- A
Apply a shape average policy on the egress interface of the WAN edge router, setting the CIR and burst parameters to match the provider contract.
Shape average enforces the CIR with bursting, preventing tail drops at the provider's policer.
- B
Use a policer on the ingress interface to drop traffic exceeding the CIR.
Why wrong: Policing drops excess traffic, which is not the goal; shaping buffers excess traffic to avoid drops.
- C
Configure a shaper on the provider's device instead of the customer router.
Why wrong: The customer cannot control the provider's device; shaping must be done on the customer edge.
- D
Set the interface bandwidth to the CIR and rely on FIFO queuing.
Why wrong: This does not shape traffic; it only sets the bandwidth parameter, which does not prevent bursts from exceeding the CIR.
Quick Answer
The correct approach is to apply a shape average policy on the egress interface of the WAN edge router, setting the CIR and burst parameters to match the provider contract. This works because shape average buffers excess traffic and transmits it at the contracted CIR, while the burst parameters (Bc and Be) allow short-term spikes above CIR to accommodate real-time traffic without triggering the provider’s policer to drop packets. On the ENCOR 350-401 exam, this question tests your understanding of how WAN edge traffic shaping prevents provider policer drops by aligning egress traffic with the service-level agreement. A common trap is confusing shape average with shape peak—remember that shape average enforces a strict upper rate, whereas shape peak allows the average to exceed CIR using excess burst, which can still cause drops. Memory tip: “Shape Average = Safe Harbor; Shape Peak = Risky Spike.”
CCNP QoS Architecture Practice Question
This 350-401 practice question tests your understanding of qos architecture. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
An enterprise is designing a QoS architecture for its WAN edge routers connecting to multiple service providers. The design must support traffic shaping to avoid packet drops due to provider policers, while also prioritizing real-time traffic. Which approach should the architect use to shape traffic to the contracted CIR while still allowing bursts?
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
Apply a shape average policy on the egress interface of the WAN edge router, setting the CIR and burst parameters to match the provider contract.
Option A is correct because 'shape average' on the egress interface allows the router to buffer excess traffic and transmit it at the contracted CIR, while the burst parameters (Bc and Be) enable short-term bursts above CIR to accommodate real-time traffic spikes without drops. This prevents the provider's policer from discarding packets, as the shaper ensures the outbound traffic rate stays within the agreed contract limits.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Apply a shape average policy on the egress interface of the WAN edge router, setting the CIR and burst parameters to match the provider contract.
Why this is correct
Shape average enforces the CIR with bursting, preventing tail drops at the provider's policer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Use a policer on the ingress interface to drop traffic exceeding the CIR.
Why it's wrong here
Policing drops excess traffic, which is not the goal; shaping buffers excess traffic to avoid drops.
- ✗
Configure a shaper on the provider's device instead of the customer router.
Why it's wrong here
The customer cannot control the provider's device; shaping must be done on the customer edge.
- ✗
Set the interface bandwidth to the CIR and rely on FIFO queuing.
Why it's wrong here
This does not shape traffic; it only sets the bandwidth parameter, which does not prevent bursts from exceeding the CIR.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between shaping and policing—the trap here is that candidates may choose policing (Option B) because it seems simpler, but they overlook that shaping buffers bursts to avoid drops, which is essential when the provider enforces a policer downstream.
Detailed technical explanation
How to think about this question
Under the hood, 'shape average' uses a token bucket algorithm with configurable committed burst (Bc) and excess burst (Be) to allow short-term bursts above CIR while maintaining a long-term average rate. In real-world scenarios, mismatched burst parameters between the customer shaper and provider policer can cause tail drops even if the average rate is within contract; thus, setting Bc and Be to match the provider's policer bucket depth is critical for loss-free operation.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the 350-401 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
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FAQ
Questions learners often ask
What does this 350-401 question test?
QoS Architecture — This question tests QoS Architecture — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Apply a shape average policy on the egress interface of the WAN edge router, setting the CIR and burst parameters to match the provider contract. — Option A is correct because 'shape average' on the egress interface allows the router to buffer excess traffic and transmit it at the contracted CIR, while the burst parameters (Bc and Be) enable short-term bursts above CIR to accommodate real-time traffic spikes without drops. This prevents the provider's policer from discarding packets, as the shaper ensures the outbound traffic rate stays within the agreed contract limits.
What should I do if I get this 350-401 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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