350-501 Architecture Practice Question
A network engineer is troubleshooting QoS in a service provider environment. Customer traffic is marked with DSCP AF31 (011010) at the CE. On the PE router, the policy maps trust DSCP and sets the CoS to 4. However, core routers remark the DSCP to 0. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the misconception that MPLS EXP bits are automatically preserved end-to-end, but the trap here is that a core router's policer can independently re-mark the DSCP to 0, overriding any earlier trust or EXP mapping, especially when the traffic exceeds the contracted rate.
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
✓
A policer on the core router is marking down out-of-contract traffic
The core router is likely applying a policer that marks down traffic exceeding the committed information rate (CIR). When traffic is out-of-contract, the policer can re-mark the DSCP to 0 (best effort), which overrides the trusted DSCP AF31 and the CoS 4 set at the PE. This behavior is common in service provider environments where core routers enforce traffic contracts and use policing to penalize excess traffic.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
A policer on the core router is marking down out-of-contract traffic
Why this is correct
Policers can re-mark DSCP to 0 for excess traffic.
- ✗
The core routers use LDP labels and ignore DSCP
Why it's wrong here
LDP doesn't process DSCP; DSCP remarking must be due to a policy.
- ✗
The ingress PE did not set MPLS EXP bits
Why it's wrong here
The problem is DSCP being cleared, not MPLS EXP.
- ✗
The MPLS EXP bits are not copied from DSCP
Why it's wrong here
MPLS EXP bits are often copied from DSCP, but this does not cause DSCP to become 0.
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Written by Johnson Ajibi, MSc IT Security
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