Quick Answer
The answer is WRED, or more precisely, both RED and WRED are the two congestion avoidance mechanisms. These mechanisms work by proactively monitoring the average queue depth and randomly dropping packets before the queue becomes completely full, which prevents tail drops and the resulting global TCP synchronization. On the Cisco SPCOR 350-501 exam, this distinction is critical: congestion management tools like CBWFQ and LLQ handle packets already in a queue, while RED and WRED avoid congestion by signaling TCP senders to slow down through early drops. A common trap is confusing WRED with a queuing tool, but remember that WRED is a drop profile applied to a queue, not the queue itself. For a memory tip, think of the "W" in WRED as "Weighted" for different drop probabilities per traffic class, and keep in mind that RED is the unweighted baseline—both are proactive, not reactive.
This 350-501 practice question tests your understanding of automation and quality of service. 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 TWO QoS mechanisms are used to provide congestion avoidance? (Choose two.)
Policing
Why wrong: Policing is a traffic conditioning mechanism, not congestion avoidance.
RED
RED (Random Early Detection) is a congestion avoidance mechanism.
CBWFQ
Why wrong: CBWFQ is a queuing mechanism.
LLQ
Why wrong: LLQ is a queuing mechanism that provides strict priority.
WRED
WRED (Weighted Random Early Detection) drops packets probabilistically to avoid congestion.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
RED
RED (Random Early Detection) and WRED (Weighted Random Early Detection) are congestion avoidance mechanisms that proactively drop packets before a queue becomes full, signaling TCP senders to reduce their transmission rate. Unlike congestion management tools (like CBWFQ or LLQ) that queue packets during congestion, RED/WRED monitor average queue depth and drop packets probabilistically to prevent tail drops and global TCP synchronization.
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.
Policing
Why it's wrong here
Policing is a traffic conditioning mechanism, not congestion avoidance.
RED
Why this is correct
RED (Random Early Detection) is a congestion avoidance mechanism.
Related concept
Read the scenario before looking for a memorised answer.
CBWFQ
Why it's wrong here
CBWFQ is a queuing mechanism.
LLQ
Why it's wrong here
LLQ is a queuing mechanism that provides strict priority.
WRED
Why this is correct
WRED (Weighted Random Early Detection) drops packets probabilistically to avoid congestion.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Cisco often tests the distinction between congestion management (queuing/scheduling) and congestion avoidance (active queue management), so the trap here is that candidates confuse mechanisms like CBWFQ or LLQ (which manage congestion after it occurs) with RED/WRED (which avoid congestion by dropping packets early).
Detailed technical explanation
RED uses an exponentially weighted moving average (EWMA) to compute the average queue size, comparing it against minimum and maximum thresholds; packets are dropped with a probability that increases linearly between these thresholds. WRED extends RED by allowing different drop profiles per IP precedence or DSCP value, enabling differentiated treatment of traffic classes. In real-world deployments, WRED is often used on core routers to avoid TCP global synchronization, where multiple TCP flows simultaneously back off and then ramp up, causing oscillatory congestion.
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 350-501 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
Automation and Quality of Service — This question tests Automation and Quality of Service — Read the scenario before looking for a memorised answer..
The correct answer is: RED — RED (Random Early Detection) and WRED (Weighted Random Early Detection) are congestion avoidance mechanisms that proactively drop packets before a queue becomes full, signaling TCP senders to reduce their transmission rate. Unlike congestion management tools (like CBWFQ or LLQ) that queue packets during congestion, RED/WRED monitor average queue depth and drop packets probabilistically to prevent tail drops and global TCP synchronization.
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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Same concept, more angles
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Variation 1. Which congestion avoidance technique drops packets probabilistically before the queue becomes full?
easyWhy C: WRED (Weighted Random Early Detection) is a congestion avoidance mechanism that monitors the average queue depth and, when it exceeds a configured threshold, begins dropping packets probabilistically before the queue becomes completely full. This proactive dropping signals TCP senders to reduce their transmission rates, thereby preventing tail drop and global synchronization.
Last reviewed: Jun 11, 2026
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