The answer is the priority queue’s built-in policer, which drops voice traffic that exceeds the configured bandwidth during congestion. This occurs because Low Latency Queueing (LLQ) uses a strict priority queue that is policed at the configured rate—when the link is congested, any voice packets beyond that rate are dropped to prevent starvation of other queues. On the ENCOR 350-401 exam, this concept tests your understanding that LLQ is not a “guaranteed” bandwidth mechanism for voice; rather, it provides priority up to a limit, after which the policer enforces drops. A common trap is assuming the priority queue will always pass all voice traffic, but the built-in policer is the key differentiator. Remember the mnemonic: “Priority is policed, not promised.”
350-401 QoS Architecture Practice Question
This 350-401 practice question tests your understanding of qos architecture. 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 WAN link. The engineer notices that voice packets are being dropped during congestion. The QoS policy uses LLQ for voice traffic, but the priority queue is not providing the expected bandwidth. 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The priority queue has a built-in policer that drops traffic exceeding the configured bandwidth.
The priority queue in LLQ uses a built-in policer that drops traffic exceeding the configured bandwidth. When congestion occurs, the policer enforces the bandwidth limit by dropping excess packets, which explains why voice packets are being dropped despite the priority queue being active. This is a fundamental behavior of LLQ to prevent the priority queue from starving other queues.
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.
✗
The priority queue is not configured with a bandwidth statement.
Why it's wrong here
Incorrect because LLQ requires a bandwidth statement, but the issue is about drops due to policing.
✓
The priority queue has a built-in policer that drops traffic exceeding the configured bandwidth.
Why this is correct
Correct because LLQ uses a policer to limit the priority queue; if voice traffic exceeds the configured bandwidth, it is dropped.
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.
✗
The class-map is not matching voice traffic correctly.
Why it's wrong here
Incorrect because the scenario states the drops are occurring, which indicates matching is happening.
✗
The router is using FIFO queuing instead of LLQ.
Why it's wrong here
Incorrect because LLQ is configured, but the drops are due to policing, not wrong queuing.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that the priority queue provides unlimited bandwidth during congestion, when in fact LLQ uses a policer to enforce the configured bandwidth limit, causing drops for excess traffic.
Trap categories for this question
Scenario analysis trap
Incorrect because the scenario states the drops are occurring, which indicates matching is happening.
Detailed technical explanation
How to think about this question
Under the hood, LLQ implements the priority queue with a token bucket policer that meters traffic at the configured rate (e.g., 'priority 128' for 128 kbps). During congestion, the policer drops packets that exceed this rate, even if the priority queue has available buffers, to ensure that other queues (like data queues) are not completely starved. In real-world scenarios, this can cause voice quality issues if the priority bandwidth is set too low for the actual voice codec rate plus Layer 2 overhead.
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 practitioner preparing for the 350-401 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 350-401 question in full detail.
QoS Architecture — This question tests QoS Architecture — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The priority queue has a built-in policer that drops traffic exceeding the configured bandwidth. — The priority queue in LLQ uses a built-in policer that drops traffic exceeding the configured bandwidth. When congestion occurs, the policer enforces the bandwidth limit by dropping excess packets, which explains why voice packets are being dropped despite the priority queue being active. This is a fundamental behavior of LLQ to prevent the priority queue from starving other queues.
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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