Windows 2008R2 DHCP Split-scope

DHCP is the mechanism that gives most client these days the tools (ip address) for connectivity. Most companies however do not realize the importance of the DHCP service and do not cluster nor have another server as standby when the DHCP server fails. If the DHCP server fails, most clients will not receive a new ip address and will seize to work on the network. While clustering is improved in Windows 2008 and sort of made easy for administrators, most of them are reluctant to implement a failover cluster for DHCP. In Windows 2008 R2 we have Split-Scopes, remember the 80-20 rule for DHCP servers, it kinda the same, but then made easy. For this article we have a single domain controller and two Windows 2008R2 servers.

To install the DHCP service, use the roles wizard from the server manager or the initial wizard page. Select the network card that the DHCP must be mapped to and click next. Set the parentdomain name and the DNS server, do not select WINS and do not add any scopes yet. Disable DHCPv6 (we do not use them for this demo) and click next. Skip the authorization for now and click next to finish the install. Perform the above steps on both servers.

Now we create a scope on the first node by selecting IPv4 and then Action New Scope. Give the scope a name and select next. Now we need to enter the starting point and endpoint of the range. Give the full range of the zone (so 100%). You can add exclusions but we skip this step for now, click next and next. Configure your options like DNS, Gateway etc. Activate the scope and click next.

Now right click the newly created scope, select Advanced and then Split-scope. The wizard for the split-scope will start and will ask for the 2^nd DHCP server that will offer this scope.

Fill in the name of the second DHCP server. When you click next you will get a slide-bar with the percentages that will be applied to the DHCP scope.

Next the Delay in DHCP offerings will show up. It will be delay that will be added to one of the nodes so that they will never respond simultaneously.

Click next to review the settings and click finish.

Now the question is why to use this wizard and let that one configure the zone on both machines with exclusions for the percentage of the zone to each node. If you manually set the 80-20 (or other percentage) the way computers will receive new addresses change (eg set DHCP01 to host 10.0.0.100 �C 10.0.0.179 and DHCP02 to host 10.0.0.180-10.0.0.200).

Let’s say Client A has received an address from DHCP01 and it is time for renewal. It will send a DHCP renewal request on 50% of the IP lease time. DHCP01 is not able to respond and if DHCP02 does not have the exclusion range, it will immediately NAK the request and force the client to do a normal DISCOVER request. Therefore the 20% of the DHCP02 server will be highly utilized and all addresses will be given out within a short time. If DHCP02 will not answer directly to the renewal request the client will continue to use the IP address until the lease expires completely. After that has occurred, the client will then send a DISCOVER automatically. Hopefully the Administrator has fixed DHCP01 by that time.