VOIP QoS

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Aug 8th, 2007
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We are in the process of upgrading our bandwidth at our branch locations into 3 Mbps MPLS networ and we only run Citrix traffic and IP Voice (Interoffice calls) from our Branch locations into our HQ.


We expect Bandwith utilization to typically max out at 1.6 MBPS. Do you think we need to configure QoS for the voice traffic since the circuit will only be 40-50 percent utilized? My thinking was why should I configure QoS if the bandwidth is only at 40 - 50 percent. The voice traffic should have enough bandwidth to communicate over the wire.


Is my thinking correct or should I configure QoS across this MPLS network?


Any responses are greatly appreciated


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jwdoherty Wed, 08/08/2007 - 06:51
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I would recommend QoS for a couple of reasons.


First, congestion is often transient, and is not often seen with "normal" traffic averages. Second, Citrix can support traffic, such as printing or disk to disk copying, that can be bandwidth intensive.


For your situation, CBWFQ using LLQ for VoIP and the default class with FQ enabled should work well and is a simple configuration.


e.g.

policy-map CBWFQ

class VoIP

priority percent 50

class class-default

fair-queue


Also determine if your MPLS vendor supports QoS within their cloud. If they do, take advantage of that. I.e. mark you packets


NB: Some MPLS vendor sell, at extra cost, a real-time class, but as long as they offer separate classes and one will provide sufficient bandwidth for your VoIP, that usually works well. I.e., you might not need to buy additional RT bandwidth.

Pavel Bykov Wed, 08/08/2007 - 07:04
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If you have bought MPLS circuit from provider, than configuring QoS would be a little bit complicated, since provider would have to configure QoS, not you. You would probably have 100/1000 Mbps access speed, and qos settings would be useless. The only way you'd be able to influence it is through complicated shaping setup (simple at branch, but you'd have to create many shaping maps at aggregation point at central location).

So usually in this setup QoS is done in provider network at the last mile.



As for the QoS/Citrix mix I think you should be fine without QoS for now. But it depends if you intend to grow. Because you have high speed, you don't need fragmentation and interleaving, since serialization delay will be low even for large packets. Since you won't utilize your link higher than 40-50 percent, all queues should be almost empty, not needing management.


But Beware! Cisco itself was the first ones to discover need for QoS even in their LAN network, where typical load is below 5%. What is threatening is traffic peaks. It doesn't matter that you have 40% average load over several hours, if 5 minutes of it is 100% load. Also remember, packets are always sent at interface speed, so it's not like at lower load your interface is not loaded. It's always either loaded at 100% or at 0% at any one given microsecond, but over time, the peaks average out to some number.


If you don't have prolonged peaks, you should be fine. My experience with Citrix (banking application) tells me it should be OK.


But QoS is generally good for you. You can implement an easy setup like "ip rtp priority".


I'd create a baseline and watch for immediate peaks. If there aren't any (assuming the only app is Citrix, and it's not aggressive Citrix), and no new applications are in sight, I'd go with QoS free. But if even one of the apps is aggressive, like file storage (SMB) or FTP, or WEB with large pages, i would go for QoS.

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