- A
The policy-map is applied to the incoming interface, but MPLS EXP marking must be done on the outgoing interface.
Correct because the MPLS label is added when the packet is forwarded; the marking must be applied on the outgoing interface to set the EXP bits on the label.
- B
The router does not support setting MPLS EXP bits.
Why wrong: Incorrect because most Cisco routers support setting MPLS EXP bits.
- C
The MPLS EXP bits are set automatically based on the IP precedence.
Why wrong: Incorrect because automatic mapping may not occur unless configured; the engineer explicitly set the EXP bits.
- D
The policy-map must use 'set mpls experimental imposition 5' instead of 'set mpls experimental 5'.
Why wrong: Incorrect because the command 'set mpls experimental' sets the EXP bits on the imposed label; the issue is the interface direction.
Quick Answer
The correct answer is that the policy-map is applied to the incoming interface, but MPLS EXP marking must be done on the outgoing interface. This is because MPLS EXP bits are set at the moment of label imposition, which occurs when the packet is forwarded out of an interface with MPLS enabled; applying the policy inbound marks the IP packet before the MPLS label is added, so the EXP bits never get written on the label itself. On the ENCOR 350-401 exam, this tests your understanding of the MPLS forwarding plane and the distinction between IP QoS marking and MPLS QoS marking—a common trap is assuming inbound marking on a CE-facing interface will propagate to the MPLS header. The key is to remember that MPLS EXP marking is an egress action at the ingress LER: you apply the policy outbound on the interface toward the provider. Memory tip: “EXP is set on the way out, not on the way in—mark the label when you push it.”
CCNP QoS Architecture Practice Question
This 350-401 practice question tests your understanding of qos architecture. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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
Correct because the MPLS label is added when the packet is forwarded; the marking must be applied on the outgoing interface to set the EXP bits on the label.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The router does not support setting MPLS EXP bits.
Why it's wrong here
Incorrect because most Cisco routers support setting MPLS EXP bits.
- ✗
The MPLS EXP bits are set automatically based on the IP precedence.
Why it's wrong here
Incorrect because automatic mapping may not occur unless configured; the engineer explicitly set the EXP bits.
- ✗
The policy-map must use 'set mpls experimental imposition 5' instead of 'set mpls experimental 5'.
Why it's wrong here
Incorrect because the command 'set mpls experimental' sets the EXP bits on the imposed label; the issue is the interface direction.
Common exam traps
Common exam trap: answer the scenario, not the keyword
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.
Trap categories for this question
Command / output trap
Incorrect because the command 'set mpls experimental' sets the EXP bits on the imposed label; the issue is the interface direction.
Detailed technical explanation
How to think about this question
In MPLS networks, the EXP bits are set at the ingress LER (Label Edge Router) during label imposition. The policy-map must be applied outbound on the interface that performs MPLS encapsulation, because the MPLS header is added only when the packet exits that interface. A common real-world scenario is when a service provider requires specific EXP markings for voice traffic to ensure proper QoS treatment across the MPLS cloud; misapplying the policy-map inbound will mark the IP DSCP but leave the MPLS EXP bits at their default value (0), causing the provider to see no marking.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the 350-401 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Questions learners often ask
What does this 350-401 question test?
QoS Architecture — This question tests QoS Architecture — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this 350-401 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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