User's Manual

Table Of Contents

31-2

31.1.2 option 82 Working Mechanism

DHCP option 82 flow chart

If the DHCP Relay Agent supports option 82, the DHCP client should go through the following four steps to

get its IP address from the DHCP server: discover, offer, select and acknowledge. The DHCP protocol follows

the procedure below:

1）DHCP client sends a request broadcast message while initializing. This request message does not have

option 82.

2）DHCP Relay Agent will add the option 82 to the end of the request message it receives, then relay and

forward the message to the DHCP server. By default, the sub-option 1 of option 82 (Circuit ID) is the

interface information of the switch connected to the DHCP client (VLAN name and physical port name),

but the users can configure the Circuit ID as they wish. The sub-option 2 of option 82(Remote ID) is the

MAC address of the DHCP relay device.

3）After receiving the DHCP request message, the DHCP server will allocate IP address and other information

for the client according to the information and preconfigured policy in the option segment of the message.

Then it will forward the reply message with DHCP configuration information and option 82 information to

DHCP Relay Agent.

4）DHCP Relay Agent will peel the option 82 information from the replay message sent by DHCP server, and

then forward the message with DHCP configuration information to the DHCP client.

31.2 DHCP option 82 Configuration Task List

1． Enabling the DHCP option 82 of the Relay Agent

2． Configure the DHCP option 82 attributes of the interface

3． Enable the DHCP option 82 of server

4． Configure DHCP option 82 default format of Relay Agent

5． Configure delimiter

6． Configure creation method of option82

7． Diagnose and maintain DHCP option 82

DHCP Server

DHCP Client

DHCP Relay Agent

DHCP Request DHCP Request

Option82

DHCP Reply