VoIP Over Ethernet with QoS

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Nov 5th, 2007
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Hello All,

I am Jay Kishan. I work in an organization as a Network Associate. We are using VoIP to communicate with our remote office branchs. There is a little question that i need to make. How can we apply advance QoS on the ethernet interface of the router through which the remote sites are connected. We were using serial interface for some time for the connectivity and with that MLP was working fine for advance VoIP QoS. But with ethernet MLP doesnt work and therefore we are facing problems with voice qualirt. Please suggest some thing. Thanks in advance.


Jay Kishan

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paolo bevilacqua Tue, 11/06/2007 - 03:37
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Usually you do not need to configure any QoS on ethernet, because it's a fast media. Please describe in detail how you are connecting the offices, and what problems are you encountering.

Hi Paolo,

I believe I got the point Jay is talking about as I'm facing a similar issue.

In summary, we've around 12 sites connected via a provider MPLS cloud (check the attachment). The resources are centralised in one main site which has the biggest throughput -acting logically as a hub site. Its current link size is 50Mb connected to the MPLS cloud via a "Fast Ethernet" interface.

Also there's an output policy map (Gold-RT(15%, gold10%, silver35% and bronze leftover%), but the Main Site is bursting to the remote sites more than they can handle -as each class has a bigger bandwidth than the remote sites classes- ending up by the provider dropping the traffic at the remote site edge. All remote sites have QoS deployed -for output traffic, using similar classes of the Main Site-, making sure that each class is bursting a specific amount of data.

How can we limit the Main Site so it doesn't send more traffic than the remote sites can handle (it would be straight forward if we were using sub-interfaces) and at the same time making sure that each class -at the hub site- doesn't oversubscribe its limit as the sum of bandwidth for all sites is slightly bigger than 50 Mb. .

To answer the first point I'm thinking of using hierarchical traffic policy. In other words create one service policy with 11 classes (the number of sites) and under each class apply another service policy where we match on the destination of the subnets to that site and use MQC to prioritise different type of traffic:

interface FastEthernet 0/1

service-policy output QOS-OUT

policy-map QOS-OUT

class Site01

service-policy output Site01

class Site02

service-policy output Site02




class site11

service-policy output site11


Class Site01

match access-group name Site01-Subnets

The extended ACL Site01-Subnets will match “any” as source with destination being all Site01 subnets

And where “service-policy output Site01” is a mirror of the service policy applied on output direction at Site01 MPLS router

But then how can I respond to the other challenge? That's the total Main Site bandwidth is less than the sum of all sites bandwidth! Then how can I limit the sum of all classes -of different service policies- of one type to a certain percentage in Main Site?

Is this achievable or do you think we need a dedicated hardware to achieve this design (packeteer or similar technology)

I saw a remarkable post by Martin: http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Unified%20Communications%20and%20Video&topic=Video%20over%20IP&CommCmd=MB%3Fcmd%3Dpass_through%26location%3Doutline%40%5E1%40%40.2cbe72af/0#selected_message

but unfortunately it doesn't quite answer my situation.

Thanks in advance


jaykishan Wed, 11/07/2007 - 19:41
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Hi Paolo and Daniel,

Thanks for you help. But let me describe the situation in a bit detail. We are connected to all of our remote sites through VSAT's iDirect technology. In this setup our router gets connected to the iDirect equipment through ethernet. But the problematic thing is that the link's speed is of 64-128 KB only. And on that little link all of the services such as voice, email, fax, SAP etc work. If we could apply Advance QoS on that ethernet port, things will go fine. Due to company's some stupid policy we cant increase the bandwidth, so we will have to stick with the Advance QoS solution in any case. Please advice a workable solution. I was looking for the solution and saw something like RSVP. Do you think this will work in my situation. And if yes please advice some example configuration and a little more details that i need to look into. Thanks in advance.


Jay Kishan

paolo bevilacqua Thu, 11/08/2007 - 01:54
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Hi Jay,

what you need is a service-policy with shaping and nested LLQ. Example:

class-map match-all voice

match dscp ef

policy-map priority

class voice

priority 25

policy-map shaping

shape average 128000

service-policy priority

interface ethernet X/Y

service-policy output shaping

What this does, is first to shape all output on ethernet to 128Kbps that is you supposed uplink speed (actual speed can be even lower), the within this constrain, voice packets (that normally have DSCP set to EF), will be given absolute priority, up to a maximum of 25 Kbps (enough for a single compressed voice call in g.729).

You can define other service-polices and classes, apply them under policy map "shaping".

If you have voice going over the satellite, I suggest you investigate the possibility of using the iLBC codec that is the only one specifically designed for packet networks where delay/jitter and packet loss are present.

Hope this helps, please rate post if it does!

jaykishan Thu, 11/08/2007 - 21:16
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Hi Paolo,

Thank you very much for your help. I havent yet tried what you have suggested because i am out of my office for some time. But i will get back to you if some thing goes wrong. Thanks again.


Jay Kishan


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