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Web OS 10.0 Application Guide
DHCP Relay
Dynamic Host Configuration Protocol (DHCP) is a transport protocol that provides a frame-
work for automatically assigning IP addresses and configuration information to other IP hosts
or clients in a large TCP/IP network. Without DHCP, the IP address must be entered manually
for each network device. DHCP allows a network administrator to distribute IP addresses from
a central point and automatically send a new IP address when a device is connected to a differ-
ent place in the network.
DHCP is an extension of another network IP management protocol, Bootstrap Protocol
(BOOTP), with an additional capability of being able to dynamically allocate reusable network
addresses and configuration parameters for client operation.
Built on the client/server model, DHCP allows hosts or clients on an IP network to obtain their
configurations from a DHCP server, thereby reducing network administration. The most sig-
nificant configuration the client receives from the server is its required IP address; (other
optional parameters include the "generic" file name to be booted, the address of the default
gateway, and so forth).
Nortel Networks DHCP relay agent eliminates the need to have DHCP/BOOTP servers on
every subnet. It allows the administrator to reduce the number of DHCP servers deployed on
the network and to centralize them. Without the DHCP relay agent, there must be at least one
DHCP server deployed at each subnet that has hosts needing to perform the DHCP request.
DHCP Overview
DHCP is described in RFC 2131, and the DHCP relay agent supported on Alteon Web
switches is described in RFC 1542. DHCP uses UDP as its transport protocol. The client sends
messages to the server on port 67 and the server sends messages to the client on port 68.
DHCP defines the methods through which clients can be assigned an IP address for a finite
lease period and allowing reassignment of the IP address to another client later. Additionally,
DHCP provides the mechanism for a client to gather other IP configuration parameters it needs
to operate in the TCP/IP network.
In the DHCP environment, the Alteon Web switch acts as a relay agent. The DHCP relay fea-
ture (/cfg/ip/bootp) enables the switch to forward a client request for an IP address to
two BOOTP servers with IP addresses that have been configured on the switch.
When a switch receives a UDP broadcast on port 67 from a DHCP client requesting an IP
address, the switch acts as a proxy for the client, replacing the client source IP (SIP) and desti-
nation IP (DIP) addresses. The request is then forwarded as a UDP Unicast MAC layer mes-
sage to two BOOTP servers whose IP addresses are configured on the switch. The servers
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Chapter 1: Basic IP Routing
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212777-A, February 2002
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