The Sysadmins

Tips and tricks from the Sysadmins

Configuring DHCP Split-scope in Server 2008 R2

Split-scope is a quick and easy way to provide redundancy and load balancing for DHCP in your network. Server 2008 R2 introduces a handy wizard for creating a split-scope and saves some administrative effort, however it can only be used if both servers are running R2.

DHCP6

Here are two ways in which you can utilize split scope.

Primary / Backup

In this scenario, the 1st DHCP server will dish out all leases and the 2nd DHCP server should only be utilized if the 1st server fails. You can accomplish this with the “Delay in DHCP offer” setting when configuring split-scope (prior to to 2008 R2 you could use the “Conflict detection attempts” for the same effect). DHCP clients accept whichever server responds first to the DHCP DISCOVER packet, so we delay the response from the 2nd server which allows the 1st to respond and therefor serve the client.

Here is the 80/20 rule in action

DHCP 80_20

Now, as you’ve probably guessed the 80/20 is rather arbitrary, and can be shuffled around to suite your network.

Load balanced

For this method you’d leave the “Delay in DHCP offer” equal when configuring split scope, which would give both servers a 50% chance of dishing out leases. You’ll probably want to set the scope to 50/50 and I’d make sure that each 50% could serve the majority if not all of the clients in your network.

Configuring Split Scope

Here we will setup the 80/20 rule. In versions prior to Server 2008 R2, you would have to manually configure the scope on the 2nd server, the wizard included in R2 does this for you.

At this point you should have 2 DHCP servers configured. The 1st server should have a scope with the full range of addresses, and the 2nd server should be scopeless. In this example I’ve configured the scope on AD1 for 10.0.0.100 – 10.0.0.200 and added both DHCP servers to the DHCP MMC console.

Right click the scope, select advanced and then Split-scope

DHCP1

Add the 2nd DHCP server

DHCP2

Adjust the split, here we choose 80/20- note it will show you amount of addresses each server will have and the excluded range

DHCP3

Here is the delay in DHCP offer I mentioned earlier, for 80/20 you’ll probably want to use 1000ms for the 2nd DHCP server. If you wanted to load balance, leave both of these at 0

DHCP4

The scope will now have been added to the 2nd server, to finish the setup, right click the scope and choose activate

DHCP5

Video

In this video I will walk you through configuring the DHCP role and split scope.

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