User's Manual

User’s Manual of MGSW-MGSD Series
4.2.9 DHCP Relay
Configure DHCP Relay on this page. DHCP Relay is used to forward and to transfer DHCP messages between the clients and
the server when they are not on the same subnet domain.
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 representing the VLAN ID. The parameter of "module_id" is the third byte for the module ID (in
standalone switch it always equals 0, in stackable switch it means switch ID). The parameter of "port_no" is the fourth byte and it
means the port number.
The Remote ID is 6 bytes in length, and the value is equal the DHCP relay agent’s MAC address. The DHCP Relay
Configuration screen in Figure 4-2-12 appears.
Figure 4-2-12: DHCP Relay
Configuration Page Screenshot
The page includes the following fields:
Object Description
 Relay Mode
Indicates the DHCP relay mode operation. Possible modes are:
 Enabled: Enable DHCP relay mode operation. When enable DHCP relay
mode operation, the agent forward and to transfer DHCP messages between
the clients and the server when they are not on the same subnet domain.
And the DHCP broadcast message won't flood for security considered.
 Disabled: Disable DHCP relay mode operation.
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