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Archive for August, 2010

DHCP Scope reservations with Server 2008 R2

August 20, 2010 2 comments

Recently I helped someone with a DHCP reservation issue on Server 2008 R2. Here is the scenario:

  • 2 Windows Server 2008 R2 domain controllers, each with DHCP server enabled, this was recently migrated from Server 2003
  • The address pool for each DHCP server is different (same subnet however), each pool has about 200 addresses
    • This was done to all for continuance of DHCP services in case one DHCP server went down, 200 addresses is more than enough to handle all DHCP requests
    • The scope for each was setup with only the IP’s needed, however is setup with a /23 subnet
  • There are about 2 dozen reservations, which are mirrored on each server

Recently while trying to add a new reservation the following error was received “The specified DHCP client is not a reserved client.” When Googling this error it would lead one to believe this is because the DHCP server is not authorized, which is a valid reason for this error. However in this case the server is authorized.

Instead the problem was the Scope Settings. Previous versions of Windows Server would allow reservations outside of the scope IP address range. Server 2008 R2 no longer allows reservations outside of the IP address range. The fix to this problem was to use the entire subnet in the scope for each, and exclude everything that isn’t needed.

Here were the steps used to fix this issue:

  • Went into the scope properties and changed the start and end IP addresses so it would include the whole subnet.
  • Went into “address pool” for the scope and created a new exclusion
    • The new exclusion is for all addresses I do not want the server to dish out addresses for

After the above change, reservations were once again able to be performed on both servers.

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Categories: Microsoft Tags: ,