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:
The DHCP Relay Information Option Remote ID (RID) is the access node identifier - a string used to identify the switch to the DHCP server.
The DHCP Relay Information Option Fields are the Option 82 circuit identification fields (CID - including VLAN ID, stack unit, and port). These fields identify the requesting device by indicating the interface through which the relay agent received the request.
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.
Command Usage
You must specify the IP address for at least one active DHCP server. Otherwise, the switch's DHCP relay agent will not be able forward client requests to a DHCP server. Up to five DHCP servers can be specified in order of preference.
If any of the specified DHCP server addresses are not located in the same network segment with this switch, specify the default router through which this switch can reach other IP subnetworks.
DHCP Snooping Information Option 82 and DHCP Relay Information Option 82 cannot both be enabled at the same time.
Command Parameters
Enable DHCP Option 82 information relay. (Default: Disabled)
Specifies how to handle client requests which already contain DHCP Option 82 information:
Floods the original request packet onto the VLAN that received it instead of relaying it. (This is the default.)
Retains the Option 82 information in the client request, inserts the relay agent's address, and unicasts the packet to the DHCP server.
Replaces the Option 82 information circuit-id and remote-id fields in the client's request with information provided by the relay agent itself, inserts the relay agent's address, and unicasts the packet to the DHCP server.
Specifies whether or not to use the sub-type and sub-length fields in the circuit-ID (CID) and remote-ID (RID) in Option 82 information. (Default: Included)
Addresses of DHCP servers or relay servers to be used by the switch’s DHCP relay agent in order of preference.