350-501 Automation and Quality of Services Practice Question
A service provider is designing a QoS policy for a business VPN service that carries voice, video, and data. The policy must ensure that voice (EF) is never dropped even during congestion, while video (AF41) and data (AF21) share the remaining bandwidth in a ratio 2:1. Which queuing strategy should be used?
⚠ Common exam trap
Cisco often tests the misconception that LLQ can be applied to multiple traffic classes simultaneously without understanding that only one strict priority queue is supported, and that CBWFQ alone cannot guarantee zero-drop for real-time 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
✓
Apply LLQ for voice and CBWFQ for video and data with bandwidth ratio 2:1.
LLQ provides strict priority queuing for voice (EF), ensuring it is never dropped during congestion, while CBWFQ allows the remaining bandwidth to be shared among video (AF41) and data (AF21) in a 2:1 ratio. LLQ is designed to guarantee low latency for real-time traffic, and CBWFQ enables explicit bandwidth allocation for non-real-time classes. This combination meets the requirement of zero-drop for voice and proportional sharing for video and data.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Use only LLQ for all three classes.
Why it's wrong here
LLQ typically supports only one strict priority queue; multiple priority queues are not standard.
- ✓
Apply LLQ for voice and CBWFQ for video and data with bandwidth ratio 2:1.
Why this is correct
LLQ provides strict priority; CBWFQ allocates bandwidth as needed.
- ✗
Apply CBWFQ with bandwidth percentages to all three classes.
Why it's wrong here
CBWFQ does not provide strict priority for voice; voice could be delayed.
- ✗
Use FIFO queuing with WRED for all classes.
Why it's wrong here
FIFO does not differentiate traffic.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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