does qos only work during congestion

[sarah.moh]

I heard that QoS configuration on routers doesn't take action unless there is congestion, and by congestion I mean one class or queue is utilized

My question is : is that true or for the whole service policy configuration? or does that refer to policy or shaping only?

[Philip D'Ath]

QoS is the re-ordering or packets in a queue to meet a specific policy.

A queue can only form when their is congestion (aka packets are being queued faster than they can be sent).

If there is no waiting queue of packets, aka there is no congestion, then QoS has no packets that it can work on (it has nothing that it can re-order).

Shaping and policing will still work as that is a different mechanism.

[sarah.moh]

Hello Philip

so classifying and marking only work during congestion, and packets will always a dscp value but they wont be matched to a class map, correct?

As for shaping and policing, I thought these were congestion management tools, I can't imagine how can can they work when they are configured per class and when there's no congestion

[Philip D'Ath]

Good point.  Marking always works, regardless of congestion.

I guess when I wrote the response I was thinking of QoS in MQS (modular qualify of service) terms, where QoS is used to guarantee minimum bandwidth levels to particular classes of traffic, and priority queuing (usually used for voice).