IP Service > DHCP > Relay

Use the IP Service > DHCP > Relay page to to configure DHCP relay service for attached host devices, including DHCP option 82 information. DHCP provides an option for sending information about its DHCP clients to the DHCP server (specifically, the interface on the relay server through which the DHCP client request was received). Also known as DHCP Relay Option 82, it allows compatible DHCP servers to use this information when assigning IP addresses, or to set other services or policies for clients.

Option 82 information contains information which can identify both the relay agent and the interface through which the DHCP request was received:

If DHCP relay is enabled, and this switch sees a DHCP client request, it inserts its own IP address into the request so that the DHCP server will know the subnet where the client is located. Depending on the selected frame format set for the remote-id, this information may specify the MAC address, IP address, or an arbitrary string for the requesting device (that is, the relay agent in this context).

By default, the relay agent also fills in the Option 82 circuit-id field with information indicating the local interface over which the switch received the DHCP client request, including the VLAN ID, stack unit, and port. This allows DHCP client-server exchange messages to be forwarded between the server and client without having to flood them onto the entire VLAN.

The switch then forwards the packet to the DHCP server. When the server receives the DHCP request, it allocates a free IP address for the DHCP client from its defined scope for the DHCP client's subnet, and sends a DHCP response back to the DHCP relay agent (i.e., this switch). This switch then passes the DHCP response received from the server to the client.

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