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New Member

QoS implementation question.

I have a QoS question.

How do you prioritize voice traffic?

In my environment I have multiple VLANs and a switching fabric. I would like to start using IP telephony with IP phones ofcourse. Do I need to add a new VLAN? Based on what do I need to priotrize traffic in my switching fabric, and how?

In general how do I do it?

Thanks a bunch,

New Member

Re: QoS implementation question.

And the obvious answer will be: IT DEPENDS.

In Depends on:

• system architecture

• Network design

• Traffic types

• … Do you really want me to continue this list?

The most important advice will be:

• It is recommended to use separate VLAN for Voice

• Deploy QoS as closer to Voice sources as possible

• Use “nasty” queuing like LLQ for Voice traffic prioritization

• Do not save on Router IOS and memory: UPGRADE AND UPLOAD THEM

• Be careful with WAN bandwidth usage calculations

• Always control CAC: Call Admission Control over WAN

• Do not relay on bursts above CIR - use always rule max bursts = CIR

• Prioritize accordingly the following traffic types:

o Voice channels (UDP streams): IP Prec 5 - IP ToS(for WAN mainly)/CoS (for LAN mainly)

o Voice signaling: IP Prec 3 (H.225, H.245, TCP 1720, 2000, 2001, 2002)

• Classify, Mark, Queue

• Conduct budgetary delay calculations considering all variable on LAN/WAN

• Conduct traffic analysis and understand what it is traveling over your WAN

This is just a small introduction into QoS concepts.

Hope this will help you,

Good luck


New Member

Re: QoS implementation question.

Thanks much for the answer. I was reading all that stuff abot QoS and voice over IP but I just want to know what works for me. I want to know an expert opinion.

Here's my network anotomy.

1- I want to implement VoIP in my LAN and LAN only.

2- Switch fabric of 2 core switches in the data center and 5 edge switches on each floor.

3- 7 VLANs, one for each floor and 2 for admin.

4- End users are 200 per VLAN and they run 100Mbps links.

5- each edge switch has a Gig uplink to the core switches.

6- Implementing a seperate VLAN for voice would be hard since I want to use hand sets with built in switch to use one wall jack for a PC and a hand set.

7- Based on what do I need to prioritize traffic and how?

8- Heart felt thank you for all of your answers.

9- Thank you

New Member

Re: QoS implementation question.

It is definitely recommended to deploy separate VLAN for Voice. Switche at the back of CISCO IP Handsets will support native data VLAN for computer connected at the back and will assign different internal queues for Data and Voice packets. So your LAN switches should support on the port basis 2 VLANs: Data VLAn as Native VLAN and AUX VLAN - voice VLAN.

If you run non-CISCO switches, consult vendor on this.

You need to face the challenge of enabling Voice VLAN on all your LAN switches.

Prioritization may be done using IP CoS 5 for voice bearer channel and IP CoS 3 for IP Telephony signaling. Just reassign traffic to different queues based on CoS values on the switches (to be honestly you do not have any other option if you run CISCO switches). IP Handset will built Voice Packets and set IP CoS accordingly.



New Member

Re: QoS implementation question.

So you are recommending enabling trunking on each and every switch port that I have to support handset and a computer on each port?! right?

New Member

Re: QoS implementation question.

If you are using CISCO switches it is not trunking.

Switch ports still belong to Native Data VLAN only. But there is auxiliary VLAN which should be enabled on every port so when IP Phone plugged in switch will recognize it and will start supporting in addition to Data VLAN also Voice VLAN.

In regards Trunking, make sure your Voice VLAN is advertised in switches where you have connected IP Phones, which I presume you will enable by default.

If you are using other than CISCO switches you need to consult vendor on support of Voice and Data VLANs on the same port.



New Member

Re: QoS implementation question.

The phones use 802.1q/p tagging to separate the Voice and Data traffic, so it is trunking between the switch in the phone and the uplink switch.

Also, the phones get the voice (aux) VLAN from the uplink switch using CDP, so non-Cisco switches are not a good idea.


New Member

Re: QoS implementation question.

I'd suggest deploying a voice vlan for a few reasons.

First is, it allows you to create a separate, more secure environment for your telephony devices. By segmenting the traffic you are limiting broadcast traffic, reducing the possibility of sniffing voice conversations, creating a different way to manage your address utilization for telephony, and protect against rogue DHCP servers.

Secondly, you create a clear trust boundary. By having a data and voice vlan deployed to each closet, you can blindly trust the voice vlan and do classification on the data vlan. It greatly simplifies a QoS deployment to have that clear demarc for each environment.

I would suggest you define a clear QoS architecture. Determine what types of traffic you wish to group together and what actions you want performed against them. For some enterprises it would be 3 classes, others choose 11 or 15. Determine the impact of adding additional classes (operational cost of deployment, technical limitation of 3 bits within CoS or MPLS EXP field, etc). Then create a design to achieve your architecture.

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