350-401 Infrastructure Practice Question
An engineer is implementing a QoS policy on a Cisco IOS XE router. The requirement is to prioritize voice traffic (marked DSCP EF) and ensure that it receives strict priority scheduling with a guaranteed bandwidth of 30% of the interface capacity. Which queuing mechanism should be configured on the interface?
⚠ Common exam trap
The trap here is assuming that CBWFQ alone can provide strict priority for voice; in reality, only LLQ (CBWFQ with a priority queue) offers strict priority scheduling.
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) with a priority statement for the voice class.
Low Latency Queuing (LLQ) combines the bandwidth guarantees of CBWFQ with a strict priority queue. By configuring a priority statement for the voice class, packets marked DSCP EF are placed in a low-latency queue that is serviced before other queues, ensuring minimal delay and jitter. The priority bandwidth allocation of 30% guarantees that voice traffic receives the necessary bandwidth even during congestion.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
First-In, First-Out (FIFO) queuing with a rate limit for voice traffic.
Why it's wrong here
FIFO queuing treats all packets equally and does not provide priority or bandwidth guarantees. A rate limit could restrict voice traffic but would not prioritize it over other traffic. This approach would introduce delay and jitter for voice, as voice packets would wait behind other packets in the single queue. It does not satisfy the requirements for strict priority and guaranteed bandwidth.
- ✓
Low Latency Queuing (LLQ) with a priority statement for the voice class.
Why this is correct
LLQ is an extension of CBWFQ that adds a strict priority queue for delay-sensitive traffic such as voice. Configuring a priority statement for the voice class ensures that packets marked DSCP EF are dequeued first, up to the specified bandwidth (30%). This provides strict priority scheduling and guarantees bandwidth, meeting both requirements for voice traffic.
- ✗
Weighted Random Early Detection (WRED) with a precedence-based drop policy for voice.
Why it's wrong here
WRED is a congestion avoidance mechanism that drops packets based on queue thresholds and precedence. It does not provide strict priority scheduling or bandwidth guarantees. Using WRED for voice would not ensure low latency and jitter; it is typically used for TCP-based traffic to prevent global synchronization. This option fails to meet the strict priority requirement.
- ✗
Class-Based Weighted Fair Queuing (CBWFQ) with a bandwidth guarantee for the voice class.
Why it's wrong here
CBWFQ provides weighted fair queuing with minimum bandwidth guarantees, but it does not offer strict priority scheduling. Voice traffic would compete with other classes based on weights, potentially introducing latency and jitter. For strict priority, Low Latency Queuing (LLQ) must be used, which is CBWFQ with a priority queue. This option alone does not meet the strict priority requirement.
Go deeper
Related to this question
About these practice questions
This 350-401 question is part of Courseiva's 1,923-question bank — original exam-style content with full explanations and wrong-answer analysis, never real exam questions or exam dumps. Learn why practice questions differ from exam dumps →
JA
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
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.