350-501 Architecture Practice Question
Which QoS mechanism is most effective for preventing congestion on a service provider edge link when traffic exceeds the link capacity?
⚠ Common exam trap
Cisco often tests the distinction between congestion management (queuing/scheduling) and congestion avoidance (WRED) versus rate limiting (policing/shaping), and the trap here is that candidates confuse policing (which drops) with shaping (which buffers), or assume WRED prevents congestion rather than just avoiding tail drops.
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
✓
Traffic shaping
Traffic shaping is the most effective QoS mechanism for preventing congestion on a service provider edge link when traffic exceeds link capacity because it buffers excess packets and smooths the transmission rate to match the configured committed information rate (CIR). Unlike policing, which drops or re-marks packets, shaping avoids tail drops and packet loss by queuing bursts, making it ideal for edge links where preserving traffic integrity is critical. Cisco IOS uses the Generic Traffic Shaping (GTS) or Class-Based Shaping (CB-Shaping) to enforce a rate limit while maintaining TCP fairness and reducing retransmissions.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Priority queuing
Why it's wrong here
May starve other traffic classes.
- ✗
Traffic policing
Why it's wrong here
Drops excess traffic, not preventing congestion.
- ✓
Traffic shaping
Why this is correct
Shapes traffic to avoid line-rate bursts and drops.
- ✗
WRED
Why it's wrong here
WRED manages congestion but doesn't prevent it.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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