350-501 Automation and Quality of Service Practice Question
A service provider needs to prioritize voice traffic over best-effort data in an MPLS VPN. The PE router uses a QoS policy applied to the ingress interface. Which action ensures that voice packets are marked with the correct DSCP value before entering the MPLS core?
⚠ Common exam trap
Cisco often tests the distinction between IP-layer marking (DSCP) and MPLS-layer marking (EXP), so the trap here is that candidates may choose 'set mpls experimental 5' thinking it achieves the same result, but the question explicitly requires DSCP marking before MPLS encapsulation.
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 policy-map that matches voice traffic using a class-map and the 'set dscp ef' action.
The question specifies that the PE router must mark voice packets with the correct DSCP value before they enter the MPLS core. The 'set dscp ef' action in a policy-map applied to the ingress interface explicitly sets the DSCP field to EF (46) for voice traffic matched by a class-map, ensuring proper classification and treatment across the MPLS network. This is the standard method for marking IP packets at the edge before MPLS encapsulation.
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 policy-map that matches voice traffic using a class-map and the 'set dscp ef' action.
Why this is correct
Sets DSCP EF for voice packets on ingress.
- ✗
Configure the ingress interface with 'mls qos trust dscp' to preserve the customer marking.
Why it's wrong here
Trust is used to honor existing markings; not to set new ones.
- ✗
Use a policy-map with the 'set mpls experimental 5' command.
Why it's wrong here
This sets MPLS EXP bits, not IP DSCP.
- ✗
Apply a police action to drop traffic exceeding the voice bandwidth.
Why it's wrong here
Policing does not mark packets; it only enforces rate limits.
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