The answer is that the switch drops video packets because it uses a single queue for all traffic by default, and configuring a queue-limit on a class does not create a separate egress queue. On a Cisco switch, unless you explicitly configure multiple egress queues—for example, with the priority-queue out command or by mapping DSCP values to specific queues—all traffic, including AF41 video, competes within the same single FIFO queue. Setting a queue-limit on a class within a single-queue system only controls how many packets of that class can be enqueued, but it does not isolate the traffic; during congestion, the single queue fills up and drops packets indiscriminately. On the ENCOR 350-401 exam, this tests your understanding that QoS classification and queuing are separate actions—a common trap is assuming a policy-map automatically allocates a dedicated queue. Remember the mnemonic: "One queue, one drop zone; multiple queues, traffic alone."
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.
A network engineer is configuring QoS on a Cisco switch to ensure that video traffic (DSCP AF41) is not dropped during congestion. The engineer creates a policy-map that sets the queue-limit for the AF41 class. However, the switch is still dropping video 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The switch uses a single queue for all traffic unless multiple queues are configured.
By default, Cisco switches use a single queue for all traffic. Creating a policy-map that sets a queue-limit for the AF41 class does not automatically create a separate queue for that class; the switch must have multiple egress queues configured (e.g., via the 'priority-queue out' command or by mapping DSCP values to specific queues). Without multiple queues, all traffic shares the same queue, and setting a queue-limit on a class within a single-queue system does not prevent drops during congestion.
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 queue-limit is set too low, causing tail drops.
Why it's wrong here
Incorrect because the engineer set the queue-limit, but the issue may be that the queue is not being used.
✓
The switch uses a single queue for all traffic unless multiple queues are configured.
Why this is correct
Correct because by default, switches may use a single queue; the engineer must configure multiple queues and assign the class to a specific queue.
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 video traffic is not being marked with DSCP AF41.
Why it's wrong here
Incorrect because the scenario assumes the marking is correct.
✗
The policy-map must be applied to the output direction.
Why it's wrong here
Incorrect because the policy-map is likely applied correctly, but the queue configuration is missing.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that creating a class-map and policy-map with a queue-limit automatically creates a separate queue for that traffic, when in fact the switch must have multiple queues explicitly configured to isolate traffic classes.
Trap categories for this question
Scenario analysis trap
Incorrect because the scenario assumes the marking is correct.
Detailed technical explanation
How to think about this question
Cisco switches use a class-of-service (CoS) or DSCP-to-queue mapping to assign traffic to hardware queues. Without explicit configuration of multiple queues (e.g., using 'mls qos srr-queue output' or 'priority-queue out'), all traffic defaults to a single egress queue. The 'queue-limit' command only sets the maximum depth of that single queue, so even if the limit is high, congestion still causes drops across all traffic classes. In real-world scenarios, engineers must first enable QoS globally and configure queue mappings (e.g., via 'mls qos trust dscp') before policy-map queue-limit settings have any effect on traffic isolation.
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.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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 switch uses a single queue for all traffic unless multiple queues are configured. — By default, Cisco switches use a single queue for all traffic. Creating a policy-map that sets a queue-limit for the AF41 class does not automatically create a separate queue for that class; the switch must have multiple egress queues configured (e.g., via the 'priority-queue out' command or by mapping DSCP values to specific queues). Without multiple queues, all traffic shares the same queue, and setting a queue-limit on a class within a single-queue system does not prevent drops during congestion.
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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