How do IP phones know what VLAN to be in

Unanswered Question
Feb 8th, 2007

I was asked the other day about my Cisco VIOP phone system and was asked a question that I didn't know the answer to. How do 7960 phones know to be in the voice VLAN. Furthermore how do they know what VLAN to let the computer attached to it to be in? Is there something in the phone? Can anyone answer this question for me so that I might have a better understanding of the proceess? Thanks!

I have this problem too.
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mparekh Thu, 02/08/2007 - 18:23

Generally on the switch port you would configure a voice vlan(for voice traffic) and access vlan(for data traffic) on a port that an IP Phone will be connecting to. Using CDP the switch can detect if an IP Phone is connected to its port, if it is then it will instruct the IP Phone to send the voice traffic in the voice VLAN and the PC traffic in the data vlan.

Hope that clarifies your question.

icabrera Fri, 02/09/2007 - 03:38

Hi, only one thing more. Through cdp it's the way the phone it's informed about its vlan, so if the switch it's not from cisco you'll need to configure the voice vlan manually at the phone.


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Zin.Karzazi Fri, 02/09/2007 - 04:17

If you really want to understand the Proccess from A to Z, install Ethereal on a PC which is connected to an IP Phone. Unplug the Cable from the Phone, start Ethereal and plug the cable into the Phone. You ll see the whole process described here and on Cisco Docs, bit for bit (Vlan/DHCP/TFTP image...etc).

Zin-Elabidine Karzazi


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