Traditionally, DHCP server availability has been implemented on Windows Server–based networks using one or more of the following methods:
- Split scopes This approach involves splitting the IP address pool of a scope between two DHCP servers, typically by assigning the primary server 80 percent of the addresses in the scope and the secondary server the remaining 20 percent of the addresses. That way, if the primary server goes offline for any reason, DHCP clients on the subnet can still respond to lease renewal requests from the secondary server.
- Server cluster This approach involves using the Failover Clustering feature of Windows Server 2008 or Windows Server 2008 R2 to cluster DHCP servers so that if the primary DHCP server in a cluster fails, the secondary server can take up the slack and continue leasing addresses to clients.
- Standby server This approach uses a hot standby DHCP server with scopes and options configured identically to your production DHCP server.
Each of the preceding approaches has the following disadvantages, which make them of limited usefulness in ensuring DHCP server availability:
- The split-scope approach provides limited IP availability during outages. As a result, some clients might not receive addresses during a long-term DHCP server outage. In addition, if your DHCP server scope is currently running at high utilization—which is common for Internet Protocol version 4 (IPv4) networks—splitting the scope might not be feasible.
- The DHCP server-cluster approach has only one DHCP database located on the cluster shared storage. That means there is a single point of failure for DHCP services on your network. In addition, implementing Failover Clustering requires relatively complex setup processes and maintenance tasks.
- The hot-standby approach requires both careful configuration of the standby DHCP server and manual intervention on the part of the administrator to ensure the failover transition when your production DHCP server fails or goes offline. There is also additional complexity in this approach when DHCP is configured to automatically update DNS records, as is recommended in an Active Directory environment.
DHCP failover is a new approach to ensuring DHCP availability that is included in Windows Server 2012. With this approach, two DHCP servers can be configured to provide leases from the same pool of addresses. The two servers then replicate lease information between them, which enables one server to assume responsibility for providing leases to all clients on the subnet when the other server is unavailable. The result of implementing this approach is to ensure DHCP service availability at all times, which is a key requirement for enterprise networks.
This tip is excerpted from my latest book Training Guide: Installing and Configuring Windows Server 2012 from Microsoft Press.
Mitch Tulloch is a nine-time recipient of the Microsoft Most Valuable Professional (MVP) award and a widely recognized expert on Windows administration, deployment and virtualization. For more information see http://www.mtit.com.
