- A
The server NIC is set to 10 Mbps half-duplex, causing a speed mismatch.
Why wrong: A speed mismatch would prevent the link from coming up properly—the interface would be down/down or show no connectivity at all—not intermittent connectivity with high input errors.
- B
The server NIC is auto-negotiating to 100 Mbps half-duplex, resulting in a duplex mismatch.
With the switch forced to full-duplex and the server using auto-negotiation, the server defaults to half-duplex. The duplex mismatch leads to collisions on the half-duplex side, generating runts, CRC errors, and input errors on the switch port.
- C
The cable connecting the server to the switch is faulty, introducing excessive noise.
Why wrong: A defective cable can cause CRC errors, but the combination of runts, input errors, and minimal output errors is a strong indicator of a duplex mismatch. Cable issues usually affect all traffic consistently and produce errors on both ends.
- D
The switch port is configured with an incorrect native VLAN, causing duplex negotiation issues.
Why wrong: VLAN configuration does not affect physical layer error counters such as CRC or runts. Incorrect VLANs cause Layer 2 forwarding problems (e.g., traffic not reaching the correct VLAN), not interface input errors.
Quick Answer
The correct answer is that the server NIC is auto-negotiating to 100 Mbps half-duplex, causing a duplex mismatch. This is because when the switch port is hardcoded to 100 full-duplex and the server negotiates to half-duplex, the full-duplex side never detects collisions, so it transmits while the half-duplex side is also sending, resulting in corrupted frames that appear as runts, CRC errors, and input errors on the switch interface. On the CCNA 200-301 v2 exam, this scenario tests your ability to interpret show interfaces output and distinguish duplex mismatch symptoms from cable faults or speed mismatches—a common trap is assuming speed must differ, but here speed matches at 100 Mbps, isolating the issue to duplex. Remember the key indicator: high input errors with minimal output errors point to a mismatch, not a bad cable. For a quick memory tip, think “Full sends freely, Half fights back—runts and CRCs are the attack.”
CCNA Network Infrastructure and Connectivity Practice Question
This 200-301 practice question tests your understanding of network infrastructure and connectivity. 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 network engineer notices that users in VLAN 10 report intermittent connectivity and slow file transfers to a server on the same switch. The engineer issues the show interfaces fa0/1 command on the switch port connected to the server and observes a high number of runts, input errors, and CRC errors, while output errors are minimal. The interface configuration shows speed 100 and duplex full.
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
The server NIC is auto-negotiating to 100 Mbps half-duplex, resulting in a duplex mismatch.
Option B is correct because the symptoms—high runts, input errors, and CRC errors with minimal output errors—are classic indicators of a duplex mismatch. When the switch port is hardcoded to 100 Mbps full-duplex and the server NIC auto-negotiates to 100 Mbps half-duplex (a common fallback when one side is manually set), collisions occur on the full-duplex side, corrupting frames and causing input errors. The speed matches (both 100 Mbps), so the issue is purely duplex-related.
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.
- ✗
The server NIC is set to 10 Mbps half-duplex, causing a speed mismatch.
Why it's wrong here
A speed mismatch would prevent the link from coming up properly—the interface would be down/down or show no connectivity at all—not intermittent connectivity with high input errors.
- ✓
The server NIC is auto-negotiating to 100 Mbps half-duplex, resulting in a duplex mismatch.
Why this is correct
With the switch forced to full-duplex and the server using auto-negotiation, the server defaults to half-duplex. The duplex mismatch leads to collisions on the half-duplex side, generating runts, CRC errors, and input errors on the switch port.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The cable connecting the server to the switch is faulty, introducing excessive noise.
Why it's wrong here
A defective cable can cause CRC errors, but the combination of runts, input errors, and minimal output errors is a strong indicator of a duplex mismatch. Cable issues usually affect all traffic consistently and produce errors on both ends.
- ✗
The switch port is configured with an incorrect native VLAN, causing duplex negotiation issues.
Why it's wrong here
VLAN configuration does not affect physical layer error counters such as CRC or runts. Incorrect VLANs cause Layer 2 forwarding problems (e.g., traffic not reaching the correct VLAN), not interface input errors.
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 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓The server NIC is auto-negotiating to 100 Mbps half-duplex, resulting in a duplex mismatch.Correct answer▾
Why this is correct
With the switch forced to full-duplex and the server using auto-negotiation, the server defaults to half-duplex. The duplex mismatch leads to collisions on the half-duplex side, generating runts, CRC errors, and input errors on the switch port.
✗The server NIC is set to 10 Mbps half-duplex, causing a speed mismatch.Wrong answer — click to see why▾
Why this is wrong here
Misconception that speed mismatch produces specific error counters; in reality, incompatible speeds cause link failure, not runts and CRC errors.
✗The cable connecting the server to the switch is faulty, introducing excessive noise.Wrong answer — click to see why▾
Why this is wrong here
Misconception that CRC errors alone point to a bad cable; duplex mismatch is one of the most common causes of runts and CRC errors on forced-full interfaces.
✗The switch port is configured with an incorrect native VLAN, causing duplex negotiation issues.Wrong answer — click to see why▾
Why this is wrong here
Misconception that a VLAN mismatch can trigger interface errors; these errors are purely physical/data-link layer phenomena unrelated to VLAN settings.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that speed and duplex mismatches always occur together; the trap here is that a duplex mismatch can exist even when speed matches, and the error pattern (high input errors, low output errors) specifically points to duplex, not cabling or VLAN issues.
Trap categories for this question
Command / output trap
A speed mismatch would prevent the link from coming up properly—the interface would be down/down or show no connectivity at all—not intermittent connectivity with high input errors.
Detailed technical explanation
How to think about this question
When one side of an Ethernet link is manually set to full-duplex and the other side auto-negotiates, the auto-negotiating side may fail to detect the manual configuration and fall back to half-duplex per IEEE 802.3. This causes the full-duplex side to transmit without checking for collisions, while the half-duplex side detects collisions and retransmits, leading to late collisions, runts, and CRC errors. The show interfaces command's 'input errors' counter includes runts, CRC errors, and frame errors, which are all elevated in this scenario.
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 help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
What to study next
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FAQ
Questions learners often ask
What does this 200-301 question test?
Network Infrastructure and Connectivity — This question tests Network Infrastructure and Connectivity — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The server NIC is auto-negotiating to 100 Mbps half-duplex, resulting in a duplex mismatch. — Option B is correct because the symptoms—high runts, input errors, and CRC errors with minimal output errors—are classic indicators of a duplex mismatch. When the switch port is hardcoded to 100 Mbps full-duplex and the server NIC auto-negotiates to 100 Mbps half-duplex (a common fallback when one side is manually set), collisions occur on the full-duplex side, corrupting frames and causing input errors. The speed matches (both 100 Mbps), so the issue is purely duplex-related.
What should I do if I get this 200-301 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Same concept, more angles
1 more ways this is tested on 200-301
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 switch port and a host NIC have a duplex mismatch. Which symptom is most likely?
medium- ✓ A.Increased late collisions and poor performance
- B.Incorrect VLAN tagging on trunks
- C.OSPF area mismatch errors
- D.A change in the subnet mask on the host
Why A: A duplex mismatch often causes collisions, frame errors, and degraded throughput, especially on the half-duplex side. It is a classic physical/link layer performance problem.
Last reviewed: Jun 25, 2026
This 200-301 practice question is part of Courseiva's free Cisco 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 200-301 exam.
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