such as subnet mask, the IP address of the default
gateway, and DNS servers is the same for the whole
subnet, so these configuration parameters apply to the
whole subnet rather than to each end host device. In
some cases, the DHCP client and the server are located in
different subnets. In such a case, a DHCP relay agent can
be used to relay the DHCP packets between clients and
servers. Any host on the network can act as a relay agent,
but in most cases, the default router for the client subnet
acts as a DHCP relay agent. Forwarding of DHCP
messages between the clients and the servers by the relay
agents is different from regular routing and forwarding.
While regular forwarding is transparent for the
endpoints involved in the exchange of data, with DHCP
forwarding, DHCP relay agents receive inbound DHCP
messages on the client interface and generate new DHCP
messages on the interface connecting to the server. The
DHCP relay agent effectively becomes a man-in-the-
middle for the DHCP traffic between clients and servers.
Figure 18-1 illustrates DHCP relay agent functionality.
Figure 18-1 DHCP Relay Agent
DHCP operations fall into the following four phases (see
Figure 18-2):
Server discovery
Lease offer
Lease request
Lease acknowledgment