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350-401 Practice Question: Is configuring QoS on a Cisco switch to ensure…

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?

⚠ Common exam trap

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.

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 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.

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
Tail drops occur only when a specific queue is both selected by the classification and saturated. The queue-limit set by the engineer applies to a user-defined queue, but if the switch never assigns the video class to that queue—because no explicit queue configuration exists—the queue-limit is never exercised. In this scenario, the video is likely tail-dropped in the default single queue, not in the queue whose limit was configured.
✓
The switch uses a single queue for all traffic unless multiple queues are configured.
Why this is correct
Modern switching platforms often use a single FIFO or default priority queue for all traffic until multiple egress queues are explicitly configured through QoS profile or class-map-to-queue mappings. If the engineer only attached a policy-map that sets DSCP/CoS but did not map the class to one of the available hardware queues, the video traffic remains in the default queue and receives no dedicated bandwidth or drop protection. Therefore, the root cause is the absence of a multi-queue schedule, not the marking or policy-map direction.
✗
The video traffic is not being marked with DSCP AF41.
Why it's wrong here
The question states the video is marked with DSCP AF41, and both the ingress classification and the remarking are assumed to be valid. If marking were absent, the policy-map would not classify video into the appropriate class, but the problem description explicitly indicates the marking is correct. The failure to achieve the desired QoS behavior is therefore unrelated to marking; even properly marked traffic cannot be prioritized if the switch lacks the queue structure to act on it.
✗
The policy-map must be applied to the output direction.
Why it's wrong here
Applying the policy-map in the output direction is a prerequisite for egress queuing, and the scenario already has that in place. The real issue is that an output policy-map that only sets DSCP/CoS or bandwidth without configuring the underlying queue assignments does not solve the problem. Traffic can still be dumped into a single default queue, and no amount of direction changes will fix the absence of explicit queue mapping.

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350-401 Practice Question: Is configuring QoS on a Cisco switch to ensure…

Why each option matters

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