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.

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.