- A
The switch port is configured for full-duplex
A duplex mismatch (switch full-duplex, device half-duplex) causes excessive collisions because the full-duplex switch transmits without checking for carrier sense, colliding with the half-duplex device's transmissions.
- B
The cable length exceeds 100 meters
Why wrong: Excessive cable length can cause signal degradation and late collisions, but the primary symptom is usually late collisions, not a general high collision count, and the scenario does not mention cable length.
- C
The device is using an incorrect VLAN
Why wrong: Incorrect VLAN assignment would cause connectivity issues but not increase collisions. Collisions are layer 1/2 issues related to timing, not VLAN membership.
- D
The switch port is configured for 1000BASE-T but the device only supports 100BASE-TX
Why wrong: Speed mismatch would cause the link to negotiate down or fail, but not typically result in a high collision count. Most modern switches auto-negotiate speed and duplex, and mismatches usually prevent link from coming up.
Quick Answer
The answer is a duplex mismatch caused by the switch port being configured for full-duplex while the connected device operates in half-duplex. This mismatch occurs because a full-duplex port transmits data without using CSMA/CD, meaning it never checks for a clear channel before sending, while the half-duplex device must listen for silence before transmitting. When the switch sends frames without sensing the carrier, the half-duplex device, believing the line is clear, sends its own frames simultaneously, resulting in collisions on the port. On the CompTIA Network+ N10-009 exam, this scenario tests your understanding of how duplex settings directly impact collision domains—a common trap is assuming cabling or speed issues are the cause. A quick memory tip: remember that full-duplex is “collision-free” by design, so if you see high collisions on a single port, always suspect a half-duplex neighbor forcing a mismatch.
N10-009 Networking Concepts Practice Question
This N10-009 practice question tests your understanding of networking concepts. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 switch is experiencing a high number of collisions on a specific port. The connected device is configured for half-duplex. Which of the following 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
The switch port is configured for full-duplex
When a switch port is configured for full-duplex but the connected device operates in half-duplex, a duplex mismatch occurs. The switch transmits without checking for collisions (as full-duplex does not use CSMA/CD), while the half-duplex device expects to sense the carrier before sending, leading to collisions on the port. This is the most common cause of excessive collisions on a single switch port.
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 configured for full-duplex
Why this is correct
A duplex mismatch (switch full-duplex, device half-duplex) causes excessive collisions because the full-duplex switch transmits without checking for carrier sense, colliding with the half-duplex device's transmissions.
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 cable length exceeds 100 meters
Why it's wrong here
Excessive cable length can cause signal degradation and late collisions, but the primary symptom is usually late collisions, not a general high collision count, and the scenario does not mention cable length.
- ✗
The device is using an incorrect VLAN
Why it's wrong here
Incorrect VLAN assignment would cause connectivity issues but not increase collisions. Collisions are layer 1/2 issues related to timing, not VLAN membership.
- ✗
The switch port is configured for 1000BASE-T but the device only supports 100BASE-TX
Why it's wrong here
Speed mismatch would cause the link to negotiate down or fail, but not typically result in a high collision count. Most modern switches auto-negotiate speed and duplex, and mismatches usually prevent link from coming up.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume collisions are always caused by cable length or physical issues, overlooking the duplex mismatch as the primary cause when one side is half-duplex and the other is full-duplex.
Trap categories for this question
Scenario analysis trap
Excessive cable length can cause signal degradation and late collisions, but the primary symptom is usually late collisions, not a general high collision count, and the scenario does not mention cable length.
Detailed technical explanation
How to think about this question
In half-duplex Ethernet, the device uses CSMA/CD (Carrier Sense Multiple Access with Collision Detection) to manage transmissions. A full-duplex switch port does not perform carrier sensing or collision detection, so it may transmit while the half-duplex device is sending, causing a collision. This mismatch also leads to excessive deferred transmissions and late collisions, which can degrade throughput to near zero in severe cases.
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
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Questions learners often ask
What does this N10-009 question test?
Networking Concepts — This question tests Networking Concepts — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The switch port is configured for full-duplex — When a switch port is configured for full-duplex but the connected device operates in half-duplex, a duplex mismatch occurs. The switch transmits without checking for collisions (as full-duplex does not use CSMA/CD), while the half-duplex device expects to sense the carrier before sending, leading to collisions on the port. This is the most common cause of excessive collisions on a single switch port.
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.
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Last reviewed: Jun 11, 2026
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