The answer is to mark voice traffic with DSCP EF and place it in a strict priority queue. This is correct because voice traffic is extremely sensitive to delay and jitter, and the DiffServ model uses DSCP EF (Expedited Forwarding, per RFC 3246) as the standard per-hop behavior for real-time flows, while a strict priority queue—often implemented as Low Latency Queuing (LLQ)—ensures that voice packets are serviced before any other class, guaranteeing minimal latency. On the ENCOR 350-401 exam, this concept tests your understanding of QoS design for converged networks, and it often appears in scenario-based questions where you must choose the correct marking and queuing strategy for voice. A common trap is confusing DSCP EF with AF41 (video) or using a non-priority queue, which would introduce jitter. Remember the memory tip: “EF for voice, strict for choice”—EF is the only DSCP value that demands strict priority treatment.
CCNP Network Function Virtualization Practice Question
This 350-401 practice question tests your understanding of network function virtualization. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 team must design QoS for a campus network that carries voice, video, and data traffic. The design must use the DiffServ model and ensure that voice traffic is prioritized over all other traffic classes. Which DSCP marking and queuing strategy should be used for voice?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Mark voice with EF and place in a strict priority queue.
Option B is correct because voice traffic requires strict priority to ensure minimal jitter and latency. DSCP EF (Expedited Forwarding, per RFC 3246) is the standard marking for real-time traffic like voice, and placing it in a strict priority queue (LLQ) guarantees that voice packets are serviced before any other queue, which is essential for meeting QoS requirements in a DiffServ model.
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.
✗
Mark voice with AF41 and place in a weighted fair queue.
Why it's wrong here
AF41 is for video, and weighted fair queue does not provide strict priority.
✓
Mark voice with EF and place in a strict priority queue.
Why this is correct
EF (DSCP 46) is the standard marking for voice, and strict priority queue ensures minimal delay.
Related concept
Read the scenario before looking for a memorised answer.
✗
Mark voice with CS3 and place in a low-latency queue.
Why it's wrong here
CS3 is used for broadcast video, not voice; low-latency queue is synonymous with strict priority, but the marking is wrong.
✗
Mark voice with BE and rely on WRED for drop precedence.
Why it's wrong here
Best-effort marking provides no priority, and WRED is for congestion avoidance, not prioritization.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse AF41 (used for video) with voice marking, or assume that any low-latency queue (LLQ) works regardless of DSCP value, but Cisco specifically tests that voice must use EF and strict priority queue, not just any low-latency queue.
Detailed technical explanation
How to think about this question
Under the hood, DiffServ uses the 6-bit DSCP field in the IP header; EF is marked as 101110 (46 decimal) and is policed to a configured rate to prevent starvation of other traffic. In a real-world scenario, if voice is not placed in a strict priority queue, even a small amount of congestion can cause voice packets to be queued behind data packets, leading to unacceptable delay (over 150 ms one-way) and jitter, which breaks call quality. Cisco IOS implements this with the 'priority' command under a class map in MQC (Modular QoS CLI), which automatically creates a strict priority queue (LLQ) for the EF-marked traffic.
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 practitioner preparing for the 350-401 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
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.
Network Function Virtualization — This question tests Network Function Virtualization — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Mark voice with EF and place in a strict priority queue. — Option B is correct because voice traffic requires strict priority to ensure minimal jitter and latency. DSCP EF (Expedited Forwarding, per RFC 3246) is the standard marking for real-time traffic like voice, and placing it in a strict priority queue (LLQ) guarantees that voice packets are serviced before any other queue, which is essential for meeting QoS requirements in a DiffServ model.
What should I do if I get this 350-401 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
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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 team must design a QoS policy for a WAN link that carries voice, video, and data. The policy must ensure that voice traffic is never dropped, even during congestion. Which queuing mechanism should be used for the voice class?
easy
A.Class-based weighted fair queuing (CBWFQ).
✓ B.Low-latency queuing (LLQ).
C.Weighted random early detection (WRED).
D.First-in, first-out (FIFO) queuing.
Why B: Low-latency queuing (LLQ) is the correct choice because it combines strict priority queuing with CBWFQ, allowing voice traffic to be placed in a strict priority queue that is serviced first before any other queues. This ensures that voice packets are never dropped due to congestion, as long as the configured policer rate is not exceeded, meeting the requirement that voice traffic is never dropped.
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Question Discussion
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