QoS over slow wireless link

[brian-andrews51]

I have a pair of WS-C2960-24TT-L running 12.2.55.SE11. They are connected by a slow wireless link that runs at 40 Mbps. A service that uses that link requires 10 Mbps. This link is also shared by a remote office site. My issue is that when someone in the office hogs bandwidth, it brings down the service that requires 10 Mbps. 

The switches are connected via a trunk. Is it possible to limit the trunk to 40 Mbps and guarantee the traffic that requires 10 Mbps? I have done something similar over a layer 3 link with class maps and a policy map.

I have looked at the following document Cisco 2960 QoS configuration.

It mentions to create an access list to match traffic and then police it. However, I am concerned that with the port being a trunk, it will never see layer 3 headers to process the access list. 

My idea was to tag important traffic and then define a QoS policy on the wireless link to limit it to 40 Mbps and allow the important traffic its 10 Mbps during congestion. Is this possible and thanks for any insight. 

[Joseph W. Doherty]

Is it possible to limit the trunk to 40 Mbps and guarantee the traffic that requires 10 Mbps?

More or less, yes it is possible.

What you want to do is limit the egress trunk port to 40 Mbps (using the srr-queue bandwidth limit interface command).  Then you want to divide your traffic into at least two queues, one being used for your service traffic that needs 10 Mbps.  For the latter, you guarantee it 25% of the port bandwidth using the srr-queue bandwidth share interface command.

[brian-andrews51]

That makes sense. I did read up on srr-queue bandwidth limit.

From my understanding, the four values assigned to srr-queue bandwidth share relate the the 4 output queues. 

What is the best approach to divide traffic into two queues? 

[Joseph W. Doherty]

You could use a specific DSCP marking for your "special" traffic, and have that marking, and only that marking, go to a queue where you've guaranteed 25% (of the 40 Mbps).

You might see if any of the RFC or Cisco DSCP markings apply to your "special" traffic.  If not, don't forget DSCP markings skip every other marking, to allow you to use those markings for your own purpose.  If you use one of those, try to keep in the CS group for the kind of traffic each CS group tends to deal with.