The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Network Configuration and Management Protocols (BOOTP, DHCP, SNMP and RMON)            9  Host Configuration and TCP/IP Host Configuration Protocols (BOOTP and DHCP)                 9  TCP/IP Bootstrap Protocol (BOOTP)

BOOTP Vendor-Specific Area and Vendor Information Extensions 1 2 3 4 TCP/IP Dynamic Host Configuration Protocol (DHCP)

The Function of BOOTP Relay Agents

To make this all work, then, we need something to act as an intermediary between the client and the server: a BOOTP relay agent. The job of a BOOTP relay agent is to sit on a physical network where BOOTP clients may be located and act as a proxy for the BOOTP server. The agent gets its name because it relays messages between the client and server, and thus enables them to be on different networks.

In practice, a BOOTP relay agent is not usually a separate piece of hardware. It's a software module that runs on an existing piece of hardware that performs other functions. It is common for BOOTP relay agent functionality to be implemented on an IP router. In that case, the router is acting both as a “regular” router and also playing the role of a BOOTP agent. The forwarding functions required of a BOOTP relay agent are distinct from the normal IP datagram forwarding tasks of a router (though there are certain similarities as we will see.)

Naturally, the placement of the client and server on different networks and the presence of a relay agent changes the normal request/reply process of BOOTP significantly. A couple of specific fields in the BOOTP message format are used to control the process. RFC 951 was rather vague in describing how this process works, so RFC 1542 described it in much more detail.

BOOTP Vendor-Specific Area and Vendor Information Extensions 1 2 3 4 TCP/IP Dynamic Host Configuration Protocol (DHCP)