CCNP Infrastructure Practice Question
A network engineer is implementing QoS on a Cisco IOS router. The engineer wants to ensure that VoIP traffic is marked with DSCP EF and that the router prioritizes this traffic during congestion. Which mechanism should be used to provide priority queuing for VoIP?
⚠ Common exam trap
The trap here is assuming that CBWFQ alone can provide priority, when it only guarantees bandwidth without strict priority.
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 QoS mechanism that provides strict priority queuing, ensuring that VoIP traffic marked with DSCP EF is serviced before other traffic during congestion. This minimizes latency and jitter, which are critical for voice quality. LLQ is configured using the 'priority' command within a policy map.
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 bandwidth guarantees to classes but does not offer strict priority. During congestion, CBWFQ queues are serviced in a weighted fair manner, which can introduce latency and jitter for VoIP. VoIP requires strict priority to ensure low latency, so CBWFQ alone is not sufficient.
- ✗
Weighted Random Early Detection (WRED)
Why it's wrong here
WRED is a congestion avoidance mechanism that drops packets based on queue thresholds to prevent tail drop. It does not provide priority queuing. While it can be used in conjunction with LLQ, WRED alone does not prioritize VoIP traffic; it merely manages congestion.
- ✗
Traffic Shaping
Why it's wrong here
Traffic shaping delays packets to smooth out bursts and conform to a specified rate. It does not provide priority queuing; instead, it can introduce additional delay. For VoIP, shaping is not appropriate as it can increase latency and jitter, which are detrimental to voice quality.
- ✓
Low Latency Queuing (LLQ)
Why this is correct
LLQ combines CBWFQ with a strict priority queue. It allows VoIP traffic to be placed in a priority queue that is serviced before other queues, ensuring minimal delay and jitter. LLQ is the recommended mechanism for prioritizing real-time traffic like VoIP during congestion.
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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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