Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 311
BOOTP Relay Agents (or BOOTP Gateway)

The BOOTP operation also considers the possibility that clients and servers may not be on the same IP network or subnet (those network segments not separated by a router). Therefore, some kind of relay agent is needed to forward the BOOTP messages. This type of service is most commonly found as part of the router function; however, it is more than a router simply receiving a BOOTP message and forwarding it on to the next segment. Basically, the router receives the message, processes it as if the message was addressed to it, and then sends out a new BOOTP message to the appropriate forwarded port.

The forwarding (scope) of a BOOTP or DHCP packet can be limited by configuring the router to limit the scope. Each router increments the TTL field. If the received packet has its limit already set in the TTL field, the packet will be discarded. Also, the router must be configured as to which ports the router should forward this packet on. It is not simply forwarded to one port. When the router forwards the packet on, if it is the first router to do so, it will place its address in the Giaddr field of the packet. It knows that it is the first router because this field is set to 0.0.0.0. When a router forwards the packet, it forwards it set to Broadcast.

Cisco enables this feature as IP helper-address. This feature allows for more than just BOOTP packets to be forwarded across the router. Basically, any UDP broadcast address can be forwarded across the router.