350-501 Automation and Quality of Service Practice Question
A service provider operates a large MPLS network with hundreds of PE routers. They have deployed QoS policies to ensure real-time traffic (voice and video) receives priority. The policy uses LLQ with a strict priority queue for voice (DSCP EF) and another for video (DSCP AF41). Each priority queue has a policer to prevent starvation of other classes. Recently, a customer reports that their video calls are experiencing jitter and packet loss. The engineer examines the PE router connected to this customer. The interface is GigabitEthernet0/0/1 with the policy applied in the output direction. Show commands indicate that the video priority queue is frequently dropping packets, while the voice queue has no drops. The police for the video queue is set to 10 Mbps. The customer's contract guarantees 20 Mbps for video traffic. However, the actual video traffic is averaging 15 Mbps with bursts to 18 Mbps. The engineer notes that the video class also has a bandwidth command of 10 Mbps under the priority statement. What is the most likely cause of the video packet loss?
⚠ Common exam trap
Cisco often tests the distinction between the bandwidth command (which reserves bandwidth for CBWFQ) and the policer rate (which enforces a hard limit on priority traffic), leading candidates to mistakenly think increasing bandwidth will solve the 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
✓
The police rate for the video queue is too low; increase it to at least 20 Mbps.
The video priority queue is dropping packets because the police rate (10 Mbps) is lower than the actual traffic rate (15 Mbps average, bursts to 18 Mbps). In LLQ, the priority queue uses a policer to enforce a maximum rate; traffic exceeding the police rate is dropped. The bandwidth command under priority only reserves bandwidth for the queue in the absence of congestion or for shaping purposes, but the policer is the active enforcement mechanism that causes drops. Increasing the police rate to at least 20 Mbps aligns with the customer's guaranteed rate and prevents drops.
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 command under priority is too low; increase it to 20 Mbps.
Why it's wrong here
Bandwidth under priority does not limit the queue; it affects the remaining queues.
- ✗
The video queue is using the wrong queue limit; increase the queue limit to accommodate bursts.
Why it's wrong here
Queue limit affects tail drops; but the loss is due to policing, not queue fullness.
- ✗
The video traffic is not being classified correctly; check the class-map match criteria.
Why it's wrong here
Traffic is being classified (drops indicate matching), so classification is correct.
- ✓
The police rate for the video queue is too low; increase it to at least 20 Mbps.
Why this is correct
The policer drops traffic exceeding 10 Mbps, causing loss for 15-18 Mbps traffic.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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