While we’re on the topic of DHCP and split-scopes, it would be good to mention reservations. Reservations are used to assign a DHCP client a specific IP address, controlled via MAC address. The value of this is that you get the benefit of dynamically assigning dedicated IP addresses while maintaining manageability of global options such as designated DNS servers. It’s much easier to change the IP address of a DNS server in a single DHCP scope than it is to change on dozens or hundreds of hosts. That said, I still firmly believe in static IP assignments for critical server functions. I wouldn’t want my production SQL cluster or domain controllers running with DHCP reservations, but for ILO/IPMI, less critical hosts, etc, why not? I have had great success using reservations on management devices.

The complication occurs when you throw split-scopes into the mix. Even though a given DHCP scope is carved among several servers and the reservations will fall within one of those splits, you still need to provide redundancy for the reserved hosts. This can be accomplished by entering the reservations on each DHCP server. This works because DHCP servers can still serve reservations that fall outside of their active address pool.

You could manually enter each reservation on each DHCP server, backup/restore the DHCP database, or write your own script. Luckily the Microsoft DHCP team has already written a script for us: the DHCP Reservation Tool. It’s a simple VB script that given the right arguments can copy, sync, dump, or enter from input file all defined DHCP reservations between the servers you designate. Download the script below and rename to .vbs. I annotated it to include the command syntax. The process is called by invoking the script with the Rmanager command followed by specifying the operation and arguments. See the remarks in the script or the DHCP team post for more specifics. This worked fine in my Server 2008 R2 environment. Example:

cscript dhcp_res_mgr.vbs Rmanager –sync <source IP> <Destination IP> <all>

   

References:

DHCP Team Blog

Splitting DHCP scopes is a best practice that has been around for a very long time. The purpose is to spread a given IP scope across multiple DHCP servers in multiple locations for redundancy and load balancing. Let’s say that your company has two locations, A and B; Location A has two DHCP servers, location B has one. The IP schemes are different at each location but you want to ensure that DHCP services are available for all IP scopes at both should any of the DHCP servers ever have a problem. The best way to achieve this is to spread the scopes across all three servers. This is done by defining the scope on each server then configuring exclusions for the IP range that each server should not serve, to avoid overlap. While this was all possible in previous versions of Windows Server it had to be configured manually.

Server 2008 R2 makes this easier by adding a split-scope wizard in the DHCP MMC. First configure a DHCP scope as you normally would with the first and last IP address that you want to be dynamically assigned. Leave out the range of the IP space you intend to use for static assignments as we are only concerned with dynamic assignments. Once the scope has been defined and created it can be split by invoking the split-scope wizard in the advanced sub menu of the scope.

Once you have Selected the authorized server that you would like to add to the scope, the wizard will allow you to define the percentage of the address space that you want to give to the new server. Move the slider left and right to adjust the pool given to the new server you are configuring. If you plan to split the scope across 3 or more servers, do some quick math to determine how many addresses each should serve if you want to split the scope evenly. In the example below I am keeping 33% of the address space on the original server and giving 67% to the new server because I plan to split the scope again. The bottom portion of the window displays the exclusions that will be created on both the original host and new server.

Next you can configure a server offer delay so you can help shape which servers you want to be able to respond in which order. I intend to make my third server primary for this scope so will configure delays for both the host and added DHCP servers.

Before executing the split you are presented with a summary screen to ensure that your configuration is correct. Once you execute, the wizard will create an exclusion on the host server as well as create the new scope with exclusion on the added server. A time saver for sure!

The wizard runs through each required step and displays the result of each.

On the newly added server the scope is not automatically activated so if you made a mistake you can easily start over by deleting the new scope on the added server and the exclusion on the host server. Repeat this process on the second server to split the scope again ensuring an even split across all 3 servers. Once your configuration is solid all you need to do is activate the scope on each server you added it to.

Here is how the scope and exclusions look on each server after the split.

Original host:

2nd added server:

3rd added server:

As you can see best practice is automatically applied by defining the entire DHCP range on each server, then limiting the assignable range on each through an exclusion. Future adjustments can be made easily by changing the exclusion.

As a final step, make sure you have IP helpers configured in your switch on the proper VLAN interfaces so it knows where to send your client’s DHCPDISCOVER messages.