Quick Answer
The answer is Weighted Random Early Detection (WRED), the correct QoS mechanism for preventing congestion by dropping packets before a queue becomes full. WRED achieves this by monitoring the average queue depth and randomly dropping packets with a probability that increases as the queue grows, which proactively signals TCP senders to slow down before tail drop occurs. This prevents the global synchronization problem that plagues traditional drop-at-full approaches. On the ENCOR 350-401 exam, this concept tests your ability to distinguish congestion avoidance (WRED) from congestion management (like CBWFQ or PQ), which only act after the queue is full. A common trap is confusing WRED with policing or shaping—remember, WRED drops early to avoid the queue filling, not to enforce a rate limit. Memory tip: think "WRED = Weighted Random Early Drop" to recall it drops before the queue is full, using randomness and weighting to avoid synchronized TCP backoffs.
This 350-401 practice question tests your understanding of qos architecture. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
Which QoS mechanism is used to prevent congestion by dropping packets before a queue becomes full?
Weighted Random Early Detection (WRED)
WRED proactively drops packets to avoid tail drop and global synchronization.
Priority Queuing (PQ)
Why wrong: PQ is a scheduling mechanism, not a congestion avoidance mechanism.
Class-Based Weighted Fair Queuing (CBWFQ)
Why wrong: CBWFQ is a scheduling mechanism that guarantees bandwidth.
Tail Drop
Why wrong: Tail drop drops packets only when the queue is full, not before.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Weighted Random Early Detection (WRED)
Weighted Random Early Detection (WRED) is a congestion avoidance mechanism that proactively drops packets before a queue becomes full. By monitoring the average queue depth and dropping packets with a probability that increases as the queue depth grows, WRED signals TCP senders to reduce their transmission rate, thereby preventing tail drop and global synchronization. This differs from congestion management mechanisms like PQ or CBWFQ, which only act on packets after the queue is full.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Weighted Random Early Detection (WRED)
Why this is correct
WRED proactively drops packets to avoid tail drop and global synchronization.
Related concept
Read the scenario before looking for a memorised answer.
Priority Queuing (PQ)
Why it's wrong here
PQ is a scheduling mechanism, not a congestion avoidance mechanism.
Class-Based Weighted Fair Queuing (CBWFQ)
Why it's wrong here
CBWFQ is a scheduling mechanism that guarantees bandwidth.
Tail Drop
Why it's wrong here
Tail drop drops packets only when the queue is full, not before.
Common exam traps
Cisco often tests the distinction between congestion management (queuing/scheduling) and congestion avoidance (drop policy), so the trap here is that candidates confuse mechanisms like CBWFQ or PQ (which manage queues after they form) with WRED (which prevents queues from filling up in the first place).
Detailed technical explanation
WRED uses an exponentially weighted moving average (EWMA) to calculate the average queue depth, comparing it against minimum and maximum thresholds. When the average queue depth is between these thresholds, packets are dropped with a probability that increases linearly from 0 to the configured mark probability denominator (e.g., 1/10). This probabilistic dropping allows WRED to selectively drop packets from lower-priority traffic or specific DSCP/CoS values, providing differentiated drop behavior per class. In real-world deployments, WRED is often configured on core routers to avoid the performance degradation caused by tail drop and to maintain consistent TCP throughput.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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
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FAQ
QoS Architecture — This question tests QoS Architecture — Read the scenario before looking for a memorised answer..
The correct answer is: Weighted Random Early Detection (WRED) — Weighted Random Early Detection (WRED) is a congestion avoidance mechanism that proactively drops packets before a queue becomes full. By monitoring the average queue depth and dropping packets with a probability that increases as the queue depth grows, WRED signals TCP senders to reduce their transmission rate, thereby preventing tail drop and global synchronization. This differs from congestion management mechanisms like PQ or CBWFQ, which only act on packets after the queue is full.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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