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Cisco Catalyst 3850 layer2 QOS


We had a small meeting this morning with our admin. We are upgrading from 3com to Cisco switches. He said we have a few things to address in the new setup.

We have an I.P based PA system and he said that in small segments it works fine, campus wide there is an issue with the broadcasts and the latency makes things in the speakers really quiet. This is an internal only setup and I guess one of the server (in house) has some app on it that makes the broadcasts. That should just need it's own VLAN too and make it a trunk to the server.
This issue I get as nobody had yet setup a VLAN for them so they share the data Vlan. 

He said the phones act similarly though. Those are on a separate Vlan and basically it just goes out of out core switch on a dedicated link and connects to our ISPs router on the same vlan. So that is basically it's own network I think that vlan does have a layer3 interface, but As far as I know it does nothing. The phones are Cisco SPA504G models.
For the new Cisco switches I setup the phones using the switch port voice vlan command I don't know for sure if that helps with QOS or if it is more just making a trunk.

That makes me wonder if the issue is just related to 3com as the phones on that use hybrid trunks (or some other setting) and Cisco may be better. Or if we need to look at layer2 QOS as well. 

I have CCNA route/switch, but i'm not real familiar with QOS besides what it does. 
I see Cisco has 2 versions a plain QOS and an auto QOS. To me auto QOS sounds simpler and all we probably need to do is tell the switches to modify the processing and forwarding for the proper vlans. To me it would seem like I could just put in something like 3lan 9 QOS 1 and that would then get the first priority and all the ports that connect to that even trunks then get the result. Then data vlan like vlan 10 QOS 5 and it's priority is less than the 1 so it is processed as data.

Suggestions do we need layer2 QOS, if so auto or plain QOS, or is there a simpler way?

1 Reply 1

Mark Malone


...For the new Cisco switches I setup the phones using the switch port voice vlan command I don't know for sure if that helps with QOS or if it is more just making a trunk.

This is the recommended way keep doing it that way , setting it as trunk is legacy , it does not have any effect on qos though

Qos is really only needed it if the switch is under pressure/congested but qos can be good to have in place too , but its not a fix for all and can add to issues sometimes but I find its best to have the phones marked correctly and the switches trusting that up to your layer 3 MQC qos at the wan

Now 3850s ios-xe support DSCP by default , so if your phone is Cisco and its marking it with EF 46 then the switchport will honour and carry it right through

the other option is to enable auto qos which will generate a class-map with policy-maps etc , these will vary on 3850 depending on what way you set the access port then you can tweak them either if you needed

take a look at this doc each variation of port qos will generate diff class-maps and it gives examples

or connect a phone span the port and the egress port , check wireshark make sure packet is being marked with EF and then check platform command , if everything is ok you don't need to apply the auto qos for voice but the decision is up to you whether you really want to deploy

#sh platform qos dscp-cos counters gigabitEthernet 1/0/1 | i DSCP46
Ingress DSCP46 612103879     0
Egress DSCP46 823031803     0

My 3850s are interconnects no direct phones but I have confirmed multiple ways they are carrying the packets correctly , if I was directly connecting the phones to the 3850s and they were acting as access switches I would set the auto qos and see how it goes -- auto qos voip trust is what I would use for those phones or auto qos voip cisco-phone

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