
Operation Manual – DHCP
H3C S3610&S5510 Series Ethernet Switches
Chapter 5 DHCP Snooping Configuration
5-2
II. Ensuring DHCP clients to obtain IP addresses from valid DHCP servers
If there is an unauthorized DHCP server on a network, the DHCP clients may obtain
invalid IP addresses. With DHCP snooping, the ports of a device can be configured as
trusted or untrusted, ensuring the clients to obtain IP addresses from authorized DHCP
servers.
z
Trusted: A trusted port is connected to a valid DHCP server directly or indirectly. It
forwards DHCP messages normally, guaranteeing that DHCP clients can obtain
valid IP addresses.
z
Untrusted: An untrusted port is connected to an invalid DHCP server. The
DHCP-ACK or DHCP-OFFER packets received from the port are discarded,
preventing DHCP clients from receiving invalid IP addresses.
5.1.2 DHCP Snooping Support for Option 82
Option 82 records the location information of the DHCP client. The administrator can
locate the DHCP client to further implement security control and accounting. For more
Relay agent option (Option 82)
.
If DHCP snooping supports Option 82, it will handle a client’s request according to the
contents defined in Option 82, if any. The handling strategies are described in the table
below.
If a reply returned by the DHCP server contains Option 82, the DHCP snooping device
will remove the Option 82 before forwarding the reply to the client. If the reply contains
no Option 82, it forwards it directly.
If a client’s
requesting
message has…
Handling
strategy
Padding
format
The DHCP snooping device will…
Drop
Random
Drop the message.
Keep Random
Forward the message without
changing Option 82.
normal
Forward the message after replacing
the original Option 82 with the Option
82 padded in normal format.
Option 82
Replace
verbose
Forward the message after replacing
the original Option 82 with the Option
82 padded in verbose format.
—
normal
Forward the message after adding the
Option 82 padded in normal format.
no Option 82
—
verbose
Forward the message after adding the
Option 82 padded in verbose format.