3Com Switch 8800 Configuration Guide Chapter 13 DHCP Configuration
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13.4 Configuring DHCP Relay

13.4.1 Introduction to DHCP Relay

This is a world where networks are ever-growing in both size and complexity, and the
network configuration is getting more and more complex. As is often the case, the
number of hosts in a network exceeds that of the available IP addresses, and position
changes of hosts (when users carry their laptops from here to there, or move to a
wireless network) require reassigned new IP addresses. Dynamic host configuration
protocol (DHCP) is designed to accommodate this context. DHCP adopts client/server
model, where DHCP clients send requests to the DHCP server dynamically and the
DHCP server in turn returns corresponding configuration information according to the
policies configured for it.
Early implementations of DHCP only work when DHCP clients and DHCP servers are
in the same subnet. That is, they cannot work across networks. So, to implement
dynamic host configuration, you must deploy at least one DHCP server in each subnet,
and this is obviously uneconomical. DHCP Relay is designed to resolve this problem.
Through a DHCP relay, DHCP clients in a LAN can communicate with DHCP servers in
other subnets to acquire IP addresses. This enables DHCP clients of multiple networks
to share a common DHCP server and thus enables you to save your cost and perform
centralized administration. Figure 13-3 illustrates a typical DHCP Relay application.
Ethe rne t Inter net
DHCP clie nt
DHCP clientDHCP client
DHCP client
Switch ( DHCP Relay )
DHCP Ser ver
Ethe rne t Inter net
DHCP clie nt
DHCP clientDHCP client
DHCP client
Switch ( DHCP Relay )
DHCP Ser ver
Figure 13-3 Network diagram for DHCP Relay
The dynamic host configuration procedure with DHCP relay is as follows:
z A DHCP client broadcasts configuration request packet in the local network when
it starts up and initializes the configuration.
z If a DHCP server exists in the network, it processes the configuration request
packet directly without the help of a DHCP Relay.
z If no DHCP server exists in the network, the network device serving as a DHCP
Relay in the network appropriately processes the configuration request packet
and forwards it to a specified DHCP server located in another network.