3com WX3000 User Manual

 

1-2 

Figure 1-1 Network diagram for IP phones 

DHCP 

Server1

DHCP 

Server2

Call 

agent

IP Phone

②

①

③

 

 

As shown in 

, 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.