- A
At the P routers in the core based on routing protocols.
Why wrong: P routers typically trust the EXP bits already set; they do not map DSCP to EXP.
- B
At the ingress PE router before the packet enters the MPLS core.
The ingress PE maps DSCP to EXP bits.
- C
At the egress PE router when packets are forwarded to the CE.
Why wrong: EXP bits are set at the ingress PE, not egress.
- D
At the CE router before sending to the PE.
Why wrong: CE may set DSCP but not MPLS EXP bits; EXP bits are set at the PE.
Question 417 of 1,000
350-501 Automation and Quality of Services Practice Question
This 350-501 practice question tests your understanding of automation and quality of services. 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.
In the context of MPLS QoS, at which point are EXP bits typically set based on the DSCP value of the IP packet?
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
✓
At the ingress PE router before the packet enters the MPLS core.
In MPLS QoS, the EXP bits are set at the ingress PE router because this is where the IP packet first encounters the MPLS domain. The ingress PE maps the incoming IP packet's DSCP value to the MPLS EXP bits using a configured trust boundary or marking policy (e.g., 'policy-map' with 'set mpls experimental imposition'). This ensures that the QoS treatment is applied consistently across the MPLS core, as P routers typically forward based on the EXP bits without inspecting the original IP header.
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.
- ✗
At the P routers in the core based on routing protocols.
Why it's wrong here
P routers typically trust the EXP bits already set; they do not map DSCP to EXP.
- ✓
At the ingress PE router before the packet enters the MPLS core.
Why this is correct
The ingress PE maps DSCP to EXP bits.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
At the egress PE router when packets are forwarded to the CE.
Why it's wrong here
EXP bits are set at the ingress PE, not egress.
- ✗
At the CE router before sending to the PE.
Why it's wrong here
CE may set DSCP but not MPLS EXP bits; EXP bits are set at the PE.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that EXP marking happens at the egress PE or that P routers can modify EXP bits based on routing, but the key is that EXP bits are set only at the ingress PE when the label is imposed, and P routers treat the EXP field as read-only unless explicit EXP manipulation is configured (e.g., via 'mpls experimental' in a policy-map on the P router, which is rare).
Detailed technical explanation
How to think about this question
The ingress PE uses the 'set mpls experimental imposition' command in a Cisco IOS policy-map to copy the DSCP value (or a mapped value) into the EXP field of the imposed label stack. This mapping can be explicit (e.g., 'set mpls experimental 5') or derived from a table (e.g., 'set mpls experimental imposition dscp'). In real-world deployments, this is critical for DiffServ-aware MPLS Traffic Engineering (DS-TE) where EXP bits determine per-hop behavior (PHB) and class-based forwarding in the core, ensuring that voice traffic (EF, DSCP 46) gets priority over best-effort data.
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 350-501 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
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FAQ
Questions learners often ask
What does this 350-501 question test?
Automation and Quality of Services — This question tests Automation and Quality of Services — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: At the ingress PE router before the packet enters the MPLS core. — In MPLS QoS, the EXP bits are set at the ingress PE router because this is where the IP packet first encounters the MPLS domain. The ingress PE maps the incoming IP packet's DSCP value to the MPLS EXP bits using a configured trust boundary or marking policy (e.g., 'policy-map' with 'set mpls experimental imposition'). This ensures that the QoS treatment is applied consistently across the MPLS core, as P routers typically forward based on the EXP bits without inspecting the original IP header.
What should I do if I get this 350-501 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
This 350-501 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-501 exam.
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