350-501 Automation and Quality of Service Practice Question
A network administrator configures a class map to match VoIP traffic using 'match ip dscp ef' on a Cisco router. However, the QoS policy is not applying the expected marking to VoIP packets. What is a possible reason?
⚠ Common exam trap
Cisco often tests the misconception that a QoS policy can re-mark traffic regardless of the original packet markings, when in fact the class map must first match the existing DSCP value for the policy to take effect.
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 VoIP traffic is not marked with DSCP EF from the source.
The 'match ip dscp ef' command in the class map checks the DSCP value already present in the incoming VoIP packets. If the source device (e.g., an IP phone) does not mark the packets with DSCP EF (46), the class map will not match, and the QoS policy will not apply the expected marking. The policy can only re-mark packets that are already matched by the class map.
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 policy is applied in the output direction instead of input.
Why it's wrong here
The direction would affect all traffic, not just VoIP.
- ✓
The VoIP traffic is not marked with DSCP EF from the source.
Why this is correct
If the source does not set DSCP EF, the match will fail and the traffic will not be classified.
- ✗
The policy is applied to the wrong interface.
Why it's wrong here
If the policy were on the wrong interface, no traffic would be affected.
- ✗
The class map uses the wrong match type.
Why it's wrong here
The match type is correct for DSCP.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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