QOS Best Practices

[jkleberg]

2 questions:

1. Is there a best practice for mapping dscp or cos to voip control and voip media traffic? I am looking for a doc that says voice control in most cases should receive dscp X and cos of X. The same for voip control. I understand that it could vary on each network but there must be a general idea? I am not looking for commands and it doesnt neccessarily need to be cisco, I am just looking for a "best practices".

2. Assuming I use Cos3 for control and Cos5 for media. I was working on a 3550 that has 4 queues, with one of them for priority. Having only 1 queue for priority, I think I can only match one COS label to it. This is a problem if I saturate a layer2 trunk upstream (since only media is getting sent as priority). Does anyone have any thoughts on what to do here? Is it possibly to use dscp and set min bw for both if I create a layer3 trunk?

Thanks

jason

[wong34539]

Check the below for more infmn on DSCP

Implementing Quality of Service Policies with DSCP

http://www.cisco.com/en/US/tech/tk543/tk757/technologies_tech_note09186a00800949f2.shtml

[ptredway]

To answer your first question, as a standard media traffic uses CoS 5 and DSCP 46(EF) and control uses CoS 3 and DSCP 24(CS3). You'll find that in the past that DSCP 26(AF31) was used for call control. I defintely recommend that you match both DSCP 24 and 26 at the access layer and reset the values to 24. You'll also need to make sure that you dscp-cos mapping is updated as well.

As for your second question, you want to reserve your priority queue for real-time data (Voice) only. You shouldn't see a problem during congestion periods as the voice is a minimal percentage of the bandwidth in comparison to most data. (roughly 80k per call with G.711). So even if you have 40 calls traversing an uplink that's still only 3.2Mb out of either 100Mb or 1000Mb depending on your port.

If you'd like you can tweak the values for what queue gets what bandwidth on an uplink. However, that is platform specific...so if you want to let me know what switch you're talking about...I can give you some sample numbers/configs.

Please let me know if I need to clarify at all.

Thanks!

Patrick

[jadecerega]

Hi,

First the priority setup is defined PHB ; You must define a translation rule within the switch so in the case of voice media that is delay-sensitive traffic, bits 5 & 7 of the EF DSCP value are "101" wich matches IP precedence 5 and allows backward compatibility . Within the switch you can display the default Dscp-cos map ( show mls qos maps ) , even though you can modify it.

References from Cisco can be found:

1. www.ietf.org/rfc/rfc2597.txt

2. www.ietf.org/rfr/rfc2598.txt

3. www.ietf.org/rfc/rfc2474.txt