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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
CBWFQ is a class-based scheduling mechanism that builds on per-flow WFQ by classifying packets into user-defined classes, each configured with a minimum bandwidth (as an absolute rate, percentage, or relative weight). When congestion occurs, the scheduler services classes in proportion to their configured weights or bandwidth guarantees, ensuring each class receives its minimum share while still allowing idle classes' bandwidth to be redistributed. This guarantee only applies during congestion; under light load, classes can exceed their configured bandwidth.
- ✓
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
LLQ implements a strict priority queue inside CBWFQ by serving the 'priority' class whenever it has traffic, before any bandwidth-based class queues are considered. Because the priority queue is always drained first, a sustained high-rate flow in that queue can consume the entire link and starve all other classes, which is why Cisco recommends attaching a policer (e.g., 'police' or 'priority percent' with burst parameters) to cap the priority-class rate. The policer guarantees that non-priority queues receive service by limiting the amount of traffic that can enter the priority queue.
- ✗
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
Tail drop is a passive congestion management behavior, not a proactive avoidance mechanism, and it operates at the tail of the queue: when the queue buffer is completely full, all newly arriving packets are dropped. The existing note correctly identifies this as the opposite of dropping from the front, but the more critical flaw is that tail drop treats all packets equally and provides no differentiation based on IP precedence or DSCP, causing global TCP synchronization and poor link utilization across multiple flows.
- ✓
WRED can be configured per class within a policy map using the 'random-detect' command under the class.
Why this is correct
In Cisco Modular QoS CLI (MQC), WRED is enabled per class under a policy map using the 'random-detect' command, which can be followed by 'precedence' or 'dscp' to define per-QoS-marking thresholds. This per-class configuration allows a network engineer to set different minimum and maximum thresholds as well as mark-probability denominators for each traffic class, so higher-priority classes experience a lower drop probability during congestion. WRED can be applied to both TCP and non-TCP traffic, though its congestion-avoidance benefit is most pronounced with TCP because TCP endpoints back off in response to loss.
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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: Strict priority queuing (option A) is correct because it places designated traffic such as voice into a high-priority queue that is always serviced first, ensuring low latency and jitter for delay-sensitive flows. Weighted Round Robin (option B) is correct because WRR services non-priority queues in a round-robin manner, allocating bandwidth proportionally to each queue's configured weight. Option C is correct because on Cisco Catalyst switches the default queue (queue 1) is typically assigned to best-effort traffic, while other queues handle priority or controlled traffic. Option D is incorrect because tail drop is a congestion-avoidance/drop mechanism, not a scheduling algorithm that selects the next queue to service. Option E is incorrect because queuing and scheduling occur on the egress interface after the forwarding/routing decision, not on the ingress interface before routing.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-401 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-401 exam.