350-501 Automation and Quality of Service Practice Question
An SP's core router experiences excessive packet drops on a congested link. The QoS policy uses CBWFQ with 4 queues. The drops occur only in the queue for a premium business class. Which is the most likely cause?
⚠ Common exam trap
Cisco often tests the distinction between policing (which drops excess traffic regardless of congestion) and queuing (which drops only when buffers are full), leading candidates to mistakenly attribute drops to congestion management mechanisms like tail-drop or WRED when the real cause is a police action configured with a drop exceed-action.
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
✓
The premium class is configured with a police action that drops traffic when it exceeds the committed information rate.
The premium business class queue is experiencing excessive packet drops because a police action configured with a 'drop' exceed-action is discarding traffic that exceeds the committed information rate (CIR). Unlike congestion management (queuing), policing is a congestion avoidance mechanism that drops packets immediately when the traffic rate surpasses the configured CIR, regardless of the queue depth or available bandwidth. This explains why drops occur only in the premium class queue, as the policer is explicitly dropping excess packets before they even enter the CBWFQ scheduler.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The 'bandwidth remaining' command is missing from the policy-map.
Why it's wrong here
That affects sharing of excess bandwidth, not dropping within a class.
- ✗
The interface is using MLP with interleaving.
Why it's wrong here
MLP interleaving is for link fragmentation and interleaving, not queue drops.
- ✗
The queue is using tail-drop without WRED.
Why it's wrong here
Tail-drop can cause drops, but the question specifies only that queue; WRED might help.
- ✓
The premium class is configured with a police action that drops traffic when it exceeds the committed information rate.
Why this is correct
Policing drops excess traffic; if the police rate is lower than the guaranteed bandwidth, drops occur.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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