DHCP Split-Scope wizard
Today, I would like to introduce new feature of DHCP role in Windows Server 2008 R2. This feature is called “Split-Scope“.
This is great improvement in that role for DHCP administrators. In earlier Windows Server versions, administrators needed to calculate by themselves how to split DHCP scope(s) between two DHCP servers for redundancy. In small networks where basic IP addressing scheme was used, that was not big problem. In large networks where VLANs were used and IP addressing scheme was more advanced, that was horrible.
DHCP servers redundancy in many companies is very important and the lowest cost of maintaining them is also crucial. So, DHCP clusters were not an option in many organizations.
Administrators needed to split scope according to their needs or Microsoft DHCP Best practices using one of recommended rules on two separate DHCP servers in a network
- 50/50 (half addresses on one DHCP server and another half on the second server)
- 80/20 (80% addresses on one DHCP server and 20% on the second server)
This required from them making exclusion ranges in scopes on DHCP servers and duplicating reservations on both servers. That was a lot of work to do! Especially, if a network had more than one scope.
When you wanted to check from which DHCP server computer received a lease you had to verify both DHCP servers lease database or run ipconfig /all command on a client to see which DHCP server isssued the lease.
DHCP servers offer leases in the same time and computer could select any of them. Mostly, the first offered lease is accepted but you cannot be sure which server offered it as the first.
From now, you don’t have to worry for all of these “limitations”. When you have both DHCP servers runnig Windows Server 2008 R2, you can simply achieve that using Split-Scope wizard. All necessary calculations will be done automatically. You can forget about making exlusion ranges in each scope on the second DHCP server and you can also forget about duplicating reservations on another server. Everything is done by Split-Scope wizard.
Another new DHCP feature is delay in lease offers. You can set up 1000 miliseconds (1 second) delay for other DHCP server and you can be sure that all of your clients will get lease from the first server.
Let’s start to show, how we can use that wizard. First, you need to install DHCP role on both Windows Server 2008 R2. Now, you can configure a scope on a server.
In this example, I will show you, how to split 192.168.1.0/24 scope between two DHCP servers (DHCP01 and DHCP02)
Log on to DHCP01 and run DHCP management console. Create a scope (if doesn’t exist) or select the existing one which you want to split.
Click right mouse button on selected scope and choose from context menu “Advanced -> Split-Scope”. You will see a wizard which helps you splitting a scope.
On “Additional DHCP Server” screen click “Add server” button and select the second DHCP server to which you want to split the scope
You can search for any DHCP server in your netwok using “Browse” button or select one of discovered from the list below
Now, you need to choose split scheme. You can choose 50/50 or 80/20 (default option) or even your own. To use other option than default, type other values (in percentage) in fields or move scroll-bar in the proper direction to adjust them ad-hoc
As I mentioned earlier, you can set up delay in leases offer to make sure that specified DHCP server will answer delayed and lease will be issued from “primary” server. To set up delay on DHCP02 in “Added DHCP Server” box, type 1000
apply settings by clicking on “Finish” button
verify if everything went well and close wizard
Check “Address pool” on current DHCP server and you will notice that DHCP Split-Scope wizard has done the job for you!
and for test, go to DHCP02 and check if scope is also split there
Notice, that scope on your DHCP02 is deactivated by default. If you wish to use it, you need to activate it first.
That’s all about configuring DHCP Split-Scope wizard.
Author: Krzysztof Pytko