1-2
Figure 1-1 Network diagram for IP phones

DHCP

Server1

DHCP

Server2

Call

agent

IP Phone

②①③
As shown in Figure 1-1, the IP phone needs to work in conjunction with the DHCP server and the NCP
to establish a path for voice data transmission. An IP phone goes through the following three phases to
become capable of transmitting voice data.
1) After the IP phone is powered on, it sends an untagged DHCP request message containing four
special requests in the Option 184 field besides the request for an IP address. The message is
broadcast in the default VLAN of the receiving port. After receiving the DHCP request message,
DHCP Server1, which resides in the default VLAN of the port receiving the message, responds as
follows:
z If DHCP Server1 does not support Option 184, it returns the IP address assigned to the IP phone
but ignores the other four special requests in the Option 184 field. Without information about voice
VLAN, the IP phone can only send untagged packets in the default VLAN of the port the IP phone is
connected to. In this case, you need to manually configure the default VLAN of the port as a voice
VLAN.
In cases where an IP phone obtains an IP address from a DHCP server that does not support Option
184, the IP phone directly communicates through the gateway after it obtains an IP address. It does not
go through step 2 and step 3 described below.
z If DHCP Server1 supports Option 184, it returns the IP address assigned to the IP phone, the IP
address of the NCP, the voice VLAN ID, and so on.
2) On acquiring the voice VLAN ID from DHCP Server1, the IP phone ignores the IP address
assigned by DHCP Server1 and sends a new DHCP request message carrying the voice VLAN tag
to the voice VLAN. After receiving the DHCP request, DHCP Server2 residing in the voice VLAN
assigns a new IP address to the IP phone and sends a tagged response message to the IP phone.
After the IP phone receives the tagged response message, it sends voice data packets tagged with
the voice VLAN tag. In this case, the port on the device connecting to the IP phone must be
configured to allow packets tagged with the voice VLAN tag to pass.