350-401 Architecture Practice Question
A network engineer is designing a QoS policy for a campus network. The requirement is that voice traffic must be guaranteed strict priority treatment over all other traffic types, even during congestion. Which queuing mechanism should be configured on the egress interfaces to meet this requirement?
⚠ Common exam trap
Test-takers frequently confuse CBWFQ with LLQ; CBWFQ provides weighted bandwidth guarantees but not strict priority, which is essential for voice traffic.
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
✓
Low Latency Queuing (LLQ)
LLQ is the Cisco IOS queuing mechanism that provides strict priority to designated classes, such as voice, while using CBWFQ for other classes. The priority queue is serviced first, ensuring minimal delay and jitter for voice even under congestion. A policer limits the priority queue to prevent it from starving other queues. CBWFQ, WRED, and FIFO do not provide strict priority scheduling, so LLQ is the correct choice.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Class-Based Weighted Fair Queuing (CBWFQ)
Why it's wrong here
CBWFQ provides weighted fair treatment among classes based on configured bandwidth guarantees, but it does not offer strict priority to any single class. During congestion, voice traffic in a CBWFQ class could be delayed behind other classes that also have bandwidth guarantees. It is suitable for fair distribution but cannot guarantee the absolute priority that voice requires. Therefore, it does not meet the strict priority requirement.
- ✓
Low Latency Queuing (LLQ)
Why this is correct
LLQ combines strict priority queuing with CBWFQ. It designates one or more classes as priority queues, which are serviced before any other queues. This ensures that voice traffic receives strict priority treatment and minimal delay and jitter, even during congestion. LLQ also includes a policer to limit the priority queue bandwidth, preventing starvation of other traffic. It directly satisfies the requirement for strict priority for voice.
- ✗
First-In, First-Out (FIFO) queuing
Why it's wrong here
FIFO queuing processes packets in the order they arrive, with no differentiation between traffic types. It offers no priority or bandwidth guarantees. During congestion, voice packets could be delayed behind large data packets, causing unacceptable jitter and latency. FIFO is the default queuing method on some interfaces but is unsuitable for guaranteeing strict priority to voice traffic in a converged network.
- ✗
Weighted Random Early Detection (WRED)
Why it's wrong here
WRED is a congestion avoidance mechanism that randomly drops packets before a queue is full, based on IP precedence or DSCP values. It does not provide strict priority scheduling. Its purpose is to prevent tail drop and manage congestion, not to guarantee priority for voice. Using WRED alone would not ensure that voice traffic is serviced ahead of other traffic during congestion, so it fails the scenario requirement.
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Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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