Re: QOS on 3550, congestion avoidance

Thomas Paine · ‎07-15-2002

I am trying to configure a 3550 in such a way that I can condition traffic leaving the gig interface to 20Mbs. Using a policy map and policing the traffic works, however, that hardcodes bandwidth. We have a mix of video / data going over the link. I have marked and classified the video from the data and mapped it to the right egress queues, but I am unable to get the wrr-queue commands to give us the desired results. Has anyone configured congestion avoidance on the 3550? Example configurations would be a great help if you have it. The documentation examples are a little vague (what’s new there). Thanks

bsivasub · ‎07-15-2002

what is vague to you ?

http://www.cisco.com/univercd/cc/td/doc/product/lan/c3550/1219ea1/3550scg/swqos.htm#xtocid35

"All packets with DSCPs assigned to the first threshold are randomly dropped when the first threshold is exceeded. However, packets with DSCPs assigned to the second threshold continue to be queued and sent as long as the second threshold is not exceeded. Each threshold percentage represents where WRED starts to randomly drop packets. By default, WRED is disabled."

Thomas Paine · ‎07-15-2002

That is if you are using WRED and not WRR. Either way that is not the issue. The problem is, how can the traffic be conditioned in such a way that the queue scheduler releases no more than 20Mbs worth of data? What is congestion avoidance otherwise? The wrr-queue queue-limit and wrr-queue bandwidth commands (according to the docs) are nothing more than ratios to their corresponding arguments. What’s vague to me is, ratios to or of what? You may only give queue number 1 x% of some buffer versus queue number 2 getting 99% of that same buffer, but if the total buffer size or bandwidth is not defined, what good is it? It would be like telling you that you will receive 25% of x where x is not defined.

One possible condition for needing QOS is when there is a speed mismatch somewhere in the network. Whether it be 100Mbs to 10Mbs, or in our case 1000Mbs to 20Mbs.

Thanks