Question 1,553 of 2,015

QoS Architecture →mediumMultiple SelectObjective-mapped

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.

Which two statements about the Cisco QoS trust boundary are true? (Choose two.)

Question 1mediummulti select

Study the full QoS explanation →

A
The trust boundary can be set at the access layer switch port connected to an IP phone.
Correct because the trust boundary is typically configured at the access layer, and can be extended to the IP phone to mark traffic from the PC.
B
The 'mls qos trust cos' command configures the interface to trust the Layer 2 CoS value.
Correct because this command sets the trust state to use the CoS value for QoS classification.
C
By default, all Cisco switch interfaces trust the incoming CoS or DSCP marking.
Why wrong: Incorrect because by default, Cisco switches do not trust markings; they set DSCP to 0 on access ports and trust CoS on trunk ports only after QoS is enabled globally.
D
The trust boundary is always located at the distribution layer switch.
Why wrong: Incorrect because the trust boundary is typically at the access layer, closest to the endpoint, to prevent unauthorized marking changes.
E
When a PC is connected to a switch port, the switch automatically trusts the DSCP value from the PC.
Why wrong: Incorrect because the switch does not automatically trust DSCP from a PC; it sets DSCP to 0 unless trust is explicitly configured.

Full breakdown with real-world context →

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 trust boundary can be set at the access layer switch port connected to an IP phone.

The trust boundary defines where the device accepts or overwrites Layer 2 CoS or Layer 3 DSCP markings. By default, Cisco switches trust the CoS value on trunk ports and set DSCP to 0 on access ports. The 'mls qos trust cos' command forces the switch to trust CoS, and 'mls qos trust dscp' forces trust of DSCP. The trust boundary can be extended to an IP phone, which then re-marks traffic from the PC. Option C is incorrect because trust is not automatically applied to all interfaces; it must be configured. Option D is incorrect because the trust boundary is at the access layer, not the core. Option E is incorrect because the switch does not automatically trust DSCP from a PC; it typically sets it to 0 unless configured otherwise.

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.

✓
The trust boundary can be set at the access layer switch port connected to an IP phone.
Why this is correct
Correct because the trust boundary is typically configured at the access layer, and can be extended to the IP phone to mark traffic from the PC.
Related concept
Access ports place end devices into a single VLAN.
✓
The 'mls qos trust cos' command configures the interface to trust the Layer 2 CoS value.
Why this is correct
Correct because this command sets the trust state to use the CoS value for QoS classification.
Related concept
Access ports place end devices into a single VLAN.
✗
By default, all Cisco switch interfaces trust the incoming CoS or DSCP marking.
Why it's wrong here
Incorrect because by default, Cisco switches do not trust markings; they set DSCP to 0 on access ports and trust CoS on trunk ports only after QoS is enabled globally.
✗
The trust boundary is always located at the distribution layer switch.
Why it's wrong here
Incorrect because the trust boundary is typically at the access layer, closest to the endpoint, to prevent unauthorized marking changes.
✗
When a PC is connected to a switch port, the switch automatically trusts the DSCP value from the PC.
Why it's wrong here
Incorrect because the switch does not automatically trust DSCP from a PC; it sets DSCP to 0 unless trust is explicitly configured.

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: The trust boundary can be set at the access layer switch port connected to an IP phone. — The trust boundary defines where the device accepts or overwrites Layer 2 CoS or Layer 3 DSCP markings. By default, Cisco switches trust the CoS value on trunk ports and set DSCP to 0 on access ports. The 'mls qos trust cos' command forces the switch to trust CoS, and 'mls qos trust dscp' forces trust of DSCP. The trust boundary can be extended to an IP phone, which then re-marks traffic from the PC. Option C is incorrect because trust is not automatically applied to all interfaces; it must be configured. Option D is incorrect because the trust boundary is at the access layer, not the core. Option E is incorrect because the switch does not automatically trust DSCP from a PC; it typically sets it to 0 unless configured otherwise.

What should I do if I get this 350-401 question wrong?

What is the key concept behind this question?

Access ports place end devices into a single VLAN.

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