[nsp] Prioritisation of VoIP

[Tim Franklin]

Hi,

cisco-nsp-bounces at puck.nether.net wrote:

> one more naive question: "How do I prioritise VoIP traffic on my cisco
> network?"
> 
> Maybe it's a little too broad question, but I don't know where to
> start from
> (even what to ask to the IP phone vendor).
> 
> So any hints, readings, good practice rules, etc. would be
> appreciated. I have heard about ToS, DiffServ. What is more commonly
> used by VoIP gear (keeping in mind that my VoIP gear will not be
> Cisco, only the transit network will)? How do you handle this with
> Ciscos? What are the pros and cons of each? Etc.

(Apologies for the brief outline, it's quite a big question)

In general you don't need the VoIP gear to do anything other than generate packets that you can recognise as being VoIP.  If your addressing plan is such that the phones are all in their own subnets, this is easy - you can identify by IP address.  Otherwise you'll have to look at ports / protocols / existing markings to spot the traffic.

On the router, you can the classify the traffic as "VoIP" or "other".  Look at CBWFQ with LLQ - the VoIP goes in the "priority" (LLQ) queue, everything else goes in one or more CBWFQ queues.  You'll probably want to mark (DSCP or IPPrecedence) on the way out so you can tell which traffic is which elsewhere in your network without having to repeat the classification ACLs you used earlier.

If your core's congested, you'll need to use those markings in the core to affect your queueing behaviour there.

If the LAN is congested, you'll need to look into 802.1p on your L2 gear and check that the VoIP gear can mark appropriately.

The Cisco Press "IP Quality of Service" (Srinivas Vegesna) is well worth a read.

Regards,
Tim.

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Tim Franklin          ____________
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