Migrate clients to a new DHCP server

We’ve decided to migrate our DHCP services off the domain controllers and onto a DHCP failover cluster.  We needed to find a way of getting the clients to migrate over without rebooting them and without the sledge hammer approach of turning off the old DHCP service.  We came up with a slow and safe method for forcing the clients to migrate with minimal impact.  

First we reduce the scope on the existing DHCP server so there are no extra IP addresses to give out.  For example we reduce the scope on the old DHCP from 1.1.1.100 – 254 to 1.1.1.100-200.  Then we create a scope on the new DHCP cluster with a range of 1.1.1.201 – 254.  Next we reduced the lease time on the old DHCP scope to 5 minutes and used PsExec to force clients to renew the lease.  Here is a link to PsExec in case you’ve never heard of it before.  http://technet.microsoft.com/en-us/sysinternals/bb897553.aspx  Now the clients will renew their leases every 5 minutes.  We only did this because we didn’t want to wait.  If you’re patient you can omit that step.  Finally, in the old DHCP server, we put in exclusions for the IP addresses of the machines we want to migrate.

When the client tries to renew the lease, it requests the same IP address.  When the old DHCP server responds that it can’t give out that address the client will broadcast a DHCP request.  Since the old DHCP server doesn’t have any free IP addresses it won’t respond, but the new DHCP server will.  The client will get a new IP, register itself in DNS and keep working with very minimal interruption.

Then it’s just a matter of  continuing to reduce the scope on the old DHCP server, widening the scope on the new server and migrating clients.