QoS Impact of Priority Queue on Class-Default Traffic

When employing QoS on a network device, configuring a priority queue for a voice-video class can impact class-default traffic, especially during congestion periods. The priority queue gets serviced first, potentially starving other queues of bandwidth, leading to increased delays and potential packet loss in the class-default queue. To mitigate this, there has to be a bandwidth limit which is policed for the priority queue. Depending on your platform and software version, this might be done automatically.

You can see a detailed explanation of examples where we test with or without LLQ and with or without congestion in the QoS LLQ Queue Starvation lesson.

https://networklessons.com/quality-of-service/qos-llq-low-latency-queueing-cisco-ios