DHCP Relay

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 condition, please make sure the switch configuration of VLAN interface IP address and PVID (Port VLAN ID) correctly.

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Object Description
Relay Mode Indicates the DHCP relay mode operation. Possible modes are:
  • Enabled: 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. The DHCP broadcast message will not be flooded for security considerations.
  • Disabled: Disable DHCP relay mode operation.
Relay Server Indicates the DHCP relay server IP address.
Relay Information Mode Indicates 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 characters are the module ID (in standalone device it is always 0; in stackable device it means the switch ID), and the last two characters are the port number.

For example, “00030108” means the DHCP message receive form VLAN ID 3, switch ID 1, port No 8. The option 82 remote ID value is equal the switch MAC address.

Possible modes are:

  • Enabled: Enable DHCP relay information mode operation. When DHCP relay information mode operation is enabled, the agent inserts specific information (option 82) into a DHCP message when forwarding to DHCP server and removes it from a DHCP message when transferring to DHCP client. It only works when DHCP relay operation mode is enabled.
  • Disabled: Disable DHCP relay information mode operation.
Relay Information Policy When DHCP relay information mode operation is enabled, if the agent receives a DHCP message that already contains relay agent information, it will enforce the policy. The Replace policy is invalid when relay information mode is disabled.

Possible policies are:

  • Replace: Replaces the original relay information when receiving a DHCP message that already contains it.
  • Keep: Keeps the original relay information when receiving a DHCP message that already contains it.
  • Drop: Drops the package when receiving a DHCP message that already contains relay information.
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