DHCP failovering at-a-glance

In the past years, DHCP failover was not possible because DHCP servers were completely independent and unaware of each other. In order to provide a redundant DHCP service, you had to setup a failover cluster and spent lots of administrative effort and hardware resources. Along with the launch of Windows Server 2012, DHCP failover feature made its appearance. Literally in a just a few clicks, you can have a nice high available DHCP service running in your organization.

 dhcp

Supported Modes explained:

  1. Hot Standby : In this mode, one server is the primary server and the other is the secondary server. The primary server handles all the load and in case of failure the secondary takes over and acts as a primary.
  2. Load Sharing : In this mode, both servers supply IP addresses to clients simultaneously.

Key Words explained:

MCLT ( Maximum Client Lead Time ) :  Is the parameter which determines the amount of time a DHCP server should wait when a partner is unavailable, before assuming control of the address range. This value cannot be zero, and the default is one hour.

Auto State Switchover Interval :  Because the server has no way of knowing what is causing the communication loss, it remains in this state until the administrator manually changes it to a partner-down state. The administrator also can enable automatic transition to partner-down state by configuring the auto state switchover interval. The default value for this interval is 10 minutes. 

Message Authentication :  Shared secret used for authentication between the two partners.  Validates that the failover message comes from the failover partner.



How to configure a DHCP failover relationship

  1. Sign in on your primary working DHCP server. Open the DHCP
    management console.
  2. In the DHCP console, right click the scope that you need to make high available and launch the Configure Failover Wizard.
  3.  Configure failover replication with the following settings:
    • Partner server: IP of the authorized partner server or choose from the list
    • Relationship Name: DHCP-HA ( or one of your preference )
    • Maximum Client Lead Time: 1 hour ( default )
    • Mode: Hot standby
    • Address reserved for standby server : 5%
    • State Switchover Interval: 1 minute
    • Message authentication shared secret: secret ( or one of your preference )

 

Hint 1 : DHCP failover is time sensitive! If the time difference between the partners is greater than one minute, the failover process will fail.

Hint 2 : Only IPv4 Scopes are supported.

George Markou

My Name is George Markou, IT Pro, Geek and Fancy Gadgets enthusiast. I am focused on Virtualization and Cloud Technologies.

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