A: Previously, to provide resiliency for DHCP scopes, the DHCP scope either had to be hosted by a cluster or it was possible to split a DHCP scope so that part of the address range was hosted on one DHCP server and the rest of the scope was hosted on a second DHCP server (Windows Server 2008 R2 added a wizard to perform this split scoping).
The problem with both these approaches was that you either needed a cluster or the actual leases of IP addresses weren't replicated to the other server that was part of the split scope, causing problems if a DHCP server actually was unavailable for a length of time, as any leases were lost.
Windows Server 2012 introduces the ability to replicate scopes between two DHCP servers. With this replication, all of the scope information is replicated between the two servers that have the replication relationship, including any leases. The relationship between the two DHCP servers can either be load balanced, where both servers are primary for a certain percentage of the scope (replicating leases between them), or put on hot standby, where only one DHCP server is actively leasing IP addresses and replicating those leases to the hot standby DHCP server that would take over in the event of a failover. The properties of the replication relationship appear in the screen shot below.
DHCP Replication Relationships
Setting up a new DHCP failover relationship is actually very simple. The only requirement is to have two Windows Server 2012 servers with the DHCP role installed on them. Additionally, the scopes that you want to replicate shouldn't have any policies defined on them, so select the Policies node of each scope and ensure there are no policies.
- The scopes should already be created; launch the Configure Failover... action on the primary server (from the protocol action menu such as IPv4).
- The Configure Failover wizard will launch, allowing the scopes to be selected that should be replicated.
- Select the server that they should be replicated to (use the Add Server button); or if you have already replicated other scopes, you can choose to use the existing failover relationship, then click Next.
- Select the options for the failover such as the failover mode (load balance vs. hot standby), distribution, if a shared secret should be used (basically a shared phrase that you configure) for authentication, and other options, then click Next.
- A summary will appear; click Finish.
The DHCP scope is now replicating and will be visible on the other server. If you have any Server Options configured (that apply to all scopes), these will need to be manually configured on the other DHCP server.
The failover options can be changed by selecting the properties for the protocol (such as IPv4) and modifying the replication relationship under the Failover tab, which also shows the basic health of the DHCP replication.
