This 350-401 practice question tests your understanding of network assurance. 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 administrator is troubleshooting a performance issue in a large enterprise campus network. The network consists of Cisco Catalyst 9300 switches acting as access switches and Cisco Catalyst 9500 switches as distribution. Users on VLAN 10 report intermittent slow file transfers to a server on VLAN 20. The administrator has verified that there are no errors on the links, CPU utilization is normal, and STP topology is stable. The administrator suspects a possible QoS issue. Upon checking the QoS configuration on the access switch, the administrator finds that the default QoS configuration is in place, which trusts the CoS value at the port level. The connected devices are IP phones and PCs; the IP phones mark voice traffic with CoS 5. The server on VLAN 20 is connected to a distribution switch. Which action should the administrator take to most likely resolve the issue?
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
✓
Configure auto QoS for VoIP on the access ports to ensure proper classification and queuing.
Option C is correct because Auto QoS for VoIP automatically configures the necessary class maps, policy maps, and trust settings to properly classify and queue voice traffic (CoS 5) while ensuring data traffic is not starved. The default QoS configuration trusts CoS at the port level, but without proper queuing and scheduling, voice and data may compete for buffers, causing intermittent slow file transfers. Auto QoS sets up strict priority queuing for voice and allocates bandwidth for data, resolving the performance issue without manual misconfiguration.
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.
✗
Apply a policy map that polices voice traffic to 128 kbps to free bandwidth for data.
Why it's wrong here
Incorrect; policing voice may cause call quality degradation.
✗
Disable QoS entirely on all switches to eliminate any potential QoS-related drops.
Why it's wrong here
Incorrect; disabling QoS may cause voice quality issues.
✓
Configure auto QoS for VoIP on the access ports to ensure proper classification and queuing.
Why this is correct
Correct.
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.
✗
Configure trust DSCP on the access ports to prioritize all traffic based on DSCP values.
Why it's wrong here
Incorrect; endpoints may not set DSCP appropriately.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that simply trusting CoS or DSCP values is sufficient to prioritize traffic, when in fact trust alone does not configure the egress queuing and scheduling policies needed to prevent congestion and ensure bandwidth allocation.
Detailed technical explanation
How to think about this question
Auto QoS for VoIP on Cisco Catalyst switches automatically generates a MQC (Modular QoS CLI) configuration that includes class maps matching CoS 5 (voice) and CoS 3 (signaling), a policy map that places voice into the priority queue (PQ) with a guaranteed bandwidth, and sets the queue-limit to prevent tail drops. Under the hood, the switch uses the egress queue structure (e.g., 1p3q8t on Catalyst 9300) where strict priority queuing ensures voice latency is minimized, while data traffic is serviced by the remaining queues with weighted round-robin (WRR). In a real-world scenario, without Auto QoS, the default CoS trust only marks packets but does not configure the egress queues, causing all traffic to share the same queue and leading to drops or delays for data when voice bursts occur.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 350-401 question in full detail.
Network Assurance — This question tests Network Assurance — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Configure auto QoS for VoIP on the access ports to ensure proper classification and queuing. — Option C is correct because Auto QoS for VoIP automatically configures the necessary class maps, policy maps, and trust settings to properly classify and queue voice traffic (CoS 5) while ensuring data traffic is not starved. The default QoS configuration trusts CoS at the port level, but without proper queuing and scheduling, voice and data may compete for buffers, causing intermittent slow file transfers. Auto QoS sets up strict priority queuing for voice and allocates bandwidth for data, resolving the performance issue without manual misconfiguration.
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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Question Discussion
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