easyMultiple ChoiceObjective-mapped
QoS for Voice: DSCP EF and Strict Priority Queue
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?
Quick Answer
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.
⚠ Common exam trap
A common mix-up: candidates 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.
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
✓
Mark voice with EF and place in a strict priority queue.
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.
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.
- ✗
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.
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Same concept, more angles
1 more way this is tested on 350-401
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.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-401 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 350-401 exam.