- A
The router does not trust the CoS marking and re-marks it to 0.
Why wrong: Incorrect because the router may trust the marking, but CoS is not preserved across Layer 3 interfaces.
- B
CoS is a Layer 2 marking and is not carried across a Layer 3 hop; the router must map CoS to DSCP.
Correct because CoS is part of the 802.1Q header, which is stripped when the packet is routed; the router needs to map CoS to DSCP to preserve the priority.
- C
The switch must be configured to set DSCP instead of CoS.
Why wrong: Incorrect because setting DSCP on the switch would also be lost if not mapped, but the issue is specifically about CoS.
- D
The router must have 'mls qos trust cos' configured on the interface.
Why wrong: Incorrect because the trust command is for accepting markings, but CoS is still a Layer 2 field.
Quick Answer
The answer is that CoS marking is lost across a router because CoS is a Layer 2 field in the 802.1Q VLAN tag, which is stripped when a frame traverses a Layer 3 hop. The router operates at Layer 3 and does not preserve the CoS value; instead, it must map CoS to DSCP to maintain QoS marking across the routed boundary. On the ENCOR 350-401 exam, this question tests your understanding of the fundamental Layer 2 versus Layer 3 QoS boundary, often appearing as a trap where candidates assume CoS persists end-to-end. A common memory tip is to remember that CoS lives in the trunk tag, DSCP lives in the IP header—when you cross a router, the tag is gone, so you must map to DSCP. Think of it as “Layer 2 stays on the switch, Layer 3 needs DSCP.”
CCNP QoS Architecture Practice Question
This 350-401 practice question tests your understanding of qos architecture. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 implementing QoS on its campus network. The network engineer configures a policy-map that sets the CoS value for voice traffic to 5 on a switch interface. However, when the traffic reaches the router, the CoS marking is lost. 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
CoS is a Layer 2 marking and is not carried across a Layer 3 hop; the router must map CoS to DSCP.
CoS (Class of Service) is a Layer 2 marking field in the 802.1Q VLAN tag, which is stripped when a frame passes through a Layer 3 device (router). Since the router operates at Layer 3, it does not preserve the CoS value; instead, the router must map the CoS to a DSCP (Differentiated Services Code Point) value at Layer 3 to maintain QoS across the routed hop. Option B correctly identifies this fundamental Layer 2 vs. Layer 3 boundary issue.
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 router does not trust the CoS marking and re-marks it to 0.
Why it's wrong here
Incorrect because the router may trust the marking, but CoS is not preserved across Layer 3 interfaces.
- ✓
CoS is a Layer 2 marking and is not carried across a Layer 3 hop; the router must map CoS to DSCP.
Why this is correct
Correct because CoS is part of the 802.1Q header, which is stripped when the packet is routed; the router needs to map CoS to DSCP to preserve the priority.
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 switch must be configured to set DSCP instead of CoS.
- ✗
The router must have 'mls qos trust cos' configured on the interface.
Why it's wrong here
Incorrect because the trust command is for accepting markings, but CoS is still a Layer 2 field.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume CoS is preserved across routers because they see 'trust cos' on switches, but Cisco tests the understanding that CoS is a Layer 2-only marking that disappears at a Layer 3 boundary, requiring DSCP for inter-VLAN or routed QoS.
Trap categories for this question
Command / output trap
Incorrect because the trust command is for accepting markings, but CoS is still a Layer 2 field.
Detailed technical explanation
How to think about this question
In an 802.1Q trunk, the CoS field occupies 3 bits within the VLAN tag, and when a router receives a frame, it strips the entire Layer 2 header (including the 802.1Q tag) before performing Layer 3 forwarding. To preserve QoS markings across a routed hop, the router must be configured to map the incoming CoS (if available) to a DSCP value using a 'mls qos map cos-dscp' command on a switch, or the router must rely on DSCP set directly by the switch. In a real-world campus network, voice traffic often uses CoS 5 on access switches, but the distribution or core router must trust DSCP (e.g., EF for voice) to maintain priority end-to-end.
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 help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
What to study next
Got this wrong? Here's your next step.
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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: CoS is a Layer 2 marking and is not carried across a Layer 3 hop; the router must map CoS to DSCP. — CoS (Class of Service) is a Layer 2 marking field in the 802.1Q VLAN tag, which is stripped when a frame passes through a Layer 3 device (router). Since the router operates at Layer 3, it does not preserve the CoS value; instead, the router must map the CoS to a DSCP (Differentiated Services Code Point) value at Layer 3 to maintain QoS across the routed hop. Option B correctly identifies this fundamental Layer 2 vs. Layer 3 boundary issue.
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
This 350-401 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 350-401 exam.
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