Windows 2000 Administration in a Nutshell

Name

DHCP relay agent

Synopsis

An agent that listens for DHCP client requests and forwards such requests to a DHCP server on a different subnet.

Description

There are three main ways of implementing DHCP servers on an internetwork consisting of multiple physical subnets connected together by routers:

• Locate a DHCP server in each subnet. This is a straightforward solution, but it increases the complexity of the job of administering DHCP.

• Place DHCP servers in a central location and configure the routers to forward DHCP broadcast traffic between subnets. The router must be RFC 1542-compliant to do this; in other words, it must be able to pass broadcast traffic on UDP ports 67 and 68.

• Place a DHCP server in a central location and have one DHCP relay agent on every other subnet (but not on the subnet where the DHCP server is located). This way you don’t need to reconfigure routers or allow DHCP broadcast traffic to flood the internetwork unnecessarily.

A DHCP relay agent is an optional service that runs on a Windows 2000 Server and helps a client on one subnet lease and renew its address from a DHCP server on a different subnet.

How It Works

Let’s consider a DHCP client on subnet A requesting a lease from a DHCP server on subnet B via a DHCP relay agent configured on subnet A:

• The client on subnet A broadcasts a DHCPDISCOVER packet on its subnet (see DHCP in this chapter for more information about DHCP packets).

• The relay agent on subnet A hears the client’s DHCPDISCOVER broadcast, picks up the ...