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Design Option 3: BOOTP/DHCP Relay Agents

Updated: November 10, 2008

Applies To: Windows Server 2008

If a router cannot function as a BOOTP/DHCP relay agent, each subnet must have either its own DHCP server or another computer that can function as a relay agent on that subnet. In cases where it is impractical or impossible to configure routers to support BOOTP/DHCP relay, you can configure a computer running Windows Server 2008 to act as a relay agent.

A relay agent is a small program that relays BOOTP and DHCP messages between clients and servers on different subnets. BOOTP/DHCP relay agents are part of the DHCP and BOOTP standards and function according to RFC 1542.

Relay agents work like routers with BOOTP/DHCP forwarding enabled, except that the relay agents run on a workstation or server instead of on the router. Relay agents are configured to listen for DHCP requests and pass them along as unicasts (rather than broadcasts) to specific DHCP servers on other subnets, which is analogous to the configuration of BOOTP-compatible routers with a helper address.

The advantages of BOOTP/DHCP relay agents on routers include:

  • Single DHCP server can support many subnets: The use of relay agents allows a single or a small number of DHCP servers to be deployed across a routed network, even where the routers are not all RFC 1542-compliant. Even where routers are compliant, it might be simpler to configure software-based relay agents through scripts or automated operating system builds.

  • Control of DHCP-related network traffic: The use of relay agents allows very precise control of the DHCP network traffic and can reduce the number of broadcast-based DHCP packets on the network.

The disadvantages of BOOTP/DHCP relay agents on routers include:

  • Support overhead: Relay agents require configuration and support, although it might be possible to automate their deployment through administrative scripts.

  • Potential point of failure: If the workstation or server running the agent fails, the effects could be critical for clients on that subnet. You must clearly document which computers are acting as relay agents so that they are not taken offline without planning for DHCP relay coverage. For this reason, there should always be at least two relay agents on all but the smallest subnets.

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