Voice VLAN

(Rene Molenaar) #1

This topic is to discuss the following lesson:

IOS command on VOIP tutorial

Hi ,

As i understand the IP PHONE working as a switch and separate traffic between voice and data and the connections between IP PHONE-COMPUTER and IP PHONE-VOICE(PHONE) are like access port and tagged with corresponding vlan that learning with cdp from the switch and then send to the trunk port . Is it wright ?
Please let me now



(Abhishek D) #3

Hi Rene,

So does that mean even though we configure ‘switch-port mode access’ , but for VOICE VLAN that same port works as trunk port because it carries 2 VLAN?


(Lazaros Agapides) #4

Hello Dionisis

Yes you are correct in your description. Any traffic coming from the PC through the phone will be untagged and will go to the access VLAN configured on the port of the switch. All voice traffic is tagged and will go to the voice vlan configured on the port of the switch.

Keep in mind that the voice vlan configuration not only configures the appropriate VLANs for data and voice, but also sends voice traffic with a QoS based on IEEE 802.1p CoS. In other words, a Quality of Service technique is automatically incorporated into the functionality of the voice VLAN.

I hope this has been helpful!


1 Like
(Lazaros Agapides) #5

Hello Abhishek

In the past, before the advent of the switchport voice vlan command you would have to configure a port as a trunk port that included both the voice and data VLANs. It was around 10 years ago when I was first learning about Cisco IP telephony where we would use such a configuration. Back then, connections to IP phones were literally trunks.

Today however, we use ports configured with voice VLANs. Such a port will have both the switchport mode access command as well as the switchport voice vlan command. So is this an access port or a trunk?

They are often referred to as “a special type of 802.1q trunk”. This makes sense since 802.1q tagging is included in the frames that are in the voice VLAN. However, Cisco has (more correctly in my opinion) referred to such a port as a @multi-VLAN access port."

Really, you can take your pick of what to call it, just realise that 802.1q tagging takes place on the voice frames and does not on the data frames.

I hope this has been helpful!


(aniket g) #7

Hi Rene,

How does switch differentiate between DATA traffic and VOICE traffic ? Consider IP phone is non-cisco

(Lazaros Agapides) #8

Hello Aniket

Let me quote Rene from his lesson:

This is the image he uses:

Behind the scenes, we have a trunk between our switch and IP phone. The port on the IP phone that connects to the computer is an access port. The IP phone will forward all traffic from the computer to the switch untagged, traffic from the IP phone itself will be tagged. The only two VLANs that are allowed though, are the access and voice VLAN.

So, all traffic to and from the phone will be tagged (voice traffic) and all traffic to and from the PC will not be tagged (data). For more information about tags and how they work, take a look at this lesson:

I hope this has been helpful!


(aniket g) #9

Thanks for the explanation Lazaros,
However I have seen a working scenario wherein DATA VLAN is 135 & VOICE VLAN is 137, also switch port is not TRUNK, Native VLAN is 1. So how could DATA Vlan traffic will go untagged as DATA Vlan being 135… Also under switchport we are giving commnad as "Switchport mode access " for DATA vlan, then how can we put this interface in Trunk mode? (as per my understanding switchport would be either access or trunk port)

(Lazaros Agapides) #10

Hello Aniket

I think you had the same post on another thread, but that’s ok, cause I can add something here.

If the native VLAN on a trunk is 1, then this VLAN will definitely function as the data VLAN in a scenario where a PC is connected to an IP phone connected to an access switch. VLAN 135 was not being piped through the phone to the connected PC in this particular instance. In order to answer more specifically, we’d need to see the actual configuration.

That’s the beauty of the voice VLAN configuration. It provides a third alternative to just access or trunk by allowing you to configure an access port but tag only voice packets so an IP phone can intercept them. See more about this in my reply to your other post:

I hope this has been helpful!


(David C) #11

I’m working on the Voice VLAN tutorial in one of the lessons - following the tutorial using packet tracer. Everything is working but one command is not. That is the show interfaces GigabitEthernet 0/1 trunk command. It has a marker ^ at trunk. Any ideas why this won’t work?


(Lazaros Agapides) #12

Hello David

I tried issuing the command you stated on packet tracer using the 3560-24PS switch and indeed it didn’t have it available. This could be because you’re using packet tracer. You can use the show interfaces trunk command instead to show you all the information about all trunks. Or you can use the show interfaces GigabitEthernet 0/1 switchport command to show info about the switchport configuration of that particular trunk.

Remember that Packet Tracer has a subset of all of the available commands on a real device. In order to get the same availability of commands, I suggest you use GNS3.

I hope this has been helpful!