The use of VLAN has implications on DHCP if DHCP is being used for support of IP phones and or PCs. The table below details the available options when using a single port for PC and IP Phones on a VLAN enabled network.
DHCP Option
Description
None (Static addressing)
Manual configuration of each IP Phone
Separate DHCP Servers
Two PCs, one for each VLAN
Multihomed DHCP Server
A single PC with two NIC Cards; one for each VLAN
DHCP Relay
The option must be supported by the Ethernet switch
If using DHCP, when the IP phone starts it first makes a DHCP request without a VLAN tag.
If the DHCP reply contains a new VLAN setting as part of the SSON scope, the phones will release all its existing IP address and makes a new DHCP request using the newly supplied VLAN ID
If the IP Phone does not get a new VLAN ID, it will continue with the settings provided in the original DHCP reply
A VLAN ID can also be passed to a phone through the settings file that it loads. Again the IP phone will release all its existing IP parameters and then make a new DHCP request using the newly supplied VLAN ID.
In the example below, the when the IP phones receives a DHCP response from the DHCP server on the data VLAN, that response contains the VLAN ID of the voice VLAN. The phone then releases the original data VLAN settings it obtained and sends a new DHCP request to the voice VLAN.
Option
Data VLAN DHCP Settings
Voice VLAN DHCP Settings
IP Address
192.168.43.x
192.168.202.x
Mask
255.255.255.0
255.255.255.0
Router
192.168.43.1
192.168.202.1
SSON
Scope
L2Q=1, L2QVLAN=202,
VLANTEST=0
MCIPADD=192.168.202.1, MCPORT=1719,
HTTPSRVR=192.168.202.X VLANTEST=0
The VLANTEST parameter is the length of time the IP Phone should make DHCP requests in a VLAN (0 means unlimited time).
