How can I tell that AutoQOS config is adequate on a video conference switch

Unanswered Question
Feb 13th, 2009

I have a Tandberg video conference bridge and gateway connected to a 3750 switch. The audio is marked EF and the video CS3. I configured AutoQos on the switch and trust DSCP on the ports. When we reach a large video conference of 25 or more attendies, we begin to have problems. The Tandberg starts to ratchet some of the video streams down, some to below 180kps, and the conference quality begins to suffer.

The only change to the default AutoQOS config I've made is changing the "mls qos trust dscp" on the uplinks, adding a QOS policy on the Tandberg ports to mark the video with CS3 and trust the audio DSCP (Tandberg had a bug that didn't mark the video packets) and adding

queue-set 2

priority-queue out

to the uplinks.

Will the default AutoQOS queue/buffer/SRR share and shape commands be adequate for 25 to 30 simultaneous video streams? We allow about 500k per stream.

I read through the 3750 QOS guide, but am a bit confused with the explanations of the queuing and SRR workings. I know that CS3 is mapped to COS-3, but don't know which queue it goes in and whether or not it gets shaped/disgarded when too much video is present.

Once I am sure that the 3750 QOS is working OK, and not causing problems with our large conference calls, then I'll expand my troubleshooting to the rest of the network.

I've attached a text file with the 3750 config, and 'show mls qos int statistics' that show the packets are being marked.

I have this problem too.
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Aaron Harrison Fri, 02/13/2009 - 09:42

Hi

The standard setup is to assign CoS4 for video, CoS3/DSCP CS3 being assigned for voice signalling.

If you take a look the the autoqos config, CoS3 is sent to queue 2 with smaller buffers and 10% of the available bandwidth. CoS4 would go into 3 with more buffers and more bandwidth.

I would try to stick to this, marking your video traffic as CoS4/DSCP CS4.

You should find if you are using auto qos accross your LAN this will work better.

Regards

jkeeffe Fri, 02/13/2009 - 10:42

I mistyped in my first post. I do indeed mark the video as AF41 (Cos4). It is the call control that I mark CS3 with the policy map to get around the old Tandberg bug that doesn't mark the call control.

So do you think the queue 3, where Cos4 goes to, is big enough to handle 30 video streams of 480k? If my calculation is correct that would be 14.4mbps of video on a 100mbps port.

With this config.....

srr-queue bandwidth share 10 10 60 20

srr-queue bandwidth shape 10 0 0 0

...is it telling me that queues 1 & 2 get 10% bw, queue 3 gets 60% and queue 4 gets 20% bw and is shared? If that is correct than 60% is more than enough for 14.4mbps of video.

What happens if too much video overruns the queue?

Scott Pickles Fri, 03/20/2009 - 13:20

Jim

I would think that once the queue is full, that any more packets are going to get dropped. If I understand the queueing technology after reading the same QoS doc for the 3750, I don't think it cares what type of traffic it is. All that matters is that the queue is full and can't accept more, so it would seem that the switch as no other choice but to drop it. In addition, given that we as administrators can change the bandwidth of each queue, if you find that video begins to get dropped, you can modify the bandwidth for that queue.

Regards,

Scott

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