hardMultiple Choice
350-401 Practice Question: Is implementing QoS on a Cisco router that…
A network engineer is implementing QoS on a Cisco router that connects to a service provider. The provider uses MPLS and expects the MPLS EXP bits to be set for voice traffic. The engineer configures a policy-map that sets the MPLS EXP to 5. However, the provider reports that the EXP bits are not being set. What is the most likely reason?
⚠ Common exam trap
Cisco often tests the concept that MPLS EXP marking must be applied on the outgoing interface (where MPLS encapsulation occurs), not on the incoming interface, leading candidates to incorrectly assume that inbound marking is sufficient.
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 policy-map is applied to the incoming interface, but MPLS EXP marking must be done on the outgoing interface.
The most likely reason is that the policy-map is applied to the incoming interface, but MPLS EXP marking must be applied on the outgoing interface. MPLS EXP bits are set at the imposition (ingress) of the MPLS label stack, which occurs when the packet is forwarded out of an interface that has MPLS enabled. If the policy-map is applied inbound, it marks the IP packet before MPLS encapsulation, and the EXP bits are not set on the MPLS label. The correct approach is to apply the policy-map outbound on the interface facing the service provider, so that the 'set mpls experimental' command marks the EXP bits on the imposed label.
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-map is applied to the incoming interface, but MPLS EXP marking must be done on the outgoing interface.
Why this is correct
The correct issue is that the policy-map is applied to the incoming interface, but MPLS EXP marking must be performed on the outgoing interface. When a packet enters the MPLS domain, the label is not yet imposed on the inbound interface; the label push occurs during forwarding, and the MPLS encapsulation is added on the outgoing interface toward the next hop. Therefore, applying 'set mpls experimental 5' inbound cannot affect the EXP bits of a label that has not yet been created—marking must be configured on the outbound interface where the label exists.
- ✗
The router does not support setting MPLS EXP bits.
Why it's wrong here
This claim is incorrect because the router's inability to set MPLS EXP bits is not the root cause; even entry-level Cisco routers support MPLS EXP marking through Modular QoS CLI (MQC) policy-maps. The command 'set mpls experimental 5' is perfectly valid and the hardware can perform the marking, so the failure stems from the policy being attached to the wrong interface, not from a lack of router capability.
- ✗
The MPLS EXP bits are set automatically based on the IP precedence.
Why it's wrong here
This statement is false because Cisco IOS does not automatically map IP precedence to MPLS EXP bits unless explicitly configured with commands like 'set mpls experimental' or 'mpls qos pre-classify'. The engineer deliberately set the EXP bits to 5 in the policy-map, indicating that automatic mapping is either not in effect or would be overridden by the explicit marking. Even if some default behavior existed, the intended EXP value would not be applied without the explicit command, so this is not the reason the marking failed.
- ✗
The policy-map must use 'set mpls experimental imposition 5' instead of 'set mpls experimental 5'.
Why it's wrong here
The command 'set mpls experimental 5' is actually correct for setting the EXP bits on an imposed label; 'set mpls experimental imposition' is a variant used on the outgoing interface of an ingress PE to explicitly mark the top label's EXP bits during label imposition. The real problem is not the command syntax but the interface direction: the policy-map is applied to the incoming interface, where no label is yet present, so even the correct command cannot take effect at that point.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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