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New Member

voip phones getting ip address

Hi all, can anyone tell me how my voip phones get addresses from my dhcp server, the pc plugs into the phone using untagged vlan, and the phone plugs into the switch, i have helper addresses setup on my switches for both vlans, my question is, how does the switch know which vlan the phone is in initally, do we need to set the vlan in the phone first ?

New Member

Re: voip phones getting ip address

It's good that you had the ip helper address setup in your switches already.

The switch knows which vlan the phone is in by this command "switchport voice vlan xxx".

Is your phone ip address in the same subnet as the pc?


Re: voip phones getting ip address

The phones send out a CDP to the switch it's connected to, requesting what VLAN it is supposed to be in (which is why on Cisco switches you have the "switchport voice vlan" command).

The switch responds, letting the phone know what vlan it is supposed to be in, then the phone receives DHCP addresses based on that.

If your phones are connected to a Cisco switch, the PC's traffic is tagged with the vlan that you specify with the "switchport access vlan" command; the vlan info is stripped or added when it gets to the phone.

Look at section 2-9. It'll tell you how the phones start up.