Sunday, January 5, 2014

Windows Server 2012 DHCP Failover Not Adhering to Scope Times

When setting up Windows Server 2012 DHCP Failover you may notice the lease duration provided to clients may not adhere to the lease duration configured on the DHCP scope.  This is by design and a normal behaviour in a DHCP failover relationship.

By default when a lease is first issued to a client, it adheres to the Maximum Client Lead Time (MCLT) value, not the value on the DHCP scope.  This is considered a temporary lease period given by a failover server to a new client.  In addition to providing a temporary lease to client, it specifies the amount of time for which a DHCP lease may be renewed by either failover peer without contacting the other.  It also specifies the amount of time that either DHCP server will wait in a “partner down” state before assuming control of the entire IP address range within the scope.  ( default = 1 hour ).

Maximum Client Lead Time (MCLT) can be found when configuring DHCP failover in the following location in the setup wizard.

 
After the initial lease, when the client attempts to renew its IP address it will then inherit the default scope lease time.
 
Key points to take away:
  • Windows Server 2012 DHCP Failover, an initial lease always = the MCLT (Maximum Client Lead Time)
  • Afterwards the same client will renew and get whatever is defined in the scope definition.
So if you see clients receiving 1 hour leases from DHCP, relax its by design!  They will receive a full lease when they next attempt to renew DHCP.

Sources:

http://technet.microsoft.com/en-us/library/dn338973.aspx
http://tools.ietf.org/html/draft-ietf-dhc-failover-12#section-5.2
http://support.microsoft.com/kb/2831920

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