC) errors and frame retransmissions, resulting in the reported intermittent network disconnections and poor performance for users.
✗STP reconvergence due to topology changeWrong answer — click to see why▾
Why this is wrong here
CRC errors indicate physical-layer issues like duplex mismatch or cabling problems, not STP reconvergence. STP reconvergence causes brief connectivity loss but does not produce CRC errors.
★ When this WOULD be the correct answer
This would be correct if the question described intermittent disconnections without any CRC errors, and the cable test passed, suggesting a Layer 2 loop or topology change causing STP to reconverge.
Why candidates choose this
Candidates may associate intermittent disconnections with STP reconvergence, especially when a topology change is suspected, without considering that CRC errors point to a physical-layer issue.
✗Broadcast storm caused by a loopWrong answer — click to see why▾
Why this is wrong here
CRC errors indicate layer 1 or layer 2 issues like duplex mismatch, not a broadcast storm. A broadcast storm would cause high utilization and packet drops, not CRC errors.
★ When this WOULD be the correct answer
This option would be correct if the question described symptoms like all users on the VLAN experiencing severe slowdowns, high CPU usage on switches, and the switch logs showing excessive broadcast traffic, with a loop detected in the topology.
Why candidates choose this
Candidates may associate intermittent disconnections with broadcast storms because both can cause network instability, but they overlook that CRC errors point to physical or duplex issues rather than excessive broadcasts.
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?”
Visual reference
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
CRC
CRC (Cyclic Redundancy Check) is an error-detecting code used to detect accidental changes to raw data in digital networks.
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.