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Fault-Tolerance Options for Windows DHCP Servers

Q: What fault-tolerance options are available for a Windows DHCP server? Are there new DHCP fault-tolerance options available in Windows Server 2012?

A: The Windows DHCP server supports three fault-tolerance options. You can install DHCP on a Windows failover cluster, set up a split-scope DHCP, or set up DHCP failover, which is a new option introduced in Server 2012.

Installing DHCP on a Windows failover cluster. You can install DHCP on a two-node Windows failover cluster so that the second DHCP server takes the DHCP load if the primary DHCP server fails. This option uses shared storage and requires additional investment for ensuring storage redundancy.

Setting up a split-scope DHCP. Split-scope DHCP uses two independent DHCP servers that share responsibility for the same DHCP scope. One part of the IP address pool in the DHCP scope is assigned to the primary server, and the other part is assigned to the backup server. If clients can't reach the primary server, they'll get their IP configuration from the secondary server.

Setting up DHCP failover. With the DHCP failover option, you can replicate one or more complete DHCP scopes to another DHCP server. To set this up, you can use the new Configure Failover option that appears when you right-click a DHCP scope in the Server 2012 DHCP management interface. In the Configure Failover configuration screen, you can then choose one of the following DHCP failover modes: hot standby or load sharing (which is the default mode).

In hot standby mode, the two DHCP servers operate in a failover relationship, whereby one active DHCP server is responsible for leasing IP configuration data to all clients in a scope or subnet. The secondary DHCP server assumes this responsibility if the primary DHCP server becomes unavailable. In the context of a subnet, a DHCP server can assume the primary or secondary role, which means that a primary DHCP server in a given subnet can be the secondary DHCP server in another subnet.

In load sharing mode, the two servers simultaneously lease IP configuration data to clients on a given subnet. The requests are then load balanced and shared between the two servers.

There are two important limitations of DHCP failover: It's limited to IPv4 configuration data, and it can only support two nodes. You can find more information about DHCP failover in the Microsoft TechNet article "Step-by-Step: Configure DHCP for Failover."

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