voice vlans and data on same port

Unanswered Question
Apr 4th, 2008

Hi all, can anyone tell me how an ip phone gets its dhcp address when a pc and phone are in the same port but using voice vlans. am i right in saying that if the phone tags the packets with a cos tag it will go in the voip vlan automatically? OR does the phone traffic for dhcp first get sent through the normal data vlan and then the dhcp looks at a certain parameter to identify that its a phone and then send back dhcp with vlan id in the packet ?

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
Willem de Groot Fri, 04/04/2008 - 05:14

Hi Carl

I use Nortel IP-Phones, on a 3650E PoE.

These phones try to get a DHCP-Address from the Data-Vlan (Vlan 101). My Windows DHCP Server has a kind of a redirect Scope option in the Vlan 101 Scope

"191 Nortel VoIP Vlan Redirect", "VLAN-A:116"

Telling the phone to use Voice Vlan 116.

On DHCP Server, Vlan 116 there are some special Scope Options for the Nortel Phone like

"043 Vendor Specific Info", "Nortel-i2004-A"


"144 Nortel-i2004-A", "Nortel-i2004-A,,001,005;,001,005."

To tell the phone to use Server at IP, port 4100 and some retry's

Hope it will help you a bit.


thotsaphon Sat, 04/05/2008 - 14:01

Hi carl,

IP phone just want to know what is devices connecting to IP phone. Thing is how does it mark packets from pcs/servers. It should igore qos on header in packets of desktops/servers when you trust them. Another option is you can override ip precedence/Tos when you want to. It depends on what is your requirement. I'd rather mark the packets that I realize those packets are critical.

One thing you should very carefully think about is QOS or packets management should take into account when you want to do something.

In your case, It should be fine if you want to mark/remark packets before sending to the switch. It makes you to be comfortable when you want to do congestion management on a router/switch.

As you asked for,Tag comes into play to clarify what vlans that those packets should use when it want to communicate to other hots.

TAG just let you know how to modify packets before sending them out. If you didn't do any things about qos then the switch should mark/ornot the packets If you want to.



simon.birtles Sat, 04/05/2008 - 14:05

To add to Justins correct comment, the switch also sends a CDP frame to the Cisco IP Phone to inform it of the VLANID the tagged frame must use to match the voice vlan as confgured on the switch. This way you can use any VLAN for voice and zero touch on the IP Phone.


Justin Brenton Sat, 04/05/2008 - 16:09

Good point Simon, I forgot to add that.

You must also have a dhcp server on each vlan or use a dhcp helper address, a dhcp helper address is basically when you want to use a dhcp server that's on another subnet/network.



msooley Sat, 04/05/2008 - 17:49

you would use the voice vlan for voice and you specify the native vlan on the interface which is where the PC traffic will be directed.


This Discussion