350-501 Automation and Quality of Service Practice Question
Which TWO QoS mechanisms are used to provide congestion avoidance? (Choose two.)
⚠ Common exam trap
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).
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
✓
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.
Answer analysis
Option-by-option breakdown
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.
- ✗
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.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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