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.

For class-default queues, FIFO queuing is the default unless overridden by an explicit action, such as policing. During congestion, FIFO queuing occurs first, and policing drops packets if the queue becomes full.

https://forum.networklessons.com/t/qos-llq-low-latency-queueing-on-cisco-ios/1817/96?u=lagapidis

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