H3C S7500 Series Ethernet Switches Operation Manual

Operation Manual – DHCP
H3C S7500 Series Ethernet Switches Chapter 4
DHCP Snooping Configuration
4-3
To do… Use the command… Remarks
Enter system view
system-view
—
Enable the DHCP
snooping function
dhcp-snooping
Required
By default, the DHCP
snooping function is
disabled
Enter Ethernet port view
interface interface-type
interface-number
—
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
 Note:
z DHCP relay agent and DHCP snooping cannot be enabled at the same time. If you
have enabled DHCP relay agent on the device, you will fail to enable DHCP
snooping.
z 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.
4.2 DHCP-Snooping Option 82
4.2.1 Overview of DHCP-Snooping Option 82
I. Introduction to DHCP Option 82
For details of option 82, see Option 82 Supporting.
II. 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.