Priority-queue out or srr-queue bandwidth share?

[Chigo]

Hi All

I am looking at QoS config across our (2960-X edge, 3850 core) switch infrastructure. The main thing I need to do is ensure Teams traffic is served over all other traffic. So far I have tagged Teams video and audio traffic with DSCP 46 and 34 via GPO, I have assigned these DSCP values to queue 1 out and threshold 3.

I'm just wondering though, should I assign a srr queue bandwidth share to uplink ports or just enable priority queue out? I understand once priority queue out is enabled the srr queue bandwidth config is irrelevant.

Also, which should I assign to switchports linking to our provider MPLS router? They have QoS deployed for us prioritising the required traffic so I just need to ensure it is all prioritised within our LAN.

Thanks!

[Georg Pauwen]

Hello,

to partially answer your question: actually, both can work together. The only thing that happens when you enable the priority queue is that this queue is always served first, as long as there are packets in there.

Also, keep in mind that the priority queue does cause weight 1 to be ignored.

From the official documentation:

When you configure this command, the SRR Switch(config-if)# priority-queue out weight and queue size ratios are affected because there is one fewer queue participating in SRR. This means that weight1 in the srr-queue bandwidth shape or the srr queue bandwidth share command is ignored (not used in the ratio calculation).

[Chigo]

Thanks Georg, that makes sense.

[Joseph W. Doherty · Accepted solution]

"I'm just wondering though, should I assign a srr queue bandwidth share to uplink ports or just enable priority queue out? I understand once priority queue out is enabled the srr queue bandwidth config is irrelevant."

Generally, for real-time audio or video, PQ would be the choice. This both to minimize any packet loss for this traffic and also to minimize latency and/or jitter.

Yes, once PQ is active, it has priority over all other traffic, however the rest of the port's QoS config may not be irrelevant. Even if all other traffic maps into only one egress queue, you may want to adjust buffer settings for both the PQ and that remaining queue. If you use more than one non-PQ egress queue, bandwidth allocations might be relevant for non-PQ traffic vis-a-vis other non-PQ traffic.

"Also, which should I assign to switchports linking to our provider MPLS router?"

Often you can give those ports the same logical treatment for QoS resource allocations. What might differ, to fully leverage your MPLS provider's QoS, you might need to mark you traffic differently then you do internally.

[Chigo]

Thanks for the information Joseph, much appreciated!