CCNP Architecture Practice Question
A mid-size enterprise is deploying a new branch office with 50 users. The branch will have its own router, switch, and wireless AP. The WAN link is a 50 Mbps MPLS circuit. The company uses VoIP and requires Quality of Service. The network administrator has configured the router with a QoS policy that marks VoIP traffic with DSCP EF and all other traffic with DSCP 0. The policy also shapes traffic to 50 Mbps. After deployment, users report that voice quality is poor during peak hours. The administrator checks the router and sees that the output queue on the WAN interface is often full and drops are occurring. Which action should the administrator take to improve voice quality?
⚠ Common exam trap
Cisco often tests the misconception that increasing bandwidth or policing alone solves voice quality issues, but the trap here is that shaping without a priority queue causes all traffic to be treated equally, so VoIP suffers from jitter and delay even if the total rate is within the shaped limit.
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
✓
Configure a priority queue for DSCP EF traffic within the shaper.
The shaper is limiting traffic to 50 Mbps, but during peak hours, the aggregate traffic exceeds this rate, causing the output queue to fill and drop packets indiscriminately. By configuring a priority queue for DSCP EF (VoIP) traffic within the shaper, the router will service VoIP packets before other traffic, ensuring low latency and jitter even when the link is congested. This is the standard Cisco approach for voice quality on shaped links, as priority queuing bypasses the normal FIFO or CBWFQ behavior for marked traffic.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Increase the shaping rate to 60 Mbps to allow for burst.
Why it's wrong here
Raising the shaping rate to 60 Mbps would exceed the provider’s 50 Mbps committed rate, causing the provider’s shaping/policing at the edge to drop all excess packets—including voice—instead of allowing a controlled burst. Burst tolerance should be managed with the shaper's Bc/Be parameters, not by increasing the shaper's token bucket rate. This approach does not solve voice drops; it simply shifts the drop location from the local router to the provider edge.
- ✓
Configure a priority queue for DSCP EF traffic within the shaper.
Why this is correct
A priority queue, implemented via LLQ (Low Latency Queuing) within the CBWFQ shaper, ensures that DSCP EF voice packets are dequeued ahead of all other classes before the shaped output is released. This guarantees that voice traffic experiences low latency and jitter, and critically, that voice packets are not tail-dropped when the shaper's buffer is temporarily congested due to bursts. The priority queue's strict scheduling protects real-time traffic from the queue-full condition that causes drops in other classes.
- ✗
Replace shaping with policing to drop non-voice traffic.
Why it's wrong here
Policing is a token-bucket mechanism that immediately drops or re-marks packets that exceed a configured rate; it does not provide a buffer or queue. Unlike shaping, which holds excess traffic and allows deferred forwarding, policing will drop any burst that exceeds the rate—including short bursts of DSCP EF voice packets. Moreover, policing applies the same drop action across all classes unless complex hierarchical policies are used; it cannot perform per-class differential prioritization like a priority queue within a shaper.
- ✗
Change the marking to use CoS instead of DSCP for better QoS.
Why it's wrong here
CoS is a Layer 2 marking field in the 802.1Q VLAN header, only trusted on switchports and typically reset when frames traverse a Layer 3 routed WAN interface. The router's egress queuing decisions are based on the Layer 3 DSCP value (or the internal QoS class mapped from it), so switching to CoS would not change which queue the voice traffic uses. The reported drops are caused by the shaper's queue depth and the lack of priority treatment for EF, not by the marking type; therefore, changing from DSCP to CoS is irrelevant.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.