350-501 Architecture Practice Question
An SP is deploying a Quality of Service (QoS) architecture in its core network to support multiple services: voice, video, and data. The core routers use MPLS and implement QoS based on the MPLS EXP bits. The architecture must ensure that voice packets are never dropped, even during congestion, while allowing video to have higher priority than data. The current design marks voice with EXP 4, video with EXP 3, and data with EXP 0. During a traffic burst, voice traffic is being dropped, which should not happen. The core routers have the following queue configuration: (1) a priority queue (PQ) for EXP 4+5, (2) a bandwidth queue for EXP 2-3, (3) a default queue for EXP 0-1. What is most likely the cause of voice 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 priority queue is rate-limited by a policer that shapes traffic to a certain bandwidth.
Voice drops occur because the priority queue is often rate-limited by a policer to prevent starvation of other queues. If the policer rate is set too low, voice packets are dropped during bursts. Strict priority without policing does not drop, but policing is common in SP core to protect other queues. The bandwidth queue and default queue are not the issue.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Voice traffic is being marked with EXP 4 but the priority queue also includes EXP 5, which may cause policing.
Why it's wrong here
Incorrect. Including EXP 5 does not directly cause voice drops unless there is a policer.
- ✗
The bandwidth queue for video is configured to borrow from the priority queue.
Why it's wrong here
Incorrect. Borrowing is not a standard feature; priority queues are typically not borrowed from.
- ✗
The priority queue is not strictly priority; it shares bandwidth with other queues.
Why it's wrong here
Incorrect. A priority queue is typically strict priority, so voice should be serviced first.
- ✓
The priority queue is rate-limited by a policer that shapes traffic to a certain bandwidth.
Why this is correct
Correct. Many implementations use a policer on the priority queue to prevent starvation; if the police rate is too low, voice packets are dropped.
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