Clustering DHCP Servers

Windows Clustering allows two servers to be managed as a single system. The Windows 2000 (Advanced Server only) clustering service can be used for DHCP servers to provide higher availability, easier manageability, and greater scalability.

Windows Clustering can automatically detect the failure of an application or server and quickly restart it on a surviving server, with users only experiencing a momentary pause in service. With Windows Clustering, administrators can quickly inspect the status of all cluster resources and easily move workloads around onto different servers within the cluster. This is useful for manual load balancing and for performing rolling updates on the servers without taking important data and applications offline.

Windows Clustering allows DHCP servers to be virtualized so that if one of the clustered nodes crashes, the namespace and all the services are transparently reconstituted to the second node. This means no changes are visible to the client, which sees the same IP address for the clustered DHCP servers.

Without clustering, network administrators might split scopes between servers, so if one server goes down, at least half of the available addresses remain available. Clustering uses IP addresses efficiently by removing the need to split scopes. A database stored on a remote disk tracks address assignment and other activity so that if the active cluster node goes down, the second node becomes the DHCP server, with complete knowledge of what has been assigned and access to the complete scope of addresses. Only one node at a time runs as a DHCP server, with the Windows 2000 clustering database providing transparent transition when needed.

Example of Clustered DHCP Servers

Figure 4.15 is a generic example of clustered DHCP servers. DHCP Server 1 is the active DHCP server, and DHCP Server 2 is the backup DHCP server.

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Figure 4.15 Clustered DHCP Servers

In Figure 4.15:

• DHCP Server 1 and DHCP Server 2 have Windows 2000 DHCP and Windows Clustering services installed.
• Each DHCP server has a unique server name and IP address.
• Each DHCP server has two network interfaces—one for the cluster identity and the connection to the enterprise network and the second for server-to-server communication. This is a private link only for cluster communication. The wire runs directly between the two servers.
• Both DHCP servers are configured with identical scopes. However, on Server 2, the scopes are not activated because Server 2 is not currently functioning as the active DHCP server. DHCP Server 2 can function as a hot spare, ready in the event of a shutdown of DHCP Server 1.
• To facilitate clustering and the sharing of resources, the DHCP servers are connected to an external disk system that holds the DHCP database and log files. This allows DHCP Server 2 to access the DHCP database files if it needs to take over as the active DHCP server. The clustering service installed on each DHCP server prevents one server from trying to exclusively claim the external disk and prevent sharing of the disk system between the DHCP servers.
• The cluster itself has a unique name and IP address, so that DHCP clients can use the cluster name and IP address to connect to the cluster and request DHCP services. This prevents rejected DHCP client requests if one of the DHCP servers is turned off. For example, if the client was configured with a specific DHCP server name and IP address instead of the cluster address, the client would not receive DHCP services. However, by configuring the DHCP clients with the cluster name and IP address, the client is able to communicate with the active DHCP server in the cluster.

Before implementing a similar scenario, consider the following recommendations:

• On each DHCP server in the cluster (whether backup or primary), install the DHCP service before you install the clustering service.
• Keep the second DHCP server turned off until the first server has the clustering service installed and is configured with a new cluster name and address. When the second server is turned on (and configured with DHCP and clustering services), it joins the existing cluster.
• The cluster name and IP address must be statically configured—they cannot be configured dynamically by another DHCP server.
• If a DHCP cluster is using an external disk system to store the DHCP database files, the DatabasePath and BackupDatabasePath registry entries must be configured on both DHCP servers in the cluster. The registry entries are located in

  HKLM\SYSTEM\CurrentControlSet\Services\DhcpServer\Parameters

  These registry entries must specify the path to the external disk system.

• Permissions: Any backup DHCP servers in the cluster will not be able to successfully take over DHCP tasks if the appropriate security permissions have not been enabled. Administrators must create a new domain security group to which the servers belong. This group must have permissions of Full Control for the DNS zone object in Active Directory where DHCP clients have their A and PTR records registered and updated. Alternatively, administrators can add the second server to the DNSUpdateProxyGroup for the domain. Otherwise, name resolution failures will result.
• Use the 80/20 rule when implementing clustered DHCP servers to provide additionally enhanced "failover" (hot-spare) services. The combination of clustering DHCP servers and using the 80/20 rule to manage scopes between the clustered server enables an enhanced failover solution. See the sections "80/20 Rule" and "Best Practices" for details in specifying scopes using the 80/20 rule.

For more information, see Windows 2000 Server Help.

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