350-501 Automation and Quality of Service Practice Question
An engineer is configuring MPLS VPN and needs to ensure that customer traffic is automatically marked with a specific QoS policy based on the VPN. Which method should be used to propagate QoS markings across the MPLS network?
⚠ Common exam trap
Cisco often tests the misconception that IP ToS or DSCP markings are automatically preserved across an MPLS backbone, when in fact MPLS EXP bits are the dedicated field for QoS propagation and must be explicitly set and mapped.
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
✓
Use MPLS EXP bits to mark traffic at the ingress PE and map to QoS at egress
In an MPLS VPN environment, QoS markings must be preserved across the MPLS backbone. MPLS EXP (Experimental) bits are the standard mechanism to carry QoS information within the MPLS label stack. At the ingress PE, customer traffic is classified and marked with the appropriate EXP bits based on the VPN or other criteria. The egress PE then uses these EXP bits to map traffic to the correct QoS policy, ensuring end-to-end QoS treatment.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Use 802.1p CoS on the CE-PE link and preserve it across the MPLS backbone
Why it's wrong here
CoS is not preserved across MPLS backbone without mapping.
- ✓
Use MPLS EXP bits to mark traffic at the ingress PE and map to QoS at egress
Why this is correct
MPLS EXP bits are designed to carry QoS information across the MPLS network.
- ✗
Use IP ToS bits to mark traffic and rely on MPLS to preserve them
Why it's wrong here
IP ToS bits are often overwritten by MPLS.
- ✗
Set DSCP at the ingress PE and preserve it across the MPLS backbone
Why it's wrong here
DSCP may not be preserved across MPLS unless explicitly configured.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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