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New Member

implementing voice vlans

Hi All,

I have inherited a network where ip phones were brought in. everything phone related is seimens equipment, the switch and the phones. the ip phone setup was done very badly. the phones pull an ip from the regular pc pool and are not on their own vlan. I have all cisco switches and routers and would like to clean things up.

my questions are:

How exactly do "voice vlans" work? do they only work with cisco phones? If I assign a port a voice vlan, and a ip phone is plugged in, how does the phone know that it is supposed to use the voice vlan and not the data vlan?




Re: implementing voice vlans

Voice VLANs work using CDP Cisco Discovery Protocol. You configure the voice vlan in the switch and the switch will pass on the voice VLAN information to the Cisco IP Phones via CDP.

Not sure if Siemens phones can talk CDP and even if they can talk CDP, can they get the VLAN info via CDP. You can do a quick check by issuing the "show cdp neighbor" command on the Cisco switch and see if it shows any of the phones that are connected.

Even if the Seimens phones do not understand CDP, you can still use two different VLANs for voice and data and make this work. Lets say if your existing data vlan is 100, create another vlan, say VLAN 200, and consider this to be your voice VLAN. Configure all the ports that are connected to the Siemens IP Phones to be in VLAN 200. The configuration would look similar to any data port. Now configure a new DHCP Scope for IP Phones (VLAN 200) and a default gateway and you should be good.

Eventhough in the configuration, it doesn't say that the phones belong to voice VLAN, they are still in a separate VLAN and that is all matters. Cisco uses the term Voice VLAN to define a dedicated VLAN for all voice traffic and also to let the phones know to use that VLAN via CDP. In this case, you are defining a dedicated VLAN for voice, but you are missing the propagation of the VLAN information to the phones.

Hope that helps.



New Member

Re: implementing voice vlans

Thanks for your reply. My concern with doing your config, is that the port is also a pc port so the port is not only pulling the vlan for the voice, but the pc will also pull a "voice vlan" (200) ip address as the phones act as switches, there are mainly only one port per office. see my dilemma?

New Member

Re: implementing voice vlans

any other help on this??

Re: implementing voice vlans


the idea is to have a trunk between IP phone and switch and the phone acts like an access switch for the PC. With Cisco phones everything is learned and then configured. automatically through CDP.

In your case you need to consider your Siemens phone manuals on how to setup VoIP/data separation.

So the switch would have a trunk config for the two VLANs - voip and data - and the telephone should insert PC frames into data VLAN and it`s own traffic and VoIP traffic into voip VLAN.

Additionally you need to manually enable QoS on the switch ports and in the telephones.

Hope this helps! please rate all posts.

Regards, Martin

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