PLANET WGSW-50040 - Manual (Page 151)

20-8
Len: the number of bytes in Sub-option Value, not including the two bytes in SubOpt segment and Len
segment.
20.1.2 option 82 Working Mechanism
Figure 20-1 DHCP option 82 flow chart
If the DHCP SNOOPING 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 SNOOPING will add the option 82 to the end of the request message it receives, and perform layer
2 forwarding. 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). The sub-option 2 of option
82(Remote ID) is the CPU MAC address of the switch.
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 SNOOPING.
4）DHCP SNOOPING will peel the option 82 information from the replay message sent by DHCP server, then
the message with DHCP configuration information to perform layer 2 forwarding.
20.2 DHCP Snooping option 82 Configuration Task List
1． Enable DHCP SNOOPING
2． Enable DHCP Snooping binding function
DHCP Server
DHCP Client
DHCP Relay Agent
DHCP Request DHCP Request Option82
Option82 DHCP Reply
DHCP Reply