308
Device Untrusted port
Trusted port disabled from
recordin
Trusted port enabled to
record bindin
Switch B
GigabitEthernet
1/0/3 and
GigabitEthernet
1/0/4
GigabitEthernet 1/0/1 GigabitEthernet 1/0/2
Switch C
GigabitEthernet
1/0/1
GigabitEthernet 1/0/3 and
GigabitEthernet 1/0/4
GigabitEthernet 1/0/2
DHCP snooping support for Option 82
Option 82 records the location information about the DHCP client so the administrator can locate the
DHCP client for security and accounting purposes. For more information, see "Option 82."
D
HCP snooping uses the same strategies as the DHCP relay agent to handle Option 82 for DHCP
request messages, as shown in Table 99.
If a response returned by the DHCP server contains Option 82,
DHCP snooping removes Option 82 before forwarding the response to the client. If the response contains
no Option 82, DHCP snooping forwards it directly.
Table 99 Handling strategy
If a DHCP re
device…
Option 82
Drop Drops the message.
Keep Forwards the message without changing Option 82.
Replace
Forwards the message after replacing the original
Option 82 with the Option 82 padded in normal
format.
No Option 82 N/A
Forwards the message after adding the Option 82
padded in normal format.
Recommended configuration procedure
Task Remarks
Enabling DHCP snooping
Required.
By default, DHCP snooping is disabled.
Configuring DHCP snooping
functions on an interface
Required.
Specify an interface as trusted and configure DHCP snooping to support
Option 82.
By default, an interface is untrusted and DHCP snooping does not support
Option 82.
IMPORTANT:
You need to specify the ports connected to the authorized DHCP servers as
trusted to make sure DHCP clients can obtain valid IP addresses. The trusted
port and the port connected to the DHCP client must be in the same VLAN.
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