The answer is that auto QoS for voice does not create a priority queue; a manual policy is required. This is correct because the auto QoS voice command on a Cisco Catalyst 9300 switch primarily enables trust on access ports, allowing the switch to honor existing DSCP EF markings from IP phones, but it does not automatically apply a queuing policy to the egress interface. Without a manually configured service policy that maps EF traffic to the priority queue, voice packets marked with DSCP EF will compete with other traffic in the default queue, leading to jitter despite proper marking. On the ENCOR 350-401 exam, this tests your understanding of the distinction between trust configuration and queuing configuration—a common trap is assuming auto QoS handles both. Remember: auto QoS trusts, but you must queue. A helpful memory tip is “Trust marks, but queue the spark”—auto QoS sets the stage, but you must manually prioritize the voice traffic to use the priority queue.
CCNP QoS Practice Question
This 350-401 practice question tests your understanding of qos. 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 network engineer is troubleshooting voice quality issues on a Cisco Catalyst 9300 switch. The switch is configured with auto QoS for voice, which enabled trust on the access ports. However, voice packets are being marked with DSCP EF but are still experiencing jitter. The engineer checks the interface queue statistics and sees that the priority queue is not being used. 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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Auto QoS does not create a priority queue; a manual policy is required
The correct answer is that auto QoS for voice enables trust but does not automatically create a priority queue on the switch; the engineer must also configure a service policy that includes a priority queue for EF traffic.
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.
✓
Auto QoS does not create a priority queue; a manual policy is required
Why this is correct
Correct because auto QoS only sets trust and marks; the queuing policy must be applied separately to prioritize voice.
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 switch does not support DSCP-based queuing
Why it's wrong here
Incorrect because Catalyst 9300 switches support DSCP-based queuing when a proper policy is applied.
✗
The voice VLAN is not configured on the access port
Why it's wrong here
Incorrect because voice VLAN affects VLAN assignment, not queuing; the priority queue issue is unrelated.
✗
The switch is using default CoS-to-queue mapping which maps EF to a non-priority queue
Why it's wrong here
Incorrect because auto QoS does modify CoS-to-queue mapping, but it does not enable the priority queue without a service policy.
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.
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: Auto QoS does not create a priority queue; a manual policy is required — The correct answer is that auto QoS for voice enables trust but does not automatically create a priority queue on the switch; the engineer must also configure a service policy that includes a priority queue for EF traffic.
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.
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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