Choose your DHCP failover option: Hot Standby and load balance modes

business man in office

Believe it or not, part of my day job is to help answer questions posted on a support forum. One of the questions that came up the other day was how to you can use multiple Dynamic Host Configuration Protocol (DHCP) servers to assign addresses for the same scope or subnet using Windows Server 2012.

There were a couple of responses said it can’t be done, won’t work or the only way to do so is by using a split scope. But there is a cool new feature to Windows Server 2012: DHCP failover.

The DHCP failover feature enables two DHCP servers to provide IP addresses and optional configurations to the same subnets or scopes. In earlier versions of Windows Server, DHCP servers were not able to talk or replicate scope or lease information.

But with the new features built-in to Windows Server 2012 you can configure two DHCP servers to replicate client lease information and if one server fails the remaining DHCP server continues to assign IP address and configuration information and renew existing client leases.

There are two ways that we can configure DHCP failover: Load balance and Hot Standby.

The load balance mode was the best solution given the information provided. With load balance you have two servers who can service your client’s requests for IP addresses and lease renewals. During the first setup, all scope configuration settings are replicated between the two partners. By default the two servers will split the addresses in the scope 50:50, but you configure the percentage if you would prefer one server to handle more of the workload.

After the first replication takes place, lease information is continuously replicated between the two servers allowing one server to service the entire DHCP scope in the event of a service or server failure. How cool is that!

Load balance works great if both servers are within the same physical site. But what if you have a branch office that is part of a separate physical site with a local DHCP server, and you want to give a way for your clients to receive IP addresses or renew leases even when the local DHCP server is offline.

This is where the Hot Standby option comes into play. With DHCP Hot Standby you will configure two servers to service the client requests, but one server will act as the primary server and takes care of all requests. The secondary or standby server will only come into play if or when the primary server is unavailable. The client requests fallback to the local server once the primary server becomes available again.

In the branch office scenario you would have the local server in the branch office be the primary server and handle all client requests locally. You would then configure the secondary or standby server to be at the corporate or regional office with easy access from the branch office. By default the two servers will split the addresses in the scope 95:5. You configure the percentage if you would prefer a larger pool of address on the standby server.

Configure failover

Configure failover (Source: Microsoft)

Choose your management tool

With Windows Server 2012 you can create, manage, and remove DHCP failover settings using either the Graphical User Interface (GUI) or PowerShell. There are close to 100 PowerShell cmdlets that can be used to manage DHCP, including cmdlets specifically for DHCP failover. With PowerShell you can run Get-Command –Module DhcpServer to see the entire list.

DHCP failover is limited to IPv4 scopes and subnets and you can only configure DHCP failover between two DHCP servers. If you have multiple scopes hosted on the same DHCP server, you configure all scopes or individual scopes to be part of the failover process.

There are other new features in DHCP that are worth taking a look at and DHCP failover is just one of them. Let us know if you use DHCP failover or any of the other cool new features in Windows Server 2012.

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