- A
Verify that QoS markings and queuing are configured on the switches and routers
QoS ensures voice traffic is prioritized over data. Without proper QoS on all network devices, voice packets can be dropped when the network is congested.
- B
Check the DNS server for the phone's name resolution
Why wrong: DNS is used for call server registration, but intermittent call drops are more indicative of a Layer 2/3 quality issue rather than a name resolution issue.
- C
Replace the patch cables from the phones to the switches
Why wrong: Physical cabling issues usually cause complete disconnection or errors, not intermittent call drops without any link flapping. QoS is a more likely culprit.
- D
Increase the DHCP lease time for the phones
Why wrong: DHCP lease time affects IP address renewal, not the quality of voice traffic during a call. It would cause loss of connectivity if the lease expires.
Quick Answer
The answer is to verify that QoS markings and queuing are configured on the switches and routers. This is the most likely cause of intermittent VoIP call drops because voice traffic, carried over a dedicated voice VLAN, still competes with data traffic for bandwidth at the network layer. Without proper QoS policies—such as marking RTP packets with DSCP EF and applying strict priority queuing—latency, jitter, and packet loss will degrade call quality, even when VLANs are correctly assigned. On the CompTIA Network+ N10-009 exam, this scenario tests your understanding that VLAN separation alone does not guarantee voice quality; QoS must be consistently enforced across all Layer 3 devices. A common trap is assuming the voice VLAN assignment solves all issues, but the real culprit is often a missing or misconfigured queuing mechanism. Remember the mnemonic: “VLANs separate, but QoS prioritizes—mark it EF to keep calls deaf.”
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 network engineer is troubleshooting intermittent call drops on a VoIP deployment. The network uses separate VLANs for voice (VLAN 20) and data (VLAN 10). Switch ports connecting the IP phones are configured with the correct voice VLAN. Which of the following is the MOST likely cause to check NEXT?
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
Verify that QoS markings and queuing are configured on the switches and routers
Intermittent call drops on a VoIP deployment with separate voice and data VLANs most likely stem from insufficient or misconfigured QoS markings and queuing. Voice traffic is sensitive to latency, jitter, and packet loss, which occur when data traffic competes for bandwidth without proper prioritization. Verifying that switches and routers have consistent QoS policies (e.g., marking with DSCP EF for RTP and queuing with strict priority) is the logical next step after ensuring the voice VLAN is correctly assigned.
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.
- ✓
Verify that QoS markings and queuing are configured on the switches and routers
Why this is correct
QoS ensures voice traffic is prioritized over data. Without proper QoS on all network devices, voice packets can be dropped when the network is congested.
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.
- ✗
Check the DNS server for the phone's name resolution
Why it's wrong here
DNS is used for call server registration, but intermittent call drops are more indicative of a Layer 2/3 quality issue rather than a name resolution issue.
- ✗
Replace the patch cables from the phones to the switches
Why it's wrong here
Physical cabling issues usually cause complete disconnection or errors, not intermittent call drops without any link flapping. QoS is a more likely culprit.
- ✗
Increase the DHCP lease time for the phones
Why it's wrong here
DHCP lease time affects IP address renewal, not the quality of voice traffic during a call. It would cause loss of connectivity if the lease expires.
Common exam traps
Common exam trap: answer the scenario, not the keyword
CompTIA often tests the misconception that physical layer issues (like bad cables) are the primary cause of intermittent VoIP problems, when in fact intermittent drops are more commonly due to QoS misconfiguration in a converged network.
Detailed technical explanation
How to think about this question
VoIP call drops often occur when network devices lack proper QoS queuing mechanisms, such as Low Latency Queuing (LLQ) for voice traffic. Without DSCP EF (46) markings on RTP packets and corresponding trust configurations on switchports, voice packets may be dropped during congestion. A common real-world scenario is a switchport configured with 'mls qos trust cos' but the phone sends DSCP-marked packets, leading to re-marking and loss of priority.
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?
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: Verify that QoS markings and queuing are configured on the switches and routers — Intermittent call drops on a VoIP deployment with separate voice and data VLANs most likely stem from insufficient or misconfigured QoS markings and queuing. Voice traffic is sensitive to latency, jitter, and packet loss, which occur when data traffic competes for bandwidth without proper prioritization. Verifying that switches and routers have consistent QoS policies (e.g., marking with DSCP EF for RTP and queuing with strict priority) is the logical next step after ensuring the voice VLAN is correctly assigned.
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 30, 2026
This N10-009 practice question is part of Courseiva's free CompTIA 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 N10-009 exam.
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