- A
By default, a Cisco switch port in access mode trusts the CoS value received from the attached device.
Why wrong: Incorrect. By default, access ports do not trust any marking; they set the internal DSCP to 0 unless a trust policy is explicitly configured.
- B
On a trunk port, the switch can be configured to trust the CoS value by default.
Correct. On trunk ports, the default trust state is to trust the CoS value, as the switch expects the other switch or router to have set the marking appropriately.
- C
The trust boundary can be extended to the endpoint by configuring the interface with the 'mls qos trust' command.
Correct. Using 'mls qos trust [cos | dscp | ip-precedence]' on the switch port tells the switch to accept the marking from the connected device, extending the trust boundary to that endpoint.
- D
When a Cisco IP Phone is connected, the switch automatically trusts the CoS values from the phone but not from the PC behind the phone.
Why wrong: Incorrect. With a Cisco IP Phone, the switch can be configured to trust the phone's marking, but the PC traffic is typically re-marked or untrusted unless the phone is configured to pass through the PC marking.
- E
The 'trust device cisco-phone' command enables the switch to trust all CoS values from both the phone and the attached PC.
Why wrong: Incorrect. 'trust device cisco-phone' only enables the switch to detect the phone and apply QoS settings for voice traffic; it does not automatically trust PC markings.
350-401 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.
Which two statements about the QoS trust boundary on a Cisco switch are true? (Choose two.)
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
On a trunk port, the switch can be configured to trust the CoS value by default.
The trust boundary defines which device in the network is trusted to mark QoS values. By default, Cisco switches trust the CoS value on trunk ports but do not trust the DSCP value on access ports. The trust boundary can be extended to the endpoint by configuring the switch port as trusted, and the Cisco IP Phone can override the marking from the attached PC.
Key principle: A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
By default, a Cisco switch port in access mode trusts the CoS value received from the attached device.
Why it's wrong here
Incorrect. By default, access ports do not trust any marking; they set the internal DSCP to 0 unless a trust policy is explicitly configured.
- ✓
On a trunk port, the switch can be configured to trust the CoS value by default.
- ✓
The trust boundary can be extended to the endpoint by configuring the interface with the 'mls qos trust' command.
- ✗
When a Cisco IP Phone is connected, the switch automatically trusts the CoS values from the phone but not from the PC behind the phone.
Why it's wrong here
Incorrect. With a Cisco IP Phone, the switch can be configured to trust the phone's marking, but the PC traffic is typically re-marked or untrusted unless the phone is configured to pass through the PC marking.
- ✗
The 'trust device cisco-phone' command enables the switch to trust all CoS values from both the phone and the attached PC.
Why it's wrong here
Incorrect. 'trust device cisco-phone' only enables the switch to detect the phone and apply QoS settings for voice traffic; it does not automatically trust PC markings.
Common exam traps
Common exam trap: an active trunk can still block the VLAN you need
A trunk being up does not prove every VLAN is crossing it. Check allowed VLAN lists, native VLAN mismatch, VLAN existence and access-port assignment.
Detailed technical explanation
How to think about this question
VLAN questions usually combine access-port and trunking clues. The key is to identify whether the issue is local to one switchport, caused by the trunk, or caused by the VLAN not existing where it needs to exist.
KKey Concepts to Remember
- Access ports place end devices into a single VLAN.
- Trunk ports carry multiple VLANs between switches.
- Allowed VLAN lists decide which VLANs can cross a trunk.
- Native VLAN mismatch can create confusing symptoms.
TExam Day Tips
- Use show vlan brief to verify access VLANs.
- Use show interfaces trunk to verify trunk state and allowed VLANs.
- Do not treat every same-VLAN issue as a routing problem.
Key takeaway
A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
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.
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 350-401 questions on switching, trunking, and access-port configuration.
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FAQ
Questions learners often ask
What does this 350-401 question test?
QoS Architecture — This question tests QoS Architecture — Access ports place end devices into a single VLAN..
What is the correct answer to this question?
The correct answer is: On a trunk port, the switch can be configured to trust the CoS value by default. — The trust boundary defines which device in the network is trusted to mark QoS values. By default, Cisco switches trust the CoS value on trunk ports but do not trust the DSCP value on access ports. The trust boundary can be extended to the endpoint by configuring the switch port as trusted, and the Cisco IP Phone can override the marking from the attached PC.
What should I do if I get this 350-401 question wrong?
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 350-401 questions on switching, trunking, and access-port configuration.
What is the key concept behind this question?
Access ports place end devices into a single VLAN.
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Last reviewed: Jun 18, 2026
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