350-501 Automation and Quality of Service Practice Question
A customer is using a Cisco ASR 9000 router with hierarchical QoS (HQoS) on a subscriber interface. The parent policy sets a shape rate of 10 Mbps, but the observed traffic rate is only 8 Mbps. What is a common misconfiguration that would cause this?
⚠ Common exam trap
Cisco often tests the distinction between shaping and policing in HQoS, where candidates mistakenly think the parent shaper always controls the final rate, ignoring that a child policer can override it by dropping traffic earlier in the pipeline.
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
✓
The child policy includes a police command that is limiting traffic below the parent shaper rate
In hierarchical QoS (HQoS) on a Cisco ASR 9000, the parent policy shapes traffic to a specified rate, but if the child policy includes a police command, that policer can independently drop or mark down traffic before it reaches the parent shaper. This causes the observed traffic rate to be lower than the parent shape rate, as the child policer is the actual bottleneck. The correct answer is A because the police command in the child policy is limiting traffic below the 10 Mbps parent shaper, resulting in only 8 Mbps observed.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The child policy includes a police command that is limiting traffic below the parent shaper rate
Why this is correct
If a child policy has a police command with a rate of 8 Mbps, that overrides the parent shaper for that class, resulting in a lower observed rate.
- ✗
The child policy uses 'bandwidth remaining' instead of 'bandwidth'
Why it's wrong here
Using 'bandwidth remaining' is valid and does not cause a rate drop.
- ✗
The child policy includes a bandwidth command that exceeds the parent shaper rate
Why it's wrong here
Bandwidth commands in child policies do not limit the aggregate; the parent shaper still enforces 10 Mbps.
- ✗
The parent shaper uses 'shape peak' instead of 'shape average'
Why it's wrong here
'Shape peak' would allow bursts above the rate, potentially increasing the observed rate, not decreasing it.
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