PLANET XGS3-24040 - Manual (Page 230)

Chapter 31 DHCP Snooping Configuration
30-2
30.1.2 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.
30.2 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． Diagnose and maintain DHCP option 82.
DHCP Server
DHCP Client
DHCP Relay Agent
DHCP Request DHCP Request Option82
Option82 DHCP Reply
DHCP Reply