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350-401 Practice Question: An enterprise is redesigning its WAN QoS…
An enterprise is redesigning its WAN QoS architecture to support real-time voice, video, and critical data applications over a limited bandwidth link. The architect must ensure that voice traffic receives strict priority queuing and that video traffic is guaranteed a minimum bandwidth, while allowing best-effort traffic to use remaining capacity. Which queuing strategy should be deployed on the WAN edge routers?
⚠ Common exam trap
Cisco often tests the distinction between CBWFQ and LLQ, trapping candidates who think CBWFQ alone can provide strict priority queuing, when in fact only LLQ adds the 'priority' keyword to create a 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
✓
LLQ (Low Latency Queuing) with a strict priority queue for voice and CBWFQ for video and data
LLQ combines a strict priority queue for delay-sensitive voice traffic with CBWFQ for other classes, guaranteeing minimum bandwidth for video while allowing best-effort traffic to share remaining capacity. This satisfies the requirement for strict priority queuing for voice and bandwidth guarantees for video, which CBWFQ alone cannot provide because it lacks a priority queue.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
FIFO (First In, First Out) with tail drop
Why it's wrong here
FIFO with tail drop simply transmits packets in the order they arrive and, when the queue overflows, discards all new packets regardless of their DSCP markings or importance. This means voice packets can be dropped or delayed behind a burst of data traffic, leading to unpredictable jitter and loss that violate strict latency requirements. Since FIFO neither provides per-class treatment nor reserves bandwidth, it cannot meet the differentiated QoS needs of a converged enterprise network.
- ✗
CBWFQ (Class-Based Weighted Fair Queuing) without a priority queue
Why it's wrong here
CBWFQ classifies traffic into classes and allocates each class a dedicated queue and guaranteed bandwidth, so voice could be assigned a share of the interface capacity. However, without a strict priority queue, voice packets still experience queuing delay and jitter during congestion because the scheduler services classes in a weighted fair manner rather than immediately. The lack of a low-latency, strict-priority path for voice means the bounded delay required for real-time interactive traffic cannot be achieved, so this option fails to meet the design requirement.
- ✓
LLQ (Low Latency Queuing) with a strict priority queue for voice and CBWFQ for video and data
Why this is correct
LLQ combines a strict priority queue with CBWFQ, ensuring that voice packets are always transmitted first, which minimizes both latency and jitter for real-time traffic. The priority queue is typically policed to a configured bandwidth limit to prevent a flood of voice traffic from starving video or data. Meanwhile, video and data are placed into CBWFQ classes that receive guaranteed bandwidth under the weighted fair scheduler, allowing the design to satisfy both strict voice latency and bandwidth guarantees for other critical applications.
- ✗
WRED (Weighted Random Early Detection) with DSCP-based drop probabilities
Why it's wrong here
WRED is a congestion avoidance mechanism that monitors queue depth and randomly drops packets with a probability based on DSCP markings, thereby avoiding the global synchronization issues of tail drop. However, it does not reorder, prioritize, or reserve bandwidth; it merely manages congestion by discarding lower-priority packets more aggressively. Because WRED lacks any dedicated queue or strict priority class for voice, it cannot guarantee low latency or sufficient bandwidth, so it is not a viable queuing strategy for the stated requirements.
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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.