350-501 Automation and Quality of Service Practice Question
A service provider is deploying QoS on a Cisco ASR 9000 router to support a triple-play service (voice, video, data) over an Ethernet access network. The network engineer must ensure that voice traffic (EF) is prioritized, video traffic (AF41) is treated with low loss, and data traffic (default) gets best-effort service. Additionally, the engineer must implement a hierarchical QoS policy to shape the aggregate subscriber traffic to 100 Mbps on a GigabitEthernet interface, with the following per-class bandwidth allocations: voice 10 Mbps, video 40 Mbps, and data 50 Mbps. During testing, voice packets are being dropped under congestion, and video traffic is experiencing jitter. The current QoS configuration is as follows:
policy-map CHILD
class VOICE
priority level 1 police rate 10 mbps
class VIDEO
bandwidth remaining ratio 40
class DATA
bandwidth remaining ratio 50 ! policy-map PARENT
class class-default
shape average 100 mbps service-policy CHILD !
Which action should the engineer take to resolve the issues while maintaining the design objectives?
⚠ Common exam trap
Cisco often tests the misconception that a police rate on a priority class is necessary to protect other classes, when in fact the parent shaper already limits the aggregate, and the priority queue should be left unpoliced to avoid dropping delay-sensitive voice 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
✓
Remove the police from the voice class and add 'priority level 1' without police; this ensures voice is priority queued without dropping.
The current configuration uses a police rate of 10 Mbps under the voice priority class, which causes voice packets to be dropped when they exceed that rate, even though priority queuing should guarantee low latency. Removing the police and keeping 'priority level 1' allows voice traffic to be strictly prioritized without a hard rate limit, ensuring no drops for voice while still allowing the parent shaper to enforce the aggregate 100 Mbps. This resolves the voice drops and, by preventing voice from being policed, reduces jitter for video traffic that might otherwise be affected by bursty voice drops.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Change the video class to 'priority level 2' to give it preferential treatment over data.
Why it's wrong here
Using a second priority level for video can still cause voice jitter and is not recommended for video.
- ✓
Remove the police from the voice class and add 'priority level 1' without police; this ensures voice is priority queued without dropping.
Why this is correct
Priority queuing with no police allows voice packets to be transmitted ahead of other queues, avoiding drops.
- ✗
Add a 'police' to the video class with a CIR of 40 Mbps and 'conform-action transmit exceed-action drop' to protect voice.
Why it's wrong here
Video already has a bandwidth guarantee; policing video will not solve voice drops and may cause video loss.
- ✗
Change the voice class to use 'bandwidth remaining percent 10' and remove the police.
Why it's wrong here
Voice requires strict priority, not just bandwidth guarantee, to meet delay requirements.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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