350-501 Automation and Quality of Service Practice Question
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?
⚠ Common exam trap
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.
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 class map for video traffic does not match the traffic correctly.
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.
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.
- ✗
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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