WAN Congestion and Priority Queuing Class Based QOS

[Andrew Grech]

Hi Experts,

If there is congestion from a bidirectional Iperf test using the default fair queue class and I have a loss intolerant application that is bursty (transmits once every 5 minutes, 60 bytes then backs off) set to use the priority queue. Is there a period of time where I may see traffic drops or delay, As the link is already congested?  In this situation the default queue is not shaped or policed, it simply has a minimum bandwidth allocation (which only really takes affect during congestion of higher classes)

One thing to note in this situation the headend router is 20mbps and tail only 512kbps. This is a bdsl circuit so the provider is performing qos outbound from their PE.

HQ Router QOS Markings> -------20Mbps--------ISP PE>QOS out>------512kbps-----<Branch Out QOS [Router]

A easy fix for this situation of the default queue congesting the link seemed to shape that queue so I had bandwidth reserved for the priority class. As the priority and bandwidth commands only take affect while their is congestion and that class is in use. However it felt like I was cheating as with rose colored glasses the priority queue is supposed help guarantee delivery.  

[Joseph W. Doherty]

A very belated post . . .

Yes, if the device doesn't "see" the congestion, it's configured QoS, for congestion management, will generally be ineffective.

To deal with later-then-this-device-bandwidth-reduction, as you note, you would normally shape for that bandwidth.  Then the device can manage congestion, much as if were the device which had the reduced bandwidth connection connected.

One note, such a shaper would normally manage all the bandwidth to the far end, not just one class.  Then a child policy might be used, including LLQ, to manage congestion in detail.

I'm not quite sure I understand your remarks about rose colored glasses, or cheating.