The Dynamic Host Configuration Protocol (DHCP) is a critical network service; if your DHCP server goes offline, client devices will eventually lose their IP addresses and drop off the network. Historically, administrators split DHCP scopes (the 80/20 rule) across two servers to provide redundancy. However, starting with Windows Server 2012, Microsoft introduced native DHCP Failover, allowing two servers to synchronize lease information and provide true high availability without splitting scopes.
Understanding DHCP Failover Modes
DHCP Failover operates in one of two modes:
- Load Balance (Active-Active): Both servers actively assign IP addresses from the same scope at the same time, sharing the load based on a configurable percentage (usually 50/50). This is best for a single physical site.
- Hot Standby (Active-Passive): One server handles all requests. The secondary server only takes over if the primary server fails. This is ideal for multi-site topologies where a backup server sits in a remote data center.
Step 1: Install the DHCP Role on Both Servers
Ensure that both Windows Servers have the DHCP Server role installed and authorized in Active Directory. Create your new DHCP scope (e.g., 192.168.10.0/24) on the Primary server only. Do not configure anything on the Secondary server yet.
Step 2: Configure the Failover Relationship
- On the Primary DHCP server, open the DHCP Management Console.
- Expand the server tree and expand IPv4.
- Right-click on the scope you just created and select Configure Failover…
- The Configuration Wizard will appear. Ensure the correct scope is checked and click Next.
- In the “Partner Server” field, type the hostname or IP address of your Secondary DHCP server and click Next.
Step 3: Define the Failover Parameters
You must now define how the two servers will interact:
- Relationship Name: Enter a descriptive name (e.g., “HQ-DHCP-Failover”).
- Maximum Client Lead Time (MCLT): This defines how long a client can keep its IP address after a server failure before the lease expires. The default (1 hour) is usually sufficient.
- Mode: Select Load balance for a local network, or Hot standby for a remote site.
- Load Balance Percentage: If using Load Balance, leave it at 50% / 50%.
- State Switchover Interval: Check this box and set it to 10 minutes. This tells a surviving server to automatically assume full control if it loses contact with its partner for 10 minutes.
- Shared Secret: Enter a strong password to authenticate communications between the two servers.
Step 4: Complete and Verify
Click Next and then Finish. The wizard will create the relationship and replicate the scope configuration to the Secondary server.
You can verify the setup by opening the DHCP console on the Secondary server. You will see the new scope listed there automatically. If a client requests an IP address, both servers will now coordinate to ensure the lease is recorded in both databases, guaranteeing uninterrupted network access.