DHCP Failover in Windows Server 2012
By Daniel Muniz Previous to Windows Server 2012, there did not exist a true high availability option for the DHCP service. There were of course split scopes and clustering that could be employed, but those methods were not truly efficient in the first case and very expensive to implement in the second example (with no provision available for load balancing either). Enter Windows Server 2012, offering not only ease of implementation and configuration, but true high availability and load balancing. It should be noted that in Windows Server 2012, you can configure only two DHCP servers for failover, and only for IPv4 scopes and subnets. To configure DHCP failover, you need to establish a failover relationship between the two DHCP servers’ services. You also must give this relationship a unique name. This is all handled very succinctly and easily via a wizard driven process. You can configure failover in one of the two modes, hot standby or load balancing mode. High availability is accomplished by the replication of the DHCP database between the respective servers. In the hot standby mode one server is primary (active) and the other is secondary (passive). This is exactly the same behavior one achieves with fail-over clustering. The primary difference here is that by using the 2012 failover functionality you are not required to have the identical hardware and configuration as required by traditional failover clustering. This provides for much more flexibility and options when placing your DHCP services on your infrastructure. Failover times are also configurable. That is, how much time to allow to elapse between a possible failure of one DHCP server before the backup server takes up the issuing of IP configuration information to client machines. The Load Sharing mode is just that. Utilizing this mode allows for your DHCP servers to operate in an active/active cluster capacity. The load distribution ratio is configurable (50-50 being the default). In larger networks this is a very useful feature. Not only is high availability accomplished but the infrastructure gains the added benefit of an increase in overall performance and responsiveness. The few simple steps it takes to set up DHCP Failover:
- In the DHCP console, expand yourserver.com, select and then right-click IPv4, and then click Configure Failover.
- In the Configure Failover Wizard, click next.
- On the Specify the partner server to use for failover page, in the Partner Server field, enter your other server, and then click next.
- On the Create a new failover relationship page, in the Relationship Name field, enter “name you chose”.
- In the Maximum Client Lead Time field, set the hours to ??? And then set the minutes to ???.
- Ensure the Mode field is set to Load balance or Standby
- Ensure that the Load Balance Percentage is set to 50% (your choice).
- Select the State Switchover Interval check box. Leave the default value of 60 minutes (configurable).
- In the Enable Message Authentication Shared Secret field, type (your password), and then click next.
- Click Finish, and then click Close.