Lantronix SISPM1040-xxxx-L3 Web User Guide
33856 Rev. A https://www.lantronix.com/ 192
DHCP Relay
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) and 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 Circuit ID is 4 bytes long 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 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 agents MAC address.
DHCP Relay Configuration
A DHCP relay agent is used to forward and to transfer DHCP messages between the clients and the server when
they are not in the same subnet domain. It stores the incoming interface IP address in the GIADDR field of the
DHCP packet. The DHCP server can use the value of GIADDR field to determine the assigned subnet. For such a
condition, please make sure the switch configuration of VLAN interface IP address and PVID(Port VLAN ID)
correctly.
Relay Mode: Indicates the DHCP relay mode operation. Possible modes are:
on: Enable DHCP relay mode operation. When DHCP relay mode operation is enabled, the agent
forwards and transfers DHCP messages between the clients and the server when they are not in the
same subnet domain. And the DHCP broadcast message won't be flooded for security considerations.
off: Disable DHCP relay mode operation.
Relay Server: Enter the DHCP relay server IP address.
Relay Information Mode: Select the DHCP relay information mode option operation. The option 82 circuit ID
format as "[vlan_id][module_id][port_no]". The first four characters represent the VLAN ID, the fifth and sixth
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