350-501 Automation and Quality of Services Practice Question
An SP engineer is configuring hierarchical QoS on a PE router. The parent policy shapes the traffic to 10 Mbps, and the child policy applies LLQ for voice and CBWFQ for data. What is the primary purpose of using policing within the LLQ child policy?
⚠ Common exam trap
Cisco often tests the misconception that policing in a child policy is used to enforce the parent shape rate, but in reality, the parent shape rate is enforced by the shape command in the parent policy, while child policing controls per-class fairness within that shaped rate.
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
✓
To drop excess voice traffic and prevent starvation of other queues
In hierarchical QoS (HQoS), the parent policy shapes the aggregate traffic to 10 Mbps, while the child policy applies LLQ for voice and CBWFQ for data. The primary purpose of policing within the LLQ child policy is to enforce a strict rate limit on voice traffic, dropping excess packets to prevent voice from consuming bandwidth beyond its allocated share. This prevents voice traffic from starving other queues (e.g., data) within the shaped parent rate, ensuring fairness and protecting other classes from congestion.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
To re-mark voice traffic to a lower DSCP
Why it's wrong here
Incorrect; re-marking is not the primary purpose.
- ✓
To drop excess voice traffic and prevent starvation of other queues
Why this is correct
Correct; policing limits voice traffic in LLQ.
- ✗
To police the parent shape rate
Why it's wrong here
Incorrect; policing is applied on the child policy, not parent.
- ✗
To buffer excess voice traffic
Why it's wrong here
Incorrect; buffering is shaping, not policing.
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Senior Network & Security Engineer · founder of Courseiva
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