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QoS

V

VeeDubbs

Limp Gawd
Joined
Dec 9, 2005
Messages
398
Hi All -

Trying to configure QoS and am getting a little confused on some things.

First, we are using Avaya as our VoIP solution. We have ProCurve switches at the edge and Cisco in our core.

I think one of my main problems is I can't seem to wrap my head around DSCP. I *think* I understand why it's used -- but I'm not entirely sure I would know how to configure it. Also, it seems it's either DSCP or priority during configuration -- when would I use one or the other? And on a priority scale of 0 - 7, which is a higher priority, 0 or 7?

If someone could explain to me that'd be great. Otherwise, a link to some good reads would be good as well. I've found a lot of info out there, but I think I need to understand some of the basics first.

Thanks!
 
DSCP is a layer 3 QoS marking. If you are talking about values in the 0-7 range you are probably referring to COS which is a layer 2 marking. The higher the number the higher the priority. Typically voice is COS 5 which generally translates to DSCP EF (but really you can use switches to remark frames and packets however you want).

Typically the phones or a device would mark the frames with COS which a switch or router would then add a corresponding DSCP value to as the packet is routed through the network. Routers use the DSCP value to decide which packets get put in higher priority queues (software queues) so that small voice traffic packets don't get delayed by large FTP packets for instance.

Honestly QoS is pretty complicated when you first learn it. I suggest you track down something like CBTnuggets videos to get started on it. Don't even try to configure it if you don't know how it works unless you want a serious network outage.
 
What Matt says pretty much covers it for the basics, you can also use fragmentation to break up the larger packets into smaller ones. This way if a voice packet comes in while a larger packet is being processed it doesn't have to wait for the entire packet.
 
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