3Com Switch 8800 Configuration Guide Chapter 13 DHCP Configuration13-2113.4 Configuring DHCP Relay13.4.1 Introduction to DHCP RelayThis is a world where networks are ever-growing in both size and complexity, and thenetwork configuration is getting more and more complex. As is often the case, thenumber of hosts in a network exceeds that of the available IP addresses, and positionchanges of hosts (when users carry their laptops from here to there, or move to awireless network) require reassigned new IP addresses. Dynamic host configurationprotocol (DHCP) is designed to accommodate this context. DHCP adopts client/servermodel, where DHCP clients send requests to the DHCP server dynamically and theDHCP server in turn returns corresponding configuration information according to thepolicies configured for it.Early implementations of DHCP only work when DHCP clients and DHCP servers arein the same subnet. That is, they cannot work across networks. So, to implementdynamic 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 inother subnets to acquire IP addresses. This enables DHCP clients of multiple networksto share a common DHCP server and thus enables you to save your cost and performcentralized administration. Figure 13-3 illustrates a typical DHCP Relay application.Ethernet InternetDHCP clientDHCP clientDHCP clientDHCP clientSw itch ( DHCP Relay)DHCP ServerEthernet InternetDHCP clientDHCP clientDHCP clientDHCP clientSw itch ( DHCP Relay)DHCP ServerFigure 13-3 Network diagram for DHCP RelayThe dynamic host configuration procedure with DHCP relay is as follows:z A DHCP client broadcasts configuration request packet in the local network whenit starts up and initializes the configuration.z If a DHCP server exists in the network, it processes the configuration requestpacket directly without the help of a DHCP Relay.z If no DHCP server exists in the network, the network device serving as a DHCPRelay in the network appropriately processes the configuration request packetand forwards it to a specified DHCP server located in another network.
