In a previous article we reviewed how to configure a Nortel Ethernet Routing Switch 5520 using ADAC/LLDP (802.1ab) so a Nortel Internet Telephone could discover the voice VLAN ID automatically without manually configuration of the phone.
If you don’t have a switch that supports ADAC/LLDP you can still utilize automatic VLAN assignment using DHCP. I’ve actually used this method (before Nortel supported LLDP) and it works well but can sometimes be difficult to troubleshoot.
You’ll need to configure DHCP option 191 (string) with the following syntax;
Where: “VLAN-A” Option 191 begins with this string for all Nortel IP phones.
“vvvv” The VLAN ID for the voice VLAN in decimal
Here’s an example if I were trying to assign the phones with a voice VLAN of 31;
There must be a colon (:) seperating the VLAN-A from the VLAN ID. The string must also end in a period. It may be necessary, depending on your DHCP server, to enclose the enter string in quotation marks.
How does it actually work?
With the phone and DHCP server configured properly here’s how it will work.
- The phone will boot up and make a DHCP request for option 191 in the Default VLAN of the port connecting the phone.
- If the phone receives a response to it’s request it will issue a DHCP Release of the address it received in Step 1.
- The phone will make a second DHCP request in the VLAN that was returned in option 191. The phone will be requesting DHCP option 128 from the DHCP server, this will include the Call Server information. (Note: if you use a sniffer you will see that the DHCP packets will have an 802.1q header with the appropriate VLAN ID)
- The phone will connect to the Call Server specified in DHCP option 128 and will prompt the user for the Node ID and TN information.
With all that said we did leave out one very important piece of the pie… the network switch configuration. You’ll need to configure the VLAN and QoS settings manually depending on the switch vendor.