QoS Failure?

[jasonfaraone]

I have QoS set to prioritize traffic on my WAN, and for the most part, it appears to be working exactly as expected.

I have traffic to and from a particular server tagged, and I run ping tests from our primary location to this server. I also have ping tests running to a server whos traffic is not prioritized.

For the most part, prioritized traffic appears to have lower latency than its non-prioritized counterpart. There are exceptions, however. See the attached image for more details. Although between 13:40 and 13:50 the overall latency is lower, the duration of the latency spike is higher. Can anyone think of what might be causing this?

[Joseph W. Doherty]

There can be reasons which depend on factors about your network you haven't described.

However, assuming your WAN link is a p-2-p link, one factor can be priorization occurs with software queues that "engage" only after the interface hardware FIFO queue "overflows". Even with a minimum sized hardware FIFO queue, larger packets take longer to transmit.

I.e., background traffic can still slightly delay foreground traffic.