The answer is a class-map mismatch for video traffic. When voice is correctly marked as DSCP EF but video falls back to DSCP 0, the policy is active and applied, so the fault lies in the video class map’s match logic—it simply isn’t capturing the intended packets. This could stem from an incorrect ACL, a wrong protocol match, or a misconfigured DSCP value in the match statement. On the Cisco SPCOR / CCNP Service Provider Core 350-501 exam, this scenario tests your understanding of how class maps filter traffic before marking actions execute; a common trap is assuming the policy itself is broken when the real issue is a mismatch in the classification stage. Remember the troubleshooting flow: if one class works and another doesn’t, the problem is almost always in the failing class map’s match criteria, not the policy map or service policy. Memory tip: “Voice works, video zeros—check the match, not the markers.”
350-501 Automation and Quality of Service Practice Question
This 350-501 practice question tests your understanding of automation and quality of service. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 engineer is troubleshooting a QoS policy on a Cisco router. The policy is intended to mark voice traffic with DSCP EF and video traffic with DSCP AF41. After applying the policy, voice traffic is correctly marked, but video traffic is marked as DSCP 0. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The class map for video traffic does not match the traffic correctly.
Option A is correct because the most common reason for video traffic being marked as DSCP 0 (default) while voice traffic is correctly marked is that the class map for video traffic fails to match the intended packets. This could be due to an incorrect match statement (e.g., using the wrong ACL, protocol, or DSCP value) or a misconfigured match criterion that does not capture the video flows. Since voice traffic is marked correctly, the policy itself is applied and functional, isolating the issue to the video class map's matching logic.
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.
✓
The class map for video traffic does not match the traffic correctly.
Why this is correct
A misconfigured match statement would cause video traffic to fall into the default class, resulting in DSCP 0.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The video traffic is being policed and dropped.
Why it's wrong here
Policing would drop or mark down traffic, not reset to DSCP 0.
✗
The trust boundary is set to 'trust dscp' and the incoming video traffic is not marked.
Why it's wrong here
Trust boundary would preserve the incoming marking, not reset to 0.
✗
The policy is not applied to the correct interface direction.
Why it's wrong here
If the policy were misapplied, no traffic would be marked, not just video.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that a marking policy failure is due to interface direction or trust boundaries, when the real issue is a misconfigured class map that does not match the intended traffic, especially when one traffic type works and another does not.
Detailed technical explanation
How to think about this question
Under the hood, Cisco Modular QoS CLI (MQC) class maps use match statements that can reference access-lists, DSCP values, IP precedence, or protocol types (e.g., 'match protocol rtp'). A common misconfiguration is using 'match ip dscp af41' in the class map when the incoming video traffic is not already marked with AF41—this would cause the class map to fail to match, leaving the traffic in the default class, which typically sets DSCP 0. In real-world scenarios, video traffic might be identified by a specific UDP port range (e.g., 16384-32767 for RTP), and using an ACL that incorrectly specifies the port or protocol leads to this exact symptom.
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 security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
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: The class map for video traffic does not match the traffic correctly. — Option A is correct because the most common reason for video traffic being marked as DSCP 0 (default) while voice traffic is correctly marked is that the class map for video traffic fails to match the intended packets. This could be due to an incorrect match statement (e.g., using the wrong ACL, protocol, or DSCP value) or a misconfigured match criterion that does not capture the video flows. Since voice traffic is marked correctly, the policy itself is applied and functional, isolating the issue to the video class map's matching logic.
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.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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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