DHCP

Consider the following scenarios:

• Scenario 1
  • You have two DHCP servers that are running Windows Server 2008 R2.
  • You set up a split-scope configuration on both DHCP servers.
  • You make identical reservations on both DHCP servers.
  • A DHCP client that reserves an IP address broadcasts a DISCOVER packet and both DHCP servers respond to the DISCOVER packet by sending an OFFER packet.
  • The DHCP client receives a reserved IP address from one of the DHCP servers.

  In this scenario, reservations on the other DHCP server are deleted incorrectly. Therefore, the reserved IP addresses may not be assigned to DHCP clients that reserve the addresses.

• Scenario 2
  • You have a DHCP server that is running Windows Server 2008 R2.
  • You create reservations for DHCP clients on this DHCP server.
  • The DHCP client releases the obtained IP address multiple times.

  In this scenario, reservations on the DHCP server are deleted incorrectly. Therefore, the reserved IP addresses may not be assigned to DHCP clients that reserve the addresses.

http://support.microsoft.com/kb/981776

While creating a reservation in a DHCP scope (Windows 2008 R2) I received the error mentioned.

After some investigation I discovered that Windows 2008 R2 doesn’t allow reservations that are not in the scope, although the subnetmask (for example 255.255.0.0)  is big enough. Changing the subnetmask isn’t an option, so you have to delete and re-create the scope.

You can export the settings with the following command :

netsh dhcp server ip-adres server scope X.X.X.X dump > c:\dump,txt

Then edit the subnet mask

and import the scope with the following command :

netsh exec c:\dump.txt

Using Windows 2003 and Windows 2008 (not R2) it is possible to add reservations within the subnetmask but out of the DHCP scope!

When importing multiple scopes after each other you get the “cannot find the file specified”.

It’s a bug (not a feature :-)). Solution is to exit after each import.

Use the following steps to migrate the DHCP configuration & settings :

• Logon to the source (W2K3) server
• Start the command prompt (cmd.exe)
• netsh
• DHCP
• server 127.0.0.1
• export W2K3DHCP all or export W2K3DHCP <scope, e.q. 10.1.1.0>
• Copy the export file to the other (W2K8) server
• stop the DHCPSERVER net stop DHCPSERVER or using services.msc
• delete the DHCP.mdb (located c:\windows\system32\dhcp)
• start the DHCP server
• netsh
• DHCP
• server 127.0.0.1
• import W2K3DHCP <scope>

All done!

Look at c:\windows\system32\dhcp\DhcpSrvLog-<Day>.txt file to monitor the DHCP requests…

Don’t forget to autohorize the new DHCP server and disable to exported scope on the old server!