3com 8807 User Guide
698
C
HAPTER
57: DHCP C
ONFIGURATION
Configuring DHCP
Relay
Relay
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.
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 162 illustrates a typical DHCP Relay application.
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 162 illustrates a typical DHCP Relay application.
Figure 162 Network diagram for DHCP Relay
The dynamic host configuration procedure with DHCP relay is as follows:
■
A DHCP client broadcasts configuration request packet in the local network
when it starts up and initializes the configuration.
when it starts up and initializes the configuration.
■
If a DHCP server exists in the network, it processes the configuration request
packet directly without the help of a DHCP Relay.
packet directly without the help of a DHCP Relay.
■
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.
Relay in the network appropriately processes the configuration request packet
and forwards it to a specified DHCP server located in another network.
■
After receiving the packet, the DHCP server generates configuration
information accordingly and sends it to the DHCP client through the DHCP
Relay to complete the dynamic configuration of the DHCP client.
information accordingly and sends it to the DHCP client through the DHCP
Relay to complete the dynamic configuration of the DHCP client.
Ethernet
Internet
DHCP client
DHCP client
DHCP client
DHCP client
Sw itch ( DHCP Relay)
DHCP Server
Ethernet
Internet
DHCP client
DHCP client
DHCP client
DHCP client
Sw itch ( DHCP Relay)
DHCP Server