- A
Configure the access switches to trust DSCP on ports connected to IP phones, and apply queuing policies on distribution and core switches that match the trusted markings.
Trusting DSCP on IP phone ports preserves the correct markings; queuing at higher layers ensures consistent treatment.
- B
Remark all traffic to a single DSCP value at the access layer and apply priority queuing at the core.
Why wrong: Remarking all traffic to the same value eliminates differentiation; voice would not receive priority.
- C
Apply QoS policies only at the core layer, ignoring markings from the access layer.
Why wrong: Without classification at the access, the core cannot distinguish voice from other traffic.
- D
Use the distribution layer to reclassify traffic based on source MAC addresses of IP phones.
Why wrong: MAC-based classification is less scalable and not standard; DSCP trust is the recommended approach.
Quick Answer
The correct answer is to configure the access switches to trust DSCP on ports connected to IP phones, while applying queuing policies on distribution and core switches that match those trusted markings. This approach establishes a proper campus QoS trust boundary for voice traffic, where the IP phone is treated as a trusted endpoint that correctly marks voice packets with DSCP EF. By trusting DSCP only at the access layer port, the network prevents unauthorized devices from injecting high-priority markings, preserving end-to-end prioritization. On the ENCOR 350-401 exam, this concept tests your understanding of the hierarchical trust model and the distinction between trusting at the access layer versus the distribution or core. A common trap is assuming you must re-mark traffic at every hop, which would break the trust boundary. Remember the memory tip: "Trust the phone, queue the core" — trust DSCP only at the access edge, then apply LLQ deeper in the network.
CCNP QoS Architecture Practice Question
This 350-401 practice question tests your understanding of qos architecture. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 company is deploying a new campus network with a hierarchical design (core, distribution, access). The QoS design must ensure that voice traffic is prioritized end-to-end, and that marking is trusted only on access ports connected to IP phones. Which architectural approach should the architect take for classification and marking?
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
Configure the access switches to trust DSCP on ports connected to IP phones, and apply queuing policies on distribution and core switches that match the trusted markings.
Option A is correct because it aligns with the Cisco QoS trust boundary model for campus networks. IP phones are trusted endpoints that mark voice traffic with the correct DSCP values (e.g., EF for voice, AF41 for video). By configuring the access switch port to trust DSCP from the IP phone, the marking is preserved end-to-end. Distribution and core switches then apply queuing policies (e.g., LLQ) based on these trusted markings, ensuring voice traffic receives priority treatment across the entire network.
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.
- ✓
Configure the access switches to trust DSCP on ports connected to IP phones, and apply queuing policies on distribution and core switches that match the trusted markings.
Why this is correct
Trusting DSCP on IP phone ports preserves the correct markings; queuing at higher layers ensures consistent treatment.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Remark all traffic to a single DSCP value at the access layer and apply priority queuing at the core.
Why it's wrong here
Remarking all traffic to the same value eliminates differentiation; voice would not receive priority.
- ✗
Apply QoS policies only at the core layer, ignoring markings from the access layer.
Why it's wrong here
Without classification at the access, the core cannot distinguish voice from other traffic.
- ✗
Use the distribution layer to reclassify traffic based on source MAC addresses of IP phones.
Why it's wrong here
MAC-based classification is less scalable and not standard; DSCP trust is the recommended approach.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the concept that the trust boundary must be set at the access layer, not at the distribution or core, and that trusting DSCP from IP phones is the correct method, while remarking all traffic to a single value or ignoring markings entirely are common misconceptions that break end-to-end QoS.
Detailed technical explanation
How to think about this question
In a Cisco campus QoS design, the trust boundary is typically set at the access layer switch port connected to an IP phone using the 'mls qos trust device cisco-phone' command, which automatically trusts the CoS from the phone and maps it to DSCP. The distribution and core switches then use MQC (Modular QoS CLI) policies with class maps matching DSCP EF and AF41 to apply priority queuing (LLQ) and bandwidth guarantees. A subtle behavior is that if a PC is daisy-chained through the IP phone, the switch must apply a separate policy to re-mark untrusted traffic from the PC while preserving the phone's markings.
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
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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: Configure the access switches to trust DSCP on ports connected to IP phones, and apply queuing policies on distribution and core switches that match the trusted markings. — Option A is correct because it aligns with the Cisco QoS trust boundary model for campus networks. IP phones are trusted endpoints that mark voice traffic with the correct DSCP values (e.g., EF for voice, AF41 for video). By configuring the access switch port to trust DSCP from the IP phone, the marking is preserved end-to-end. Distribution and core switches then apply queuing policies (e.g., LLQ) based on these trusted markings, ensuring voice traffic receives priority treatment across the entire network.
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.
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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