I recently stood up a secondary DC. It authenticates users to the domain when i turned the primary DC off, but I didn't have the DHCP server running on the secondary. How would i use the secondary DC as a backup DHCP server?
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What I meant about rebuilding the server was in terms of domain controllers. Since active directory information is replicated throughout the domain, if a machine were to fail, all that would be required is to rebuild a machine and promote it to become a domain controller (and sieze and FSMO roles on the original server). But that has little to do with the issue of redundant DHCP servers.
That would be best addressed by the comments I included at the end about the 80/20 rule. Of course, if you wish to have both servers up and running full time, then you can split the pool down the middle 50/50, as long as only one DHCP server provides the address (which is handled automatically by the DHCP protocol).
Something else to be sure of though, is setting conflict detection on your DHCP servers.
Right click the server in the console, select properties, advanced, set a number for "Conflict Detection Attempts".
This tells the DHCP server to attempt to validate that the address it wants to give to your client is not already in use on the network. If it sees that another machine is on that address, it finds another one.
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by: dhoffman_98Posted on 2009-02-06 at 10:54:06ID: 23572826
Well, in terms of best practices... you wouldn't. Domain controllers typically should not host any other applications aside from DNS. The reason for this is that when you start hosting other applications on them, they become more of a management issue in terms of backing up and patching.
In some environments with multiple domain controllers, they are often not even backed up. If a DC fails, no problem, just rebuild it and promote it and it's a new domain controller again.
Probably not what you wanted to hear, so here's some more information.
For DHCP to be supported in a failover type situation, you need to cluster (again something you can't or shouldn't do with domain controllers). The reason for this all falls back to the D in DHCP... Dynamic. At any time, the lease database can be changed on a DHCP server.
Let's assume you have a client that logs into the domain and gets a DHCP lease. The database that holds that lease information remains on the DHCP server. Now say you fail over to the other DC, and even had DHCP running there. The chance that you have up-to-the-minute copies of your DHCP files is very slim. So when a client logs in and gets an address from the new server they could be assigned the same address, and thus causing conflicts for the users.
Enter the 80/20 rule...
If you run DHCP on both machines, then you can set up the allocation pool to only provide 80% of your available addresses on the main host. If the main host fails, then the secondary host can come up, and the other 20% of your available addresses would be hosted there. That way there is never a conflict because two different DHCP servers gave out the same address.
So, for example, your entire range of dynamic addresses is 192.168.1.101 to 192.168.1.200...
Your first server would set up a pool from 192.168.1.101 to 192.168.1.80
And the other server would have a pool from 192.168.1.81 to 192.168.1.200