Planning DHCP networks

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

Planning DHCP networks

This topic covers the following DHCP network planning issues.

• How to determine the number of DHCP servers to use

• How to support DHCP clients on additional subnets

• Planning for routed DHCP networks

• Other planning considerations for enterprise networks

How to determine the number of DHCP servers to use

Because there is no fixed limit to the maximum number of clients a DHCP server can service or to the number of scopes you can create on a DHCP server, the primary factors to consider when you determine the number of DHCP servers to use are network architecture and server hardware. For example, in a single subnet environment, only one DHCP server is necessary, although you may want to use two servers or deploy a DHCP server cluster for increased fault tolerance. In multiple subnet environments, routers must forward DHCP messages between subnets, so router performance can affect your DHCP service. In both cases, DHCP server hardware affects service to clients.

For more information on deploying a DHCP server cluster, see Cluster support for DHCP servers.

When you determine the number of DHCP servers to use, consider the following:

• The location of routers on the network and whether you want a DHCP server on each subnet.

  When you extend the use of a DHCP server across more than one network, you often need to configure additional DHCP relay agents and, in some cases, use superscopes as well.

• The transmission speed between the segments for which DHCP service is provided.

  If you have slower WAN links or dial-up links, you may need a DHCP server on both sides of these links to service clients locally.

• The speed of the server disk drives and the amount of random access memory (RAM) installed in the DHCP server computer.

  In order to maximize DHCP server performance, use the fastest disk drives and the most RAM possible. Carefully evaluate disk access time and average times for disk read and write operations when you plan for your DHCP server hardware needs.

• Practical constraints based on the IP address class selected for use and other server configuration details.

You can test your DHCP servers before deployment on the organization network in order to determine the limitations and abilities of your hardware and to see whether network architecture, traffic, and other factors affect DHCP server performance. Hardware and configuration tests also allow you to determine the number of scopes to configure at each server.

To provide a general idea of DHCP server performance, a DHCP server running Microsoft® Windows Server 2003 was run in a test lab environment, and a custom stress application was used against the server. The details of this lab test can help you configure your tests and determine how many DHCP servers to use on your network:

Server and network configuration

• Processors: Two x86 Family 6 Model 7 Stepping 3 GenuineIntel ~498 megahertz (MHz)

• Total physical memory: 256.00 megabytes (MBs)

• Network adapters: Three Ethernet 802.3 100 megabits per second (mbps)

• Subnets serviced: Six, four of which are separated from the test server by routers running the DHCP Relay Agent service.

• Operating system: Windows Server 2003, Enterprise Edition

• Number of scopes: 5,155

• DHCP database size at maximum load: 2 gigabytes (GBs)

• Additional factors: Several thousand exclusion ranges, option values, and reservations are configured in the scopes on the server.

Test details

The DHCP server was hit with both valid and invalid DHCP client lease and renewal requests from a client/attack simulation stress application on six subnets for 1,152 hours (48 days).

Test results

The DHCP server provided clients with the following service during the test:

Notes

• These test results are intended to provide a general idea of DHCP server performance capacity. Tests were performed on average hardware and do not imply any limitation of the DHCP service. In addition, the large size of the test DHCP server database (2 GBs) is extremely unusual and was produced due to the high volume of DHCP traffic that the test application generated. In real world environments, typical database sizes are in the tens of MBs or less.

• When you add a large number of scopes to the server, be aware that each scope creates a corresponding need for additional, incremental increases to the amount of disk space used for the DHCP server registry and for the server paging file. For more information, see Change the size of the virtual memory paging file.

• DHCP servers running Windows Server 2003 provide performance monitoring tools that you can use to test and monitor your servers. For more information, see Monitoring DHCP server performance.

A DHCP planning alternative: configuring standby servers

Most networks need one primary online DHCP server and one other DHCP server acting as a secondary or backup server. If you choose not to implement two DHCP servers using the 80/20 rule for balancing scopes but want to continue to provide a measure of potential fault tolerance, you might consider implementing a backup or hot standby DHCP server as an alternative. For more information, see the section "Using the 80/20 rule for scopes" in Configuring scopes.

In the hot standby configuration, the standby DHCP server is another server computer that is installed and configured identically to your primary DHCP server. In the case of the standby, the only difference is that the standby server and its scopes are not activated for use under normal conditions. Duplicate scopes are configured but not activated except when critically needed, such as to substitute for the primary DHCP server because it has stopped or been taken offline for an extended period of time.

Because the hot standby solution requires special attention to its configuration and also manual administration for ensuring fall-over transition for DHCP clients to use it, it is less recommended as a planning alternative than the use of two to three DHCP servers that balance active scope use.

Supporting additional subnets

For DHCP service to support additional subnets on your network, you must first determine whether the routers used to connect adjoining subnets can support relaying of BOOTP and DHCP messages. If routers cannot be used for DHCP and BOOTP relay, you can set up either of the following for each subnet:

• A computer running Windows NT® Server 4.0, Windows 2000 Server, or a Windows Server 2003 operating system configured to use the DHCP Relay Agent component.

  This computer simply forwards messages back and forth between clients on the local subnet and a remote DHCP server, using the IP address of the remote server. The DHCP Relay Agent service is available only on computers running Windows NT Server 4.0, Windows 2000 Server, or Windows Server 2003 operating systems.

• A computer running a Windows Server 2003 operating system configured as a DHCP server for the local subnet.

  This server computer must contain and manage scope and other address-configurable information for the local subnet it serves.

For more information, see Relay agent configurations.

Planning for routed DHCP networks

In routed networks that use subnets to divide network segments, planning options for DHCP services must observe some specific requirements for a full implementation of DHCP services to function. These requirements include:

• One DHCP server must be located on at least one subnet in the routed network.

• For a DHCP server to support clients on other remote subnets separated by routers, a router or remote computer must be used as a DHCP and BOOTP relay agent to support forwarding of DHCP traffic between subnets.

The following illustration shows a simple routed network with DHCP implemented.

Enterprise planning considerations

For an enterprise DHCP network, you should:

• Plan the physical subnets of the network and relative placement of DHCP servers.

• Specify the DHCP option types and their values to be predefined per scope for the DHCP clients.

  This can include planning for scopes based on the needs of particular groups of users. For example, a unit that frequently moves computers to different locations, like a marketing group that uses portable computers docked at different stations, shorter lease durations can be defined for the related scopes. This approach collects IP addresses that are changed frequently and disposed of and returns them to the pool of available addresses that can be used for new lease offerings.

• Recognize the impact that slower links have on your WAN environment.

  Place DHCP, WINS, and DNS servers to maximize response time and minimize low-speed traffic.

As one example of planning for a large enterprise network, the segmenting of the WAN into logical subnets can match the physical structure of the internetwork. Then one IP subnet can serve as the backbone and, this backbone in relation to each physical subnet maintains a separate IP subnet address.