voice and data in same port

Unanswered Question
May 3rd, 2008


I want to connect the IP phone to a switch and PC to the IP phone. If i specify the voice VLAN and data VLAN on the same port how will the phone take an IP from the voice VLAN if its dynamic DHCP configured on the phone.

I have this problem too.
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floydbass Sat, 05/03/2008 - 04:59

I may be incorrect here (and welcome the correction), but I don't believe you can have 1 switch port (mode access) on more than 1 VLAN. Only ports configured for trunking can pass data for multiple VLANs. So I don't think you would be able to use the phone pass-through in the way you're describing. To do this I believe you would have to have your voice and data on the same VLAN.

Again - I welcome the correction.

Giuseppe Larosa Sat, 05/03/2008 - 05:48


n theory you are right the definition of voice vlan and data vlan leads to a trunk port.

However, in modern switches supporting VoIP and IP phones you just need to define the voice vlan and the data vlan and an implicit 802.1Q trunk carrying this two VLANs is formed (you don't need the switchport mode trunk command, in some older switches instead this is a workaround).

Actually there are more options for tagging but the usage of full 802.1Q tags provides full QoS support.

The voice vlan needs to be associated with a L3 SVI vlan/ router interface|subif that is configured with the ip helper address for the dhcp server(s).

On the dhcp server the scope for the voice vlan subnet needs a tftp server policy that lists the call manager servers. The list is passed on the dhcp response to the phone that starts to talk with the CCM cluster.

Communication between the IP phone and the switch port allows to configure correctly the PC port in the data vlan.

CDP exchange allows also to tune the online power if it is supported and user.

I see this every day on job and it works : phones and PCs get their IP addresses via DHCP from different scopes/subnets on "access ports".

hope to help


Richard Burts Sat, 05/03/2008 - 07:56


In general you are correct that if a port is configured as an access port then it can have only a single VLAN on it. However there is an exception to this when you configure a port as an access port (its VLAN is the data VLAN) and under the port you configure voice-vlan. This allows a second VLAN to operate on that port. As Giuseppe points out this technically makes the port a trunk. But it can carry only the 2 VLANs and is not treated as a real trunk.



spiecomcisco Sun, 05/04/2008 - 10:05

hello, i have a 3560 catalyst. I configured my ports in order to have a phone and a computer on the same port. So my config is :

access vlan 40 (data) and voice vlan 1 (voix).

it works with 7960 phones but 7911 phones fail. 7911 phones stay on "configuring ip". When i plug my phone on a port with just the voice vlan configured,(mode acces vlan 1) it works.

has someone an idea? Some admin changed the number of the voice vlan and tell that it works. I can't try it because my installation is in production.


freakx123 Wed, 10/29/2008 - 07:33

we've the same problem. after a few minutes the phone shows configuring ip.

have you a solution for this problem?

burleyman Wed, 10/29/2008 - 07:43

Here is how we have out ports configured...

interface FastEthernet0/3

switchport access vlan 104

switchport voice vlan 204

no ip address

srr-queue bandwidth share 10 10 60 20

srr-queue bandwidth shape 10 0 0 0

mls qos trust device cisco-phone

mls qos trust cos

no mdix auto

auto qos voip cisco-phone

spanning-tree portfast

Hope that helps.


freakx123 Wed, 10/29/2008 - 08:09

looks like our config.

we're using ios 12.2(25)SEE?! is there a bug in that ios-version?

singhsaju Wed, 10/29/2008 - 08:19

I think the ip phone is taking up data vlan ip in your case(when you see configuring ip), check "show cdp neighbor detail" output and look for MAC address of the phone.

I think the problem is with phone firmware.

If thats the case,

1.just configure the port to which phone is connected as

"switch access vlan " .

2.shut down the port ,

3. no shut the port ,

4.the phone should come up with voice vlan , once it does ,

5.now change it back to data vlan "switch mode access vlan .

Now everthing should work as normal , i.e PC gets data vlan ip and phone gets voice vlan ip. I think phone updates its firmware through TFTP etc ,I have resolved similar cases.




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