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, methods like bandwidth allocation or policing for the voice-video class can be a good idea, to put a limit on the amount of voice-video class traffic.

If a voice-video class continuously exceeds its bandwidth allocation but class-default traffic remains under its allocation and there is no congestion, class-default traffic should not be impacted. However, during congestion, class-default traffic must wait behind the priority queue to access the transmit ring.

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