Voice VLAN?

[MrBig80]

I have been reading up on my Cat 2950 and saw that I can configure voice vlan's for my network. I am going to be rearanging my network where the 7960 IP phones will connect to my switch, and the PC will connect to my phone (one of the features recommended in the literature), and the phone can carry mixed traffic.
Not sure if anyone in here has extensive experience in voice, but, There are different ways to configure traffic.

You can configure the port to carry voice traffic in one of these ways:

Configuring Ports to Carry Voice Traffic in 802.1Q Frames

Configuring Ports to Carry Voice Traffic in 802.1P Priority Tagged Frames


You can configure the IP phone to carry data traffic in one of these ways:

Overriding the CoS Priority of Incoming Data Frames

Configuring the IP Phone to Trust the CoS Priority of Incoming Data Frames

Does anyone explain to me what each of these do, and what should be recommended for what I am doing?

Reference -

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat2950/1219ea1/scg/swvoip.htm

Thanks!

 

[VoIPGuyNSC]

As a general rule, this is the port configuration that I use for voice:

interface FastEthernet0/X
switchport trunk encapsulation dot1q
switchport mode trunk
switchport voice vlan *vlannumber*
switchport native vlan *vlannumber*
mls qos trust cos
mls qos trust device cisco-phone

This puts the switchport in trunk mode, where it will pass traffic on both the access and voice vlans. For QoS purposes, it classifies frames based on Class of Service instead of TOS, since TOS is adjustable by applications and untrustable and to make sure there aren't any rogue devices setting CoS values. Of course, QoS is a week long course in itself, but this is a very good place to start.

John Lever
Telecommunications
Richland School District Two

 

[ADB100]

I have to disagree with John here since configuring the port as a trunk is incorrect. With the configuration you have posted ALL VLANs will be allowed on the port, this could have a performance impact if you have high levels of broadcasts on your VLANs.

The correct way of configuring this on Catalyst 2950's and above is:

interface FastEthernet0/X
switchport mode access
switchport voice vlan *vlannumber*
switchport access vlan *vlannumber*


This is then a 'psuedo' trunk in that it allows the Native VLAN (the untagged Access VLAN) and only a single tagged VLAN (Voice VLAN). On older (2900XL/3500XL) switches you MUST configure the port as a trunk but even then you should manually prune off the unwanted VLANs

interface FastEthernet0/X
switchport trunk encapsulation dot1q
switchport mode trunk
switchport voice vlan *vlannumber*
switchport native vlan *vlannumber*
switchport trunk allowed vlan [native],[voice]

With regards to QoS the easiest configuration is as John has listed - trusting the device if its a Cisco IP Phone and trusting the received 802.1p CoS value. There are much more complex and better ways of configuring QoS though and I would advise you to have a read of the QoS Solution Reference Network Design (SRND) on Cisco's website which details configurations for the majority of Cisco switches.

HTH

Andy

 

[VoIPGuyNSC]

Good points Andy. Guess I'm too old school sometimes. You are absolutely right.

John Lever
Telecommunications
Richland School District Two

 

[ptdog]

Regarding qos, if the port will connect to an IP phone, use " auto qos voip trust". If it connects to another switch and needs to trust the other, use "auto qos voip trust". This command will also put in Portfast for you.

Auto qos makes things alittle simplier on the switch.

 

[ADB100]

The problem with AutoQoS is it is too generic, plus it doesn't configure the egress queueing optimally for Voice on the majority of switch platforms. I would suggest reading the QoS SRND on CCO since this is your best starting point when attempting to configure QoS.

Andy