Voice Vlan?

Mr Big

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!

Danman32

A little beyond the CCNA scope. But working with the VLAN 802.1Q, I believe you treat the phone like a switch, where you have trunking on the port the switch is connected to. The phone itself would use one VLAN, and the PC would use another, or the native VLAN. Or the other way around.

Remember, you can connect non-trunk devices to a trunk port, and it will belong to and only communicate with the native (default) VLAN, which is usually VLAN 1.

EdTheLad

Basically it all comes down to prioritizing the voice traffic above data traffic.In order to prioritize traffic at layer 2 you need to assign a CoS, the CoS field is not available in a regular ethernet frame, so to support QoS you need to use tagging.The 802.1p field in the 802.1q is a 3 bit field providing a priority ranging from 0 to 7 with 7 the highest and 0 the lowest.5 is used for Voice.Best effort will be 0 or 1 depending on you traffic planning.So if you have a phone connected to a switchport the phone will automatically assign a CoS 5 the tagged frame, when the frame arrives at the switchport since its trusted by default the switch will keep the CoS assignment.If on the otherhand you had an untrusted device sending frames with higher priority than it should be sending you can rewrite the CoS field to a lower value i.e. overriding the value.
For your setup ensure cdp is enabled on the port and that will be fine it will trust the CoS for the voice traffic, there is different setups you can have for the data coming from the pc on the same port, you can either trst or rewrite depending on what you want.

Mr Big

ed_the_lad wrote:

Basically it all comes down to prioritizing the voice traffic above data traffic.In order to prioritize traffic at layer 2 you need to assign a CoS, the CoS field is not available in a regular ethernet frame, so to support QoS you need to use tagging.The 802.1p field in the 802.1q is a 3 bit field providing a priority ranging from 0 to 7 with 7 the highest and 0 the lowest.5 is used for Voice.Best effort will be 0 or 1 depending on you traffic planning.So if you have a phone connected to a switchport the phone will automatically assign a CoS 5 the tagged frame, when the frame arrives at the switchport since its trusted by default the switch will keep the CoS assignment.If on the otherhand you had an untrusted device sending frames with higher priority than it should be sending you can rewrite the CoS field to a lower value i.e. overriding the value.
For your setup ensure cdp is enabled on the port and that will be fine it will trust the CoS for the voice traffic, there is different setups you can have for the data coming from the pc on the same port, you can either trst or rewrite depending on what you want.

Thanks for the helpful information, now when I configure ports to carry voice traffic there are 2 options - carry voice traffic in 802.1Q frames or carry voice traffic in 802.1P Priority Tagged Frames. Which is better? The difference between the two is:

802.1Q instructs the Cisco IP phone to forward all voice traffic through the specified VLAN. By default, the Cisco IP phone forwards the voice traffic with an 802.1Q priority of 5.

Sample Config -

 configure terminal
 Enter global configuration mode.
 
 interface interface-id
 Enter interface configuration mode, and enter the port to be configured.
 
 switchport voice vlan vlan-id
 Instruct the Cisco IP phone to forward all voice traffic through the specified VLAN. By default, the Cisco IP phone forwards the voice traffic with an 802.1Q priority of 5.

Valid VLAN IDs are from 1 to 4094 when the enhanced software image is installed and 1 to 1001 when the standard software image is installed. Do not enter leading zeros.
 
 end
 Return to privileged EXEC mode.
 

802.1P instructs the switch port to use 802.1P priority tagging for voice traffic and to use the default native VLAN to carry all traffic. By default, the Cisco IP phone forwards the voice traffic with an 802.1P priority of 5.

Sample config:

 configure terminal
 Enter global configuration mode.
 

 interface interface-id
 Enter interface configuration mode, and enter the port to be configured.
 

 switchport voice vlan dot1p 
 Instruct the switch port to use 802.1P priority tagging for voice traffic and to use the default native VLAN to carry all traffic. By default, the Cisco IP phone forwards the voice traffic with an 802.1P priority of 5.
 

 end
 Return to privileged EXEC mode.
 

EdTheLad

I'm only familiar with 802.1Q, with this method you configure a specific voice vlan id, this id is then sent to the phone via cdp and all traffic originating from the phone will have this tag and a priority of 5.All data traffic will use the native vlan.
My understanding of 802.1p is that you dont assign a specific voice vlan so all traffic data and voice share the same vlan id, i.e. the native vlan.
I assume 802.1Q is better to use as it gives the ability of an extra vlan and hence a larger ip address range with more flexibility.