DHCP Design Guidelines in Network Infrastructure

In a network with multiple subnets, a major concern is answering DHCP broadcast requests from all subnets. There are three ways to configure DHCP to allow clients on all subnets to get an IP address from a DHCP server.

Method	Considerations
Place a DHCP server on each subnet	The local DHCP server answers DHCP requests for its local subnet only. This solution could increase costs as you must configure a DHCP server for each subnet.
Enable BootP forwarding	Use this method if you choose to have only one DHCP server on the routed network. Place the DHCP server on the subnet with the most hosts.
Use relay agents	Configure a relay agent on each subnet that does not have a DHCP server to forward DHCP broadcasts to the remote DHCP server.

To provide fault tolerance and improve DHCP performance, you can:

• Configure split scopes. In a split scope, two DHCP servers service each subnet.
• Configure the full range of addresses in the scope on both DHCP servers.
• Exclude different ranges of addresses on each server. With both servers on the same subnet, exclude 50% of the addresses on each server. With one DHCP server on the local network, exclude addresses using the 80/20 (or 75/25) rule (exclude 80% on the remote server).
• Add relay agents to allow DHCP broadcasts to travel to remote subnets in case the local DHCP server goes down.
• Implement a DHCP cluster. Windows Server 2003 DHCP is cluster-aware. When one server goes down, the other server(s) can take over.

To decrease the time that the DHCP server takes to respond to client requests, consider the following to improve performance:

• Create a multihomed DHCP server. By doing so, the DHCP server services multiple subnets without forwarding DHCP requests through the router. A multihomed server will decrease router traffic and decrease the time it takes for clients on the remote subnet to receive their IP addresses.
• Upgrade DHCP server components. Upgrading the CPU, RAM, and network cards all affect DCHP performance, but because DHCP is a disk-intensive service, the most important upgrade is to improve disk access.
• Add additional DHCP servers. By doing so, you spread the workload between two computers.
• Modify lease lengths. Increasing the lease time means clients will not request an address as often. This reduces network traffic due to lease broadcasts, and reduces the workload on the DHCP server.

Members of the Enterprise Admins group can authorize DHCP servers (members of the Domain Admins group can administer, but not authorize, DHCP servers).