The answer is a duplex mismatch. This is the most likely cause because the switch port is manually set to 100 Mbps and full duplex, while the printer is using auto-negotiation; when one side is hard-coded and the other is set to auto, the auto-negotiating side fails to detect the manual setting and defaults to half duplex. This mismatch forces the full-duplex side to transmit while the half-duplex side is also sending, causing collisions that corrupt frames and produce the observed FCS errors. On the CompTIA Network+ N10-009 exam, this scenario tests your understanding of how duplex mismatch directly leads to frame check sequence errors, a common trap where students mistakenly blame cable faults or speed mismatches. Remember the key rule: if one side is manually configured for duplex, the other must match—auto-negotiation only works when both sides use it. A helpful memory tip is "Manual one side, auto the other? Expect FCS errors and a half-duplex default."
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 switch is connected to a network printer. The switch port is manually configured for 100 Mbps and full duplex. The printer is configured for auto-negotiation. The link is up, but there are many FCS errors on the switch port. 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.
The manually-configured full duplex setting forces the link to operate at full duplex, but the printer's auto-negotiation may negotiate half duplex, leading to a duplex mismatch and errors.
B
Bad Ethernet cable
Why wrong: While a bad cable can cause FCS errors, the specific scenario of one side manually set and the other auto-negotiating strongly points to a duplex mismatch.
C
Speed mismatch
Why wrong: The link is up, indicating both sides are likely at the same speed (100 Mbps). Speed mismatch would prevent the link from coming up.
D
Printer driver issues
Why wrong: Driver issues would not cause physical layer errors like FCS errors; they are a Layer 2/3 issue.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Duplex mismatch
The most likely cause is a duplex mismatch. The switch port is manually set to full duplex, while the printer is using auto-negotiation. When one side is manually configured and the other is set to auto-negotiation, the auto-negotiating side fails to detect the manual setting and defaults to half duplex. This mismatch causes collisions and frame check sequence (FCS) errors on the full-duplex side, as the half-duplex side does not properly handle simultaneous transmission.
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.
✓
Duplex mismatch
Why this is correct
The manually-configured full duplex setting forces the link to operate at full duplex, but the printer's auto-negotiation may negotiate half duplex, leading to a duplex mismatch and errors.
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.
✗
Bad Ethernet cable
Why it's wrong here
While a bad cable can cause FCS errors, the specific scenario of one side manually set and the other auto-negotiating strongly points to a duplex mismatch.
✗
Speed mismatch
Why it's wrong here
The link is up, indicating both sides are likely at the same speed (100 Mbps). Speed mismatch would prevent the link from coming up.
✗
Printer driver issues
Why it's wrong here
Driver issues would not cause physical layer errors like FCS errors; they are a Layer 2/3 issue.
Common exam traps
Common exam trap: answer the scenario, not the keyword
CompTIA often tests the misconception that a speed mismatch causes FCS errors, but the trap here is that a speed mismatch prevents the link from coming up, while a duplex mismatch allows the link to be up but with errors.
Trap categories for this question
Scenario analysis trap
While a bad cable can cause FCS errors, the specific scenario of one side manually set and the other auto-negotiating strongly points to a duplex mismatch.
Detailed technical explanation
How to think about this question
When a device with manual duplex (full) connects to an auto-negotiating device, the auto-negotiating device fails to receive the duplex advertisement and falls back to half duplex per IEEE 802.3. The half-duplex side uses CSMA/CD and may transmit while the full-duplex side is sending, causing collisions that the full-duplex side does not detect, leading to late collisions and FCS errors. This scenario is common in legacy environments where network administrators hard-code ports for printers or servers, forgetting that auto-negotiation requires both sides to be set to auto for proper operation.
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 practitioner preparing for the N10-009 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
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 — The most likely cause is a duplex mismatch. The switch port is manually set to full duplex, while the printer is using auto-negotiation. When one side is manually configured and the other is set to auto-negotiation, the auto-negotiating side fails to detect the manual setting and defaults to half duplex. This mismatch causes collisions and frame check sequence (FCS) errors on the full-duplex side, as the half-duplex side does not properly handle simultaneous transmission.
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.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A network technician notices that a switch port connected to a user's computer is showing a high number of CRC errors and late collisions. The link is operating at 100 Mbps, full duplex according to the switch. Which of the following is the most likely cause of these errors?
medium
A.Cable length exceeds 100 meters
✓ B.Duplex mismatch
C.Faulty switch port
D.Electromagnetic interference
Why B: CRC errors and late collisions in a full-duplex link are classic symptoms of a duplex mismatch. When one side is set to full duplex and the other to half duplex, the half-duplex side does not sense the carrier before transmitting, leading to collisions that are detected late in the frame. The switch reports full duplex, so the user's NIC is likely stuck at half duplex, causing these errors.
Variation 2. A network technician is troubleshooting a workstation that is experiencing intermittent connectivity and slow file transfers. The technician has confirmed that the patch cable passes a cable tester, the switch port is not error-disabled, and the workstation's NIC is configured for autonegotiation. The switch port is also set to autonegotiate. Which of the following should the technician check next?
medium
A.The route table on the default gateway
✓ B.The duplex and speed settings on both ends
C.The DNS server configuration
D.The MAC address filtering on the switch
Why B: Option B is correct because intermittent connectivity and slow file transfers, despite a passing cable test and autonegotiation on both ends, strongly indicate a duplex mismatch. When both sides are set to autonegotiate but one side fails to negotiate correctly (e.g., due to a faulty NIC or switch port), they may fall back to half-duplex while the other remains full-duplex, causing collisions, CRC errors, and retransmissions. The technician should verify the actual negotiated duplex and speed on both the workstation NIC and the switch port using commands like 'show interfaces' or NIC driver properties.
Last reviewed: Jun 30, 2026
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