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QoS in Campus & Branch Environment


I have a question about QoS and the best way to get started. I am familiar with the concepts that QoS is used to prioritize traffic, make sure voice traffic can have the bandwidth it needs, etc.

I am in the process of deplying a VoIP system in a campus environment with 3 buildings connection via gigabit fiber to a Catalyst 6509. The VoIP system is using Catalyst 3560 PoE switches for the infrastructure. There is some video over IP on the network and I want to make sure the voice traffic gets the priority it needs to. I also have a branch office connected to the main office campus through a T1 and want to prioritize that traffic so the voice traffic gets priority over data traffic on the T1.

All voice traffic, regular PC and server traffic, and video traffic are each separated onto their own subnet and own VLAN.

I'm not quite sure how to get started with QoS. I know I need to define policies and categorize traffic. What I basically want to do is say anything on my voice subnets gets highest priority, data next and video last (the video is in no way mission critical).

Could someone please point me in the right direction for where I could get started here? I've read up on the different QoS tools and different methods that can be used, but I'm not quite sure how to actually put it all together and get started.

Thanks for any help in advance.


Re: QoS in Campus & Branch Environment

Hello mbalik,

For any QoS deployment you need to know what your bandwith requirements are and where the business priorites lie. In your case voice has priority over data, which has priority over video.

To start with voice, you can configure auto-qos on all access layer switchports that connect to an IP phone.

This will tag voice frames with a dot1q/dot1p field and mark the frame with CoS 5 which maps to DSCP 46. DSCP 46 is handled by a strict priority-queue that transmits CoS 5/DSCP 46 before any other frames.

Data from the attached PC is handled on a best effort basis by default. You can set policies to mark traffic with route-maps. You need to specify what traffic to give priority e.g. www over ftp et, cetera.

There is no singe solution for a QoS deployment. Please read into the documentation and case studies on You'll find something that suits the needs of your business.

White Paper - Deploying Quality of Service for Converged Networks

QoS Home




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New Member

Re: QoS in Campus & Branch Environment

Hi Leon,

Thanks for taking the time to plot this out. One question I do have - we are using a non-Cisco VoIP phone system. From what I've read, I would not be able to use Auto-QoS for that, correct? What I've seen is that Auto-QoS uses CDP to detect the Cisco IP phone as a Cisco device and obviously that would not work for third party phones.

I do have a control in my phone system where I can specify the DiffServ/ToS Byte (0-255) values in my phone system for the packets to be tagged.

Thanks again for your help.


Re: QoS in Campus & Branch Environment


CDP is used to negotiate the dot1q trunk between the IP phone and switch. In that case you can tag the frames in the phone and auto-qos would take care of the rest.

You can configure the switch to trust the frames from your phone. Use the appropiate configuration guide for doing so.

Configuring Voice VLAN

Good luck!


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