3Com Switch 7750 Configuration Guide Guide

DHCP-Snooping Option 82 589
n
■ DHCP relay and DHCP snooping cannot be enabled at the same time. If you
have enabled DHCP relay on the device, you will fail to enable DHCP snooping.
■ The dhcp-snooping trust command and the dhcp-snooping command must
be configured at the same time; otherwise DHCP packets may be dropped in
actual networking.
DHCP-Snooping
Option 82
Overview of
DHCP-Snooping Option
82
Introduction to DHCP Option 82
For details of option 82, see “Option 82 Supporting”.
Working mechanism of DHCP-Snooping option 82
The process in which a DHCP client obtains an IP addresses from a DHCP server
through DHCP-Snooping is the same as the process in which a DHCP client obtains
an IP address from a DHCP server directly. The process includes four phases: IP
lease request, IP lease offer, IP lease selection, and IP lease acknowledgement. This
section only introduces the working mechanism of DHCP-Snooping option 82. The
details are as follows:
1 When a DHCP client gets online, it broadcasts an IP address request message
across the network.
2 After receiving the broadcast message, the DHCP-Snooping-enabled switch
checks whether the message contains option 82 and processes it.
■ If the message contains option 82, the switch replaces the original option 82 in
the message with its own option 82 or keeps the original option 82, and then
broadcasts the request message.
■ If the request message does not contain option 82, the
DHCP-Snooping-enabled switch inserts option 82 into the message, and then
broadcast this message.
3 By now, the request message contains the number of the switch port connected
to the DHCP client, the VLAN to which the port belongs to, and the MAC address
of the DHCP-Snooping-enabled switch.
4 After receiving the DHCP request message broadcast by the
DHCP-Snooping-enabled device, the DHCP server records the information carried
by the options in the message, and then sends the message containing DHCP
configuration information and option 82 information to the
DHCP-Snooping-enabled device.
5 After receiving the returned message from the DHCP server, the
DHCP-Snooping-enabled switch checks the option 82 field in the message.
Set the port connected to a
DHCP server to a trusted port
dhcp-snooping trust
Required
By default, all ports of a
switch are untrusted ports
Table 460 Configure the DHCP snooping function
Operation Command Description