Adding redundancy to DHCP if often overlooked. DHCP gets setup on the primary DC and it’s taken for granted that this server will always be online to hand out IP addresses when needed. Since Windows Server 2012 we’ve been able to setup DHCP failover and share an address pool across multiple servers.
DHCP Failover Modes
There are two modes to choose from when setting up DHCP failover:
Hot Standby Mode
Primary and secondary DHCP servers work in an active-passive mode. The primary server hands out all leases while the secondary server maintains a copy of the lease table. In the event that the primary server goes offline the secondary server will take over the active role.
Load Balance Mode
Both the primary and secondary server can hand out leases in an active-active mode. Since the Discover phase of the DHCP process is a broadcast packet, both server will receive this request and respond with an IP address. The client will take the lease from the first response it receives. This will then be added to the lease table and synced between both servers.
If you’re looking into setting up failover, I’m going to assume you’ve already got one DHCP server setup with at least one Scope configured. You will also need to have the DHCP role installed on a second server. Both servers should be Authorized in AD. These are the minimum requirements needed before setting up failover.
Launch DHCP on the primary server. Right click on IPv4 and select Configure Failover.
Select any and all scopes you want to configure failover for.
Enter the name/IP address of your secondary server. You can also click Add Server and select from a list.
Enter a sensible Relationship Name. Select the mode you wish to use from the drop-down menu. Add a Shared Secret key.
Confirm the config is correct before clicking Finish.
You should now see a message stating everything completed successfully.
Now jump over to the DHCP console on your secondary server. Refresh and you will see the replicated scope(s) appear. Both server are now ready to start handing out IP addresses.