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Lesson 6 Designing DHCP service for a routed network
Objective Describe how to place DHCP Relay Agents and Servers in routed network.

Designing DHCP Service for Routed network

In a routed network, the broadcast domains are restricted. As such, any DHCP solution must allow the broadcast traffic from the DHCP Clients on the subnets to reach a DHCP Server. Windows 2000 provides a DHCP Relay Agent to forward DHCP Client requests to a DHCP Server. You can place the DHCP Relay Agent in a subnet anywhere in the routed network.
DHCP Clients and Servers initially establish DHCP leases with MAC and IP broadcast packets. But to provide IP configuration to clients on multiple subnets, you must install a Relay Agent for DHCP or configure IP routers to support DHCP/Bootstrap Protocol (BOOTP) forwarding.
Let's say, for example, that a network has two physical segments separated by a router. Each segment is represented by a logical network ID. The first segment has the network ID, the second A single DHCP Server is used, and is placed on the segment.
What happens when a DHCP Client starts up? It broadcasts a DHCP Discover or DHCP Request message. If this DHCP Client is on network ID, the request is stopped at the router interface acting as the default gateway for
To solve the issues related to the broadcast nature of DHCP messages, you could turn to two solutions: the DHCP Relay Agent and BOOTP Forwarding.

DHCP Relay Agent

Windows 2000's RFC 1542-compliant DHCP Relay Agent acts as an intermediary between separated DHCP Clients and Servers. The DHCP Client communicates with the Relay Agent by using DHCP broadcast messages. The DHCP Relay Agent uses unicast packets to communicate with a DHCP Server and is transparent to a DHCP Client.
In the example of the DHCP Client on network ID, a DHCP Relay Agent on its subnet will intercept the DHCP messages and relay them to the DHCP Server on the remote subnet. The Relay Agent is configured with the IP address of the DHCP Server servicing the requests, and therefore doesn't depend on broadcast messages to communicate with the DHCP Server on the remote subnet.

The DHCP service and DHCP Relay Agent use the same User Datagram Protocol (UDP) ports. Neither service works reliably if you install them both on the same computer.

How to place a Relay Agent

Placing a Relay Agent entails several steps and considerations. For a routed network, use DHCP Relay Agents on each subnet if:
  1. There is no DHCP Server with an interface on the subnet
  2. There are computers available to use as DHCP Relay Agents
  3. There are no routers that support DHCP/BOOTP Forwarding

You may also configure the Relay Agent to meet specific forwarding needs:
  1. Delaying forwarding requests to a DHCP Server so that local DHCP Servers can respond to the request. Set the Boot threshold high enough so that the DHCP Relay Agent doesn't intercept and forward a request before a local DHCP Server can answer the request made by a local DHCP Client.
  2. Forwarding requests to multiple DHCP Servers
  3. Preventing DHCP messages from looping throughout the network. Set a Hop-count threshold to a reasonably low value.

BOOTP Forwarding

You may use BOOTP Forwarding instead of DHCP Relay Agents on your network. BOOTP Forwarding is enabled on network routers that separate the DHCP Client from the Server. DHCP messages are passed across routers, not dropped at the router interface.
The advantage to this is that the broadcast message will be available to multiple DHCP Servers, rather than preconfigured IP addresses for targeted DHCP Servers. The disadvantage is that DHCP broadcast traffic is able to traverse the interceding segments between the DHCP Client's segment and the segment that contains the DHCP Server.
Note: Remember that BOOTP Forwarding and DHCP Relay Agents perform similar functions, but they are different. Both allow DHCP Clients to access a remote DHCP Server, but the Relay Agent is typically located on a dedicated computer, whereas the BOOTP Forwarding is performed on routers.

DHCP Server placement

You should place DHCP Servers in a way that provides the best client performance and service availability. The decision to use single or multiple server solutions depends on the routing configuration, the network configuration, and the server hardware architecture.

In the next lesson, you will learn how a DHCP Server services non-Microsoft hosts.

Designing Dhcp Services Routed Network - Exercise

Click the Exercise link below to work on designing a DHCP service for a routed network.
Designing Dhcp Services Routed Network - Exercise