Question 200 of 464
Duplex Mismatch and FCS Errors: Troubleshooting Switch Ports
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?
Quick Answer
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."
⚠ Common exam trap
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.
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
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.
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.
- ✗
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.
When this WOULD be correct
A bad Ethernet cable would be correct if the question described intermittent link drops, excessive CRC errors, or the link not coming up at all, with both sides configured identically for speed and duplex.
- ✗
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.
When this WOULD be correct
If the link were down or flapping, and the switch port was set to a different speed (e.g., 1000 Mbps) while the printer auto-negotiated to 100 Mbps, speed mismatch would be the likely cause.
- ✗
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.
When this WOULD be correct
A question where a printer intermittently fails to print or prints garbled text, and the network connectivity is fine, would point to printer driver issues as the 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 mismatchCorrect answer▾
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.
✗Bad Ethernet cableWrong answer — click to see why▾
Why this is wrong here
FCS errors indicate frame corruption at Layer 2, often due to duplex mismatch causing collisions. A bad cable would cause link instability or CRC errors, but the link is up and stable, making cable issues less likely than duplex mismatch.
★ When this WOULD be the correct answer
A bad Ethernet cable would be correct if the question described intermittent link drops, excessive CRC errors, or the link not coming up at all, with both sides configured identically for speed and duplex.
Why candidates choose this
Candidates often default to blaming physical cabling for any errors, not realizing that FCS errors specifically point to duplex mismatch when one side is manually set and the other auto-negotiates.
✗Speed mismatchWrong answer — click to see why▾
Why this is wrong here
A speed mismatch would prevent the link from coming up entirely, or cause intermittent connectivity, not FCS errors on an active link. The link is up, so speed is negotiated correctly.
★ When this WOULD be the correct answer
If the link were down or flapping, and the switch port was set to a different speed (e.g., 1000 Mbps) while the printer auto-negotiated to 100 Mbps, speed mismatch would be the likely cause.
Why candidates choose this
Candidates may confuse speed and duplex negotiation, assuming that any mismatch in link parameters causes FCS errors, but speed mismatch typically prevents link establishment, not frame corruption.
✗Printer driver issuesWrong answer — click to see why▾
Why this is wrong here
Printer driver issues would not cause FCS errors on the switch port. FCS errors indicate physical layer problems like collisions or duplex mismatches, not software or driver issues.
★ When this WOULD be the correct answer
A question where a printer intermittently fails to print or prints garbled text, and the network connectivity is fine, would point to printer driver issues as the cause.
Why candidates choose this
Candidates may think that since the printer is involved, driver problems could affect network communication, but FCS errors are purely layer 1/2 issues unrelated to drivers.
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?”
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Same concept, more angles
2 more ways this is tested on N10-009
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: 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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