3Com Switch 4800G 24-Port Configuration Manual page 827

DHCP Snooping Support
for Option 82
n
Figure 251 Configure trusted ports in a cascaded network
DHCP client
Host A
DHCP snooping
Switch A
GE1/0/1
DHCP client
GE1/0/3
Host B
GE1/0/1
GE1/0/4
DHCP client
GE1/0/3
Host C
DHCP client
Host D
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 information, refer to "Relay agent option (Option 82)" on page 795.
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 Handling
message has... strategy
Option 82 Drop
Keep
Replace
no Option 82 -
-
The handling strategy and padding format for Option 82 on the DHCP-Snooping
device are the same as those on the relay agent.
GE1 /0/1
GE 1/0 /2 GE 1/0/4
DHCP snooping
GE1/0/3
GE 1/0 /2
DHCP snooping
Switch B
Padding
format The DHCP snooping device will...
Random Drop the message.
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.
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.
verbose Forward the message after adding the
Option 82 padded in verbose format.
DHCP Snooping Overview
DHCP server
SwitchD
GE 1/0 /1
GE1 /0/2
Switch C
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