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NS3503-16P-4C User Manual
when assigning IP addresses, or to set other services or policies for clients. It is also an effective tool in
preventing malicious network attacks from attached clients on DHCP services, such as IP Spoofing,
Client Identifier Spoofing, MAC Address Spoofing, and Address Exhaustion.
The DHCP option 82 enables a DHCP relay agent to insert specific information into a DHCP request
packets when forwarding client DHCP packets to a DHCP server and remove the specific information
from a DHCP reply packets when forwarding server DHCP packets to a DHCP client. The DHCP server
can use this information to implement IP address or other assignment policies. Specifically the option
works by setting two sub-options:
Circuit ID (option 1)
Remote ID (option2)
The Circuit ID sub-option is supposed to include information specific to which circuit the request came in
on.
The Remote ID sub-option was designed to carry information relating to the remote host end of the
circuit.
The definition of Circuit ID in the switch is 4 bytes in length and the format is "vlan_id" "module_id"
"port_no". The parameter of "vlan_id" is the first two bytes represent the VLAN ID. The parameter of
"module_id" is the third byte for the module ID (in standalone switch it always equal 0, in switch it means
switch ID). The parameter of "port_no" is the fourth byte and it means the port number.
After enabling DHCP snooping, the switch will monitor all the DHCP messages and implement software
transmission. The DHCP Rate Limit Setting and Config screens in Figure 4-9-44 and Figure 4-9-45
appear.
Figure 4-9-44 Option82 Global Setting Page Screenshot
The page includes the following fields:
Object
Description
State
Set the option2 (remote ID option) content of option 82 added by DHCP
request packets.
Default means the default VLAN MAC format.
User-Define
means the remote-id content of option 82 specified by users
Buttons
: Click to apply changes.