3Com WX3000 operation manual Dhcp Relay Agent Support for Option, Introduction to Option

Figure 2-1Typical 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 Obtaining IP Addresses Dynamically.

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 DHCP Packet Format for details).

DHCP Relay Agent Support for Option 82

Introduction to Option 82

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).

Padding content of Option 82

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 Figure 2-2 and Figure 2-3. (The content in brackets is the fixed value of each field.)

z

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.

Sub-option 2: Padded with the bridge MAC address of the DHCP relay agent device that received the client’s request.

2-2