DHCP secondary server is configured with 20% of address leases and if it serves the clients at the same pace it would run out of address leases and would be useless if the primary DHCP server is down. To avoid address range depletion on the secondary server one of the following two methods can be employed
· A delay is set on the RA (Relay Agent) in the proximity of the Secondary DHCP server. A delayed OFFER from the secondary DHCP server will make the client to request the lease from the primary DHCP server and will only request from secondary only the case where primary is not reachable. Setting the delay in the RA ensures that the client receives the OFFER later than it receives from the primary, but it does not delay the OFFER originating from the secondary server itself. In this kind of setup the RA should be setup mandatorily even if it is not required.
· Delay is set on the secondary Windows Server 2008 R2 DHCP server using MMC/Netsh tools. This would make the OFFER originating from the DHCP server is delayed. In this setup the RA is not mandatory.
Configuration Steps (set delay on secondary DHCP server):
Launch the scope properties and change the Delay configuration value in the Advanced tab of the scope in particular:
This can be configured also while configuring the split-scope using the split-scope configuration wizard. (refer the blog on wizard based split-scope configuration)
Hope this helps in managing your addresses. Comments, suggestions or queries are welcome.