The answer is attaching a QoS policy to a VRF interface using a policy-map that references a non-VRF-aware class-map, along with applying a policy-map in the wrong direction (input vs output) and misapplying direction-specific actions like shaping on an inbound interface. These are common QoS misconfigurations on PE routers because QoS actions such as shaping, policing, and queuing are inherently direction-specific; for instance, shaping controls outbound traffic and is typically applied on egress, while policing can be applied inbound to rate-limit incoming traffic. On the Cisco SPCOR 350-501 exam, this topic tests your understanding of how MPLS VPN environments require VRF-aware class-maps to correctly classify customer traffic, and a frequent trap is assuming a standard class-map works across VRF interfaces. A useful memory tip is “VRF needs VRF-aware, direction dictates action”—always verify the class-map references the VRF and that shaping is egress-only, or the router will silently ignore the policy.
350-501 Automation and Quality of Service Practice Question
This 350-501 practice question tests your understanding of automation and quality of service. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
Which THREE are common causes of QoS misconfiguration on PE routers? (Choose three.)
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Applying a policy-map in the wrong direction (input vs output).
Applying a policy-map in the wrong direction (input vs output) is a common QoS misconfiguration because QoS actions like shaping, policing, and queuing are direction-specific. For example, shaping is typically applied on the egress interface to control outbound traffic, while policing can be applied inbound to rate-limit incoming traffic. Misapplying a policy-map (e.g., attaching a shaper to the input direction) will either be ignored by the router or cause unexpected behavior, leading to QoS failures.
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.
✓
Applying a policy-map in the wrong direction (input vs output).
Why this is correct
Correct: Common mistake that causes policy to have no effect.
Related concept
Read the scenario before looking for a memorised answer.
✗
Using the 'bandwidth percent' command in a class that also has a priority queue.
Why it's wrong here
Valid in some implementations; not a misconfiguration per se.
✗
Insufficient bandwidth on the subscriber line.
Why it's wrong here
This is a capacity issue, not a misconfiguration.
✓
Class-map match criteria that do not correctly identify the intended traffic.
Read the scenario before looking for a memorised answer.
✓
Attaching a QoS policy to a VRF interface using a policy-map that references a non-VRF-aware class-map.
Why this is correct
Correct: Policy may not apply to VRF traffic correctly.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between QoS misconfiguration (e.g., wrong direction, incorrect match criteria) and capacity issues (e.g., insufficient bandwidth), so candidates mistakenly select 'insufficient bandwidth' as a misconfiguration when it is actually a resource constraint that QoS cannot fix.
Detailed technical explanation
How to think about this question
Under the hood, Cisco IOS uses a two-pass QoS architecture: classification occurs first via class-maps, then actions are applied via policy-maps. When a policy-map is attached in the wrong direction, the router may silently ignore unsupported actions (e.g., shaping on input) or apply them incorrectly, leading to no effect or dropped packets. In real-world scenarios, misapplying a policy-map direction on a PE router can cause MPLS VPN traffic to bypass QoS entirely, as the router processes inbound and outbound policies at different points in the forwarding pipeline (e.g., before or after MPLS label imposition).
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 350-501 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
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Automation and Quality of Service — This question tests Automation and Quality of Service — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Applying a policy-map in the wrong direction (input vs output). — Applying a policy-map in the wrong direction (input vs output) is a common QoS misconfiguration because QoS actions like shaping, policing, and queuing are direction-specific. For example, shaping is typically applied on the egress interface to control outbound traffic, while policing can be applied inbound to rate-limit incoming traffic. Misapplying a policy-map (e.g., attaching a shaper to the input direction) will either be ignored by the router or cause unexpected behavior, leading to QoS failures.
What should I do if I get this 350-501 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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Question Discussion
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