In many cases, BOOTP clients and their associated BOOTP server(s) do not reside on the same IP (sub)network. In such cases, a third-party agent is required to transfer BOOTP messages between clients and servers. Such an agent was originally referred to as a BOOTP forwarding agent. However, to avoid confusion with the IP forwarding function of a router, the name BOOTP relay agent has been adopted instead.
A BOOTP relay agent performs a task that is distinct from a router's normal IP forwarding function. While a router normally switches IP datagrams between networks more-or-less transparently, a BOOTP relay agent may more properly be thought to receive BOOTP messages as a final destination and then generate new BOOTP messages as a result. One should resist the notion of simply forwarding a BOOTP message straight through like a regular packet.
This relay-agent functionality is most conveniently located in the routers that interconnect the clients and servers (although it may alternatively be located in a host that is directly connected to the client (sub)net).
A router MAY provide BOOTP relay-agent capability. If it does, it MUST conform to the specifications in [APPL:3].
Section [5.2.3] discussed the circumstances under which a packet is delivered locally (to the router). All locally delivered UDP messages whose UDP destination port number is BOOTPS (67) are considered for special processing by the router's logical BOOTP relay agent.
Sections [220.127.116.11] and [5.3.7] discussed invalid IP source addresses. According to these rules, a router must not forward any received datagram whose IP source address is 0.0.0.0. However, routers that support a BOOTP relay agent MUST accept for local delivery to the relay agent BOOTREQUEST messages whose IP source address is 0.0.0.0.