DHCP Relay and BOOTP Relay Overview

The DHCP relay feature relays a request from a remote client to a DHCP server for an IP address. When the router receives a DHCP request from an IP client, it forwards the request to the DHCP server and passes the response back to the IP client.

Configuring DHCP relay also enables bootstrap protocol (BOOTP) relay. The router relays any BOOTP requests it receives to the same set of servers that you configured for DHCP relay. A DHCP server can respond to the BOOTP request only if it is also a BOOTP server. The router relays any BOOTP responses it receives to the originator of the BOOTP request. If you do not configure DHCP relay, then BOOTP relay is disabled.

The router must wait for an acknowledgment from the DHCP server that the assigned address has been accepted. The IP client must accept an IP address from one of the servers. When the DHCP server sends an acknowledgment message back to the DHCP client via the router, the router updates its routing table with the IP address of the client.

If a DHCP relay request is received on an unnumbered interface, the router determines the loopback address for that interface and passes that IP address to the server.

DHCP carries other important configuration parameters, such as the subnet mask, default router, and DNS server. You can also use the DHCP relay agent information option (option 82) to add information to the DHCP packets sent to DHCP servers—the additional information, in the form of suboptions to the option 82 value, helps you to manage the IP address and service level assignments granted to your subscribers. For example, you can add the E Series hostname or the virtual router name to the front of the Agent Circuit ID suboption (suboption 1) of the DHCP relay agent information option (option 82). See Configuring Relay Agent Information Option (Option 82) Suboption Values.