ip dhcp relay information option server-override

Use this command to insert the Relay Agent Information option 82 sub-options link selection (5) and server identifier override (11) into the relay agent DHCP packet.

Syntax

Parameters

None.

Defaults

None.

Mode

Configuration command, Global configuration.

Configuration command, Interface configuration.

Usage

When forwarding DHCP requests from a local client to a remote DHCP server, the DHCP relay agent needs to include information about itself in order for the DHCP server to determine which pool of client addresses to pull the lease from. Including Option 82 sub-options in the DHCP relay information provides the required DHCP relay information.

The link selection sub-option is described in RFC 3527. The link-selection sub-option is used by any DHCP relay agent that desires to specify a subnet/link for a DHCP client request that it is relaying but needs the subnet/link specification to be different from the IP address the DHCP server should use when communicating with the relay agent. By default, the link selection sub-option contains the subnet of the inbound interface to which the client is connected. This default value can be changed with the ip dhcp relay information option link-selection command.

The server identifier override sub-option is described in RFC 5107. This sub-option allows the DHCP relay agent to specify a new value for the server ID option, which is inserted by the DHCP server in the reply packet. This allows the DHCP relay agent to act as the actual DHCP server so that subsequent requests from the client will come to the relay agent rather than to the DHCP server directly. The server identifier override sub-option contains the IP address on the relay agent that is accessible from the client. Using this information, the DHCP client sends all renew and release requests to the relay agent. The relay agent adds all of the appropriate sub-options and then forwards the request packets to the original DHCP server.

The “no” form of this command removes the sending of these options/sub-options in the DHCP relay information.