What are the step to configure interface fa1 0/12 To in a voice VLAN that prioritizes voice traffic

Let’s see how we can configure a switchport connected to VoIP phone. Here is my setup for this post.

Here is the SPAN configuration.

First we will configure as a simple access vlan & see what’s  happen.

If you look at a packet capture in this scenario, you would see a CDP packets send by both Phone & Switch.

Here is the CDP information send by Switch.

Here is the information send by phone via CDP. As you can see phone will inform power requirement via CDP. Therefore it is very important to have CDP enable on these switch port where you connect VoIP phones (this applies to any cisco PoE devices like AP, Camera, etc)

Then Phone & PC get IP via DHCP on vlan 130 & start normal communication. Here is SCCP & RTP packets coming from 7965 phone in this scenario. Since switchport is access port no vlan-tag is coming in those frames.

Here is a packet coming from PC.

In the above method both Phone & PC would be on the same vlan. In best practice scenario you would like to put phones & PC in two different vlan. By using “switchport voice vlan x” command you can do this. In that scenario switchport is carry two different VLAN traffic even though we have not configured it as a trunk port.

As you can see below, switch will inform voice vlan  information to the phone via CDP. Also note that this time layer 2 vlan tagging is available in these frames

Here is a CDP packet coming from Phone is same as previous time.

Here is the SCCP packet coming from Phone this time. Note that it comes with layer2 vlan tag which include priority.

Here is the RTP traffic coming from the phone. You can see phone will set CoS value 5 for this RTP traffic in layer 2 header.

All traffic coming from PC will be on vlan 140 will be un-tagged (as Phone will only tagged it’s own traffic with layer 2 vlan)

But you can see from switch to Phone still traffic will be tagged on vlan 140.

From QoS perspective you wanted to trust priority set by phone for voice traffic. For PC traffic is “untrusted” in normal scenario you do not want to trust DSCP value of those packets. So best option is to trust CoS at the switchport. You can do this trust relationship conditionally  in order to end device directly connect to switchport & sending frame with layer 2 tag. So in this example as long as siwtch detect a Cisco-Phone via CDP it will trust CoS value set by that phone.

If you want to prioritize voice traffic (EF) over any other traffic, you have to enable priority-queue in 3750/3560/2960 switch platforms as it is not ON by default.

So final switchport configuration is looks like this.

You can verify switch port configured features by using “show interface x switchport” command.

Hope this is useful to understand switch port configuration to be done when it comes to VoIP phone connection.

Here is few reference talk about this voice vlan configuration.

1. Good Explanation of Voice Vlan
2. Switchport Voice Vlan – What does it do ?

Related Posts

1. 3750/3560/2960 Wired QoS
2. Who do you trust ? (DSCP or CoS)
3. QoS for H-REAP
4. Best Practice QoS Config
5.