Question 597 of 1,420
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350-401 Practice Question: Which three statements about queuing and…
Which three statements about queuing and congestion avoidance in a QoS architecture are true? (Choose three.)
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
✓
Class-Based Weighted Fair Queuing (CBWFQ) assigns a weight to each class and guarantees a minimum bandwidth during congestion.
Queuing manages packets when output is congested, using algorithms like CBWFQ and LLQ. Congestion avoidance techniques like WRED proactively drop packets to prevent tail drops. LLQ provides strict priority queuing for delay-sensitive traffic. WRED can be configured per class in a policy map.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Class-Based Weighted Fair Queuing (CBWFQ) assigns a weight to each class and guarantees a minimum bandwidth during congestion.
Why this is correct
Correct. CBWFQ allocates bandwidth to each class based on the configured bandwidth or weight, ensuring each class gets its minimum share when the link is congested.
- ✓
Low Latency Queuing (LLQ) provides a strict priority queue that is serviced before any other queues, which can cause starvation of other queues if not policed.
Why this is correct
Correct. LLQ places delay-sensitive traffic (e.g., voice) into a priority queue that is always served first, but without policing, it can consume all bandwidth and starve other queues.
- ✗
Weighted Random Early Detection (WRED) can be used only with TCP traffic and drops packets randomly based on the average queue depth.
Why it's wrong here
Incorrect. WRED can be used with both TCP and non-TCP traffic, though it is most effective with TCP because it leverages TCP's congestion avoidance. It drops packets based on the average queue depth and the precedence/DSCP value.
- ✗
Tail drop is a congestion avoidance mechanism that drops packets from the front of the queue when it is full.
Why it's wrong here
Incorrect. Tail drop drops packets from the tail (end) of the queue when the queue is full, not from the front. It is a simple drop mechanism, not a proactive avoidance mechanism.
- ✓
WRED can be configured per class within a policy map using the 'random-detect' command under the class.
Why this is correct
Correct. In Cisco IOS, WRED is configured per class in a policy map with the 'random-detect' command, allowing different drop probabilities for different traffic classes.
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Same concept, more angles
1 more way this is tested on 350-401
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Which three statements about Cisco QoS queuing and scheduling are true? (Choose three.)
hard- ✓ A.Strict priority queuing ensures that voice traffic is always sent before other traffic.
- ✓ B.Weighted Round Robin (WRR) is used to service non-priority queues in a round-robin fashion based on configured weights.
- ✓ C.On Cisco Catalyst switches, the default queue (queue 1) is typically used for best-effort traffic.
- D.Tail drop is a scheduling algorithm that determines which queue to service next.
- E.Queuing and scheduling are performed on the ingress interface before routing decisions.
Why A: Queuing manages packets when output is congested, and scheduling determines the order of transmission. Cisco uses multiple queues (e.g., 4 queues on Catalyst switches) with strict priority or weighted round-robin (WRR). Option A is correct because strict priority queue (PQ) ensures low-latency for voice. Option B is correct because WRR (or shaped round robin) is used for non-priority queues. Option C is correct because the default queue is usually queue 1 (or the best-effort queue). Option D is incorrect because tail drop is a congestion avoidance mechanism applied to a queue, not a scheduling method. Option E is incorrect because scheduling occurs on the egress interface, not ingress.
Last reviewed: Jun 18, 2026
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