Relay Agent Deployment

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When you have multiple DHCP servers, Microsoft recommends that you place your DHCP servers on different subnets to achieve a degree of fault tolerance, rather than having all the DHCP servers in one subnet. The servers should not have common IP addresses in their scopes (each server should have a unique pool of addresses).

If the DHCP server in the local subnet shuts down, requests are relayed to a remote subnet. The DHCP server at that location can respond to DHCP requests if it maintains a scope of IP addresses for the requesting subnet. If the remote server has no scope defined for the requesting subnet, it cannot provide IP addresses even if it has available addresses for other scopes. If each DHCP server has a pool of addresses for each subnet, it can provide IP addresses for remote clients whose own DHCP server is offline.

There are several relay agent configuration options available if you plan to incorporate a relay agent into your DHCP/BOOTP-enabled network. These include using third-party routers, Windows 2000 Routing and Remote Access, and the DHCP Relay Agent component provided in Windows NT Server 4.0. For more information about how relay agents work, see the section "Managing Relay Agents" later in this chapter.

Figure 4.18 shows a recommended relay agent implementation, which provides for the best network performance.

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Figure 4.18 Windows 2000 Recommended Relay Agent Configuration

This figure illustrates a general configuration for relay agents. For specific scenarios, see the following sections and illustrations.

Windows 2000 Server Routing and Remote Access Relay Agents

Figure 4.19 shows the Windows 2000 Server Routing and Remote Access configuration. In this example, the Windows 2000 server is acting as an IP router between Subnet 1 and 2, as well as a relay agent between the DHCP server on Subnet 1 and the DHCP clients on Subnet 2.

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Figure 4.19 Windows 2000 remote access server as a Relay Agent

The DHCP Relay Agent on the Windows 2000 server must be configured with the IP address of the DHCP server to relay DHCP requests between Subnet 1 and Subnet 2.

Windows NT Server 4.0 Relay Agents

Figure 4.20 shows a standard router configuration.

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Figure 4.20 Standard Router as a Relay Agent

This example shows how a standard IP router can be implemented on a network, in combination with a Windows NT Server 4.0 relay agent relaying DHCP requests between Subnet 1 and Subnet 2.