- A
The switch port is incorrectly configured for auto-negotiation and should be manually set to match the server's NIC.
Why wrong: Manually setting the switch port to 100 Mbps/half-duplex would resolve the mismatch, but the root cause is the mismatch itself, not the auto-negotiation. The problem is the duplex mismatch, and disabling auto-negotiation is merely a workaround.
- B
The server's NIC is failing, causing cyclic redundancy check (CRC) errors and forcing retransmissions.
Why wrong: A failing NIC might produce CRC errors and runts, but late collisions are specific to duplex mismatches. Hardware failures do not generate late collisions; they cause checksum errors, malformed frames, or physical link flapping.
- C
A duplex mismatch exists between the switch port and the server NIC.
The switch port negotiated full-duplex at 100 Mbps (as shown in the switch output), while the server NIC is hard-coded to half-duplex. This mismatch causes exactly the observed symptoms: late collisions on the full-duplex switch port and excessive retransmissions on the half-duplex server.
- D
The switch port is overloaded by a broadcast storm, causing an excessive number of collisions.
Why wrong: Broadcast storms generate a high volume of broadcast traffic, which can saturate the link and cause slow performance, but they do not cause late collisions. Late collisions are a layer-1 timing issue, not a traffic-load issue.
Quick Answer
The answer is a duplex mismatch between the switch port and the server NIC. This is correct because the switch, set to auto-negotiation, settles on full-duplex and transmits without listening for collisions, while the server’s manually configured half-duplex NIC uses CSMA/CD and detects collisions when frames arrive during its own transmission. Late collisions occur specifically because the collision is detected after the first 64 bytes, a hallmark of duplex mismatch rather than cabling issues. On the CCNA 200-301 v2 exam, this scenario tests your understanding of how auto-negotiation failures and manual speed/duplex settings create asymmetric link behavior, often appearing in troubleshooting questions about retransmissions and collision counters. A common trap is to blame the cable or speed mismatch, but the key is that speed matches at 100 Mbps while duplex differs. Memory tip: “Full sends freely, half fights back—late collisions mean duplex mismatch.”
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 a switch port connected to a legacy server is experiencing late collisions and the server reports excessive retransmissions. The switch port is configured for auto-negotiation and shows a negotiated speed of 100 Mbps and duplex full. The server's NIC is manually set to 100 Mbps and half-duplex. 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.
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 exists between the switch port and the server NIC.
Option C is correct because the switch port is auto-negotiating to full-duplex while the server's NIC is manually set to half-duplex. This creates a duplex mismatch: the switch transmits expecting no collisions (full-duplex), but the server, operating in half-duplex, detects collisions when the switch sends frames while the server is transmitting. Late collisions occur because the collision is detected after the first 64 bytes of the frame, and the server's half-duplex CSMA/CD logic forces retransmissions, matching the symptoms described.
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 switch port is incorrectly configured for auto-negotiation and should be manually set to match the server's NIC.
Why it's wrong here
Manually setting the switch port to 100 Mbps/half-duplex would resolve the mismatch, but the root cause is the mismatch itself, not the auto-negotiation. The problem is the duplex mismatch, and disabling auto-negotiation is merely a workaround.
- ✗
The server's NIC is failing, causing cyclic redundancy check (CRC) errors and forcing retransmissions.
Why it's wrong here
A failing NIC might produce CRC errors and runts, but late collisions are specific to duplex mismatches. Hardware failures do not generate late collisions; they cause checksum errors, malformed frames, or physical link flapping.
- ✓
A duplex mismatch exists between the switch port and the server NIC.
Why this is correct
The switch port negotiated full-duplex at 100 Mbps (as shown in the switch output), while the server NIC is hard-coded to half-duplex. This mismatch causes exactly the observed symptoms: late collisions on the full-duplex switch port and excessive retransmissions on the half-duplex server.
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.
- ✗
The switch port is overloaded by a broadcast storm, causing an excessive number of collisions.
Why it's wrong here
Broadcast storms generate a high volume of broadcast traffic, which can saturate the link and cause slow performance, but they do not cause late collisions. Late collisions are a layer-1 timing issue, not a traffic-load issue.
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.
✓A duplex mismatch exists between the switch port and the server NIC.Correct answer▾
Why this is correct
The switch port negotiated full-duplex at 100 Mbps (as shown in the switch output), while the server NIC is hard-coded to half-duplex. This mismatch causes exactly the observed symptoms: late collisions on the full-duplex switch port and excessive retransmissions on the half-duplex server.
✗The switch port is incorrectly configured for auto-negotiation and should be manually set to match the server's NIC.Wrong answer — click to see why▾
Why this is wrong here
Misunderstanding that auto-negotiation always causes duplex mismatches, when in fact a mismatch occurs because one side is manually configured while the other uses auto-negotiation to negotiate an incompatible mode.
✗The server's NIC is failing, causing cyclic redundancy check (CRC) errors and forcing retransmissions.Wrong answer — click to see why▾
Why this is wrong here
Confusing CRC errors with late collisions. Late collisions are a layer-1 timing issue, not a data integrity problem.
✗The switch port is overloaded by a broadcast storm, causing an excessive number of collisions.Wrong answer — click to see why▾
Why this is wrong here
Attributing all network performance problems to broadcast storms, ignoring the specific error counter 'late collisions' that points directly to a duplex mismatch.
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 concept that auto-negotiation mismatches (e.g., one side set to manual) cause duplex mismatches, and candidates mistakenly think the issue is speed mismatch or that both sides must be manually set, but the trap here is that the server's manual half-duplex setting overrides the auto-negotiation result, creating a duplex mismatch that produces late collisions.
Detailed technical explanation
How to think about this question
In Ethernet, full-duplex allows simultaneous transmission and reception without collisions, while half-duplex uses CSMA/CD where collisions are expected and detected within the slot time (512 bit times for 100 Mbps). A duplex mismatch causes the full-duplex side to never defer or back off, leading to late collisions (detected after 512 bit times) on the half-duplex side. The server's NIC, set to half-duplex, interprets the switch's simultaneous transmission as a collision and retransmits, but the switch never sees a collision, so it does not retransmit, causing frame loss and application-level retransmissions.
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 network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
What to study next
Got this wrong? Here's your next step.
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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: A duplex mismatch exists between the switch port and the server NIC. — Option C is correct because the switch port is auto-negotiating to full-duplex while the server's NIC is manually set to half-duplex. This creates a duplex mismatch: the switch transmits expecting no collisions (full-duplex), but the server, operating in half-duplex, detects collisions when the switch sends frames while the server is transmitting. Late collisions occur because the collision is detected after the first 64 bytes of the frame, and the server's half-duplex CSMA/CD logic forces retransmissions, matching the symptoms described.
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.
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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Last reviewed: Jun 25, 2026
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