3com WX3000 Manuel D’Utilisation

 

2-2 

Figure 2-1 Typical DHCP relay agent application 

 

 

In the process of dynamic IP address assignment through the DHCP relay agent, the DHCP client and 

DHCP server interoperate with each other in a similar way as they do without the DHCP relay agent. 

The following sections only describe the forwarding process of the DHCP relay agent. For the 

interaction process of the packets, see 

. 

1)  After receiving the DHCP-DISCOVER or DHCP-REQUEST broadcast from the client, the network 

device providing the DHCP relay agent function unicasts the message to the designated DHCP 

server based on the configuration.  

2)  The DHCP server selects an IP address and other parameters and sends the configuration 

information to the DHCP relay agent that relays the information to the client (the sending mode is 

decided by the flag filed in the client’s DHCP-DISCOVER packet, refer to 

 for 

details). 

Option 82 is the relay agent information option in the DHCP message. It records the location information 

of the DHCP client. With this option, the administrator can locate the DHCP client to further implement 

security control and accounting. The Option 82 supporting server can also use such information to 

define individual assignment policies of IP address and other parameters for the clients. 

Option 82 involves at most 255 sub-options. If Option 82 is defined, at least one sub-option must be 

defined. Currently the DHCP relay agent supports two sub-options: sub-option 1 (circuit ID sub-option) 

and sub-option 2 (remote ID sub-option). 

Option 82 has no unified definition in RFC 3046. Its padding information varies with vendors. Currently, 

the device that operates as a DHCP relay agent supports the extended padding format of Option 82 

sub-options. By default, the sub-options of Option 82 are padded as follows, as shown in 

 and 

. (The content in brackets is the fixed value of each field.) 

z 

Sub-option 1: Padded with the port index (smaller than the physical port number by 1) and VLAN ID 

of the port that received the client’s request. 

z 

Sub-option 2: Padded with the bridge MAC address of the DHCP relay agent device that received 

the client’s request.