The Need for DHCP High Availability
The Dynamic Host Configuration Protocol (DHCP) assigns IP addresses to every device on your network. If your single DHCP server crashes or goes offline for patching, new devices cannot connect to the network, and existing devices will eventually drop off as their leases expire. Historically, administrators split DHCP scopes 80/20 across two independent servers to provide redundancy. Modern Windows Server environments provide a much superior solution: DHCP Failover, which actively replicates lease information between two servers ensuring a seamless transition during an outage.
Step 1: Install the DHCP Role on Two Servers
You will need two Windows Servers joined to the same Active Directory domain. Open Server Manager on both servers and install the DHCP Server role via the Add roles and features wizard. Once installed, complete the post-installation configuration to authorize both servers in Active Directory.
Step 2: Create the Initial Scope on the Primary Server
Log in to your primary DHCP server (e.g., DHCP-01) and open the DHCP Management Console. Right-click IPv4, select New Scope…, and follow the wizard to define your IP address range, subnet mask, default gateway, and DNS servers. Activate the scope.
Do not create this scope on the secondary server. The failover process will automatically replicate it.
Step 3: Initiate the Failover Configuration
Still on DHCP-01, right-click the IPv4 node (or the specific scope you just created) and select Configure Failover…. The wizard will display the scopes available for high availability. Check the box to select all scopes and click Next.
Step 4: Define the Partner Server and Relationship
On the next screen, enter the hostname or IP address of your secondary DHCP server (e.g., DHCP-02) as the Partner Server. Click Next.
You must now define the relationship parameters:
- Relationship Name: Leave default or create a descriptive name.
- Mode: Choose Load Balance if you want both servers to actively hand out IPs simultaneously (default 50/50 split), or Hot Standby if you want the secondary server to remain completely idle until the primary fails. (Load Balance is generally recommended).
- Shared Secret: Enter a strong password to authenticate the replication traffic between the two servers.
Step 5: Verify Replication
Click Finish. The wizard will create the relationship and instantly copy the scope, options, and current active leases over to DHCP-02.
Log in to your secondary server and open the DHCP console. You will see the exact same scope listed. You can test the failover by shutting down the DHCP Server service on DHCP-01 and attempting to acquire a new IP address from a client machine; DHCP-02 will seamlessly handle the request.