Question 1,219 of 2,015

QoS →mediumMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the switchport is configured as an access port without the 'mls qos trust cos' command. This is the most likely cause of the IP phone voice drops because, by default, Cisco Catalyst switches treat all access ports as untrusted for QoS, meaning they re-mark any incoming CoS values to 0. When the IP phone sends voice packets marked with CoS 5, the switch overwrites that marking, causing the downstream service policy to see only CoS 0 and drop the voice traffic. On the ENCOR 350-401 exam, this scenario tests your understanding of the trust boundary—a common trap is assuming that applying a policy map alone is sufficient, when in fact you must explicitly enable trust on the port to preserve the phone’s markings. Remember the memory tip: “No trust, no voice—CoS 5 becomes 0 in a flash.”

350-401 QoS Practice Question

This 350-401 practice question tests your understanding of qos. 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.

A network engineer is configuring QoS on a Cisco Catalyst 3850 switch to prioritize voice traffic. The switch is connected to an IP phone and a PC using a single access port. The engineer applies a service policy on the access port that marks CoS 5 for voice and CoS 0 for data. However, the IP phone is not receiving any voice packets. What is the most likely cause?

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.

Question 1mediummultiple choice

Study the full QoS explanation →

A
The switchport is configured as an access port without 'mls qos trust cos'
Correct because without trusting CoS, the switch ignores the phone's markings and treats all traffic as best-effort.
B
The IP phone is not configured with the correct VLAN for voice traffic
Why wrong: Incorrect because the scenario describes a QoS marking issue, not a VLAN assignment problem; voice VLAN configuration is separate from QoS trust.
C
The service policy is applied in the output direction instead of input
Why wrong: Incorrect because the service policy direction does not affect trust; trust is a port-level attribute, not part of the policy map.
D
The switch does not support CoS marking on access ports
Why wrong: Incorrect because Catalyst 3850 switches support CoS marking on access ports when trust is enabled.

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 switchport is configured as an access port without 'mls qos trust cos'

The correct answer identifies that the switchport must be configured to trust the CoS markings from the IP phone. If the port is set to untrusted, the switch will re-mark all incoming CoS values to 0, overriding the phone's markings.

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 switchport is configured as an access port without 'mls qos trust cos'
Why this is correct
Correct because without trusting CoS, the switch ignores the phone's markings and treats all traffic as best-effort.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Access ports place end devices into a single VLAN.
✗
The IP phone is not configured with the correct VLAN for voice traffic
Why it's wrong here
Incorrect because the scenario describes a QoS marking issue, not a VLAN assignment problem; voice VLAN configuration is separate from QoS trust.
✗
The service policy is applied in the output direction instead of input
Why it's wrong here
Incorrect because the service policy direction does not affect trust; trust is a port-level attribute, not part of the policy map.
✗
The switch does not support CoS marking on access ports
Why it's wrong here
Incorrect because Catalyst 3850 switches support CoS marking on access ports when trust is enabled.

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.

Trap categories for this question

Scenario analysis trap
Incorrect because the scenario describes a QoS marking issue, not a VLAN assignment problem; voice VLAN configuration is separate from QoS trust.

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 — This question tests QoS — Access ports place end devices into a single VLAN..

What is the correct answer to this question?

The correct answer is: The switchport is configured as an access port without 'mls qos trust cos' — The correct answer identifies that the switchport must be configured to trust the CoS markings from the IP phone. If the port is set to untrusted, the switch will re-mark all incoming CoS values to 0, overriding the phone's markings.

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

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?

Access ports place end devices into a single VLAN.

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