350-401 Architecture Practice Question
A network engineer is designing a QoS policy for a Cisco Catalyst switch. The requirement is to ensure that a specific class of traffic receives a guaranteed minimum bandwidth during congestion while still allowing other classes to use excess bandwidth when available. Which queuing mechanism should the engineer configure?
⚠ Common exam trap
The trap here is choosing LLQ because it is commonly associated with QoS, but LLQ provides strict priority rather than a guaranteed minimum bandwidth with excess sharing.
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
✓
Class-Based Weighted Fair Queuing (CBWFQ)
CBWFQ guarantees a minimum bandwidth to each class during congestion and allows classes to share unused bandwidth. LLQ provides strict priority, WRED only manages congestion avoidance, and FIFO offers no guarantees. Therefore CBWFQ is the mechanism that matches the stated requirement for a guaranteed floor with excess sharing.
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
Why it's wrong here
FIFO queuing places all packets in a single queue and forwards them in arrival order. It provides no bandwidth guarantee and no differentiation between traffic classes. Under congestion, all traffic competes equally, so a specific class cannot be assured a minimum bandwidth. FIFO is the default on some interfaces but does not meet this design requirement.
- ✓
Class-Based Weighted Fair Queuing (CBWFQ)
Why this is correct
CBWFQ is correct because it provides a guaranteed minimum bandwidth to each class during congestion, based on the configured bandwidth value, while allowing classes to use unused bandwidth from others. This matches the requirement for a guaranteed floor with excess sharing. It is implemented with the bandwidth command inside a policy-map class.
- ✗
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 drop precedence. It does not provide a guaranteed minimum bandwidth to a class. It is often used inside a class configured with CBWFQ, but by itself it does not reserve bandwidth or share excess capacity as required.
- ✗
Low Latency Queuing (LLQ)
Why it's wrong here
LLQ is designed to provide strict priority to delay-sensitive traffic such as voice, not a guaranteed minimum bandwidth with excess sharing for a general class. While LLQ includes CBWFQ for non-priority classes, the priority queue itself is policed and does not share excess bandwidth in the same way. The scenario asks for a guaranteed minimum, not strict priority.
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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
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