Using Clustered DHCP Servers

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

Windows Server 2003 DHCP can use Windows Clustering, which allows two or more servers to be managed as a single system. You can increase DHCP (or multicast address dynamic client allocation protocol [MADCAP]) scalability, availability, and reliability by using the Cluster service to deploy a DHCP server cluster.

By using clustering support for DHCP, you can implement a local method of DHCP server failover. In this way, you can achieve greater fault tolerance and minimize disruptions and work stoppages. Windows Clustering can automatically detect the failure of an application or server and restart the application on or transfer the server role to an alternate server. Users experience only a brief break in service.

Windows Clustering creates a virtual DHCP server so that if one of the clustered nodes fails, the namespace and all of the services contained in that node are automatically transferred to a second node. No changes are visible to the client, which sees the same IP address for the clustered DHCP servers.

Use Windows Clustering alone to create a fault-tolerant design that makes efficient use of available IP addresses. To further enhance DHCP fault tolerance and availability, combine DHCP server clustering with a remote failover configuration — such as a split-scope configuration across different segments of your network. Although combining server clustering with a split-scope configuration increases DHCP availability, you must consider whether the benefits to your organization outweigh the hardware costs involved.

Figure 2.5 shows an example of clustered DHCP servers. DHCP Server 1 is the active DHCP server, and DHCP Server 2 is the backup DHCP server.

Figure 2.5   Clustered DHCP Servers

Example: Using a Split-Scope Configuration in Combination with a DHCP Server Cluster to Enhance Availability and Fault Tolerance

A company has its main corporate office in North America, and two European offices in Milan and Seville. The North American DHCP server cluster includes two servers, which are configured as nodes of the server cluster.

Twenty percent of the available IP leases for the Milan and Seville sites are configured on the North American server cluster. The remaining 80 percent of the available IP leases are configured on the local Milan and Seville DHCP servers.

This configuration provides the following levels of fault tolerance for the European sites:

1. Under normal circumstances clients in Milan and Seville request IP lease assignments from the local DHCP server, which contains 80 percent of the available IP addresses for the subnet.

2. If either the Milan or Seville DHCP server is slow or unavailable, European DHCP clients use the North American DHCP server, which contains 20 percent of the available IP addresses for the Milan and Seville subnets. The relay agent between the European offices and the North American office is configured to delay the DHCP messages from the European offices, allowing the local (Milan or Seville) DHCP server enough time to respond.

3. Because the North American DHCP server is configured as a server cluster, if one node of the cluster becomes unavailable, Windows Clustering automatically brings the other node online.