Server 2012, Server 2008, migrating DHCP - high and low servers

The environment is one single domain, four (4) Server 2008 domain controllers, two sites, DHCP everywhere!

Site 1 has the FSMO role holder DC in it and DHCP is running on that server issuing IPs on the high side of our subnet.  It's an 80/20 split for the most part.  The second DC in site one has DHCP running and it's issuing IPs on the low side of the subnet.

Site two is set up the same.  One DC issuing IPs on the high range and one issuing IPs on the low range.  

So, all four existing DCs run server 2008 STD and all four are DHCP servers.  UGH!  Who thinks this stuff up?

My job is to bring online four new 2012 R2 domain controllers to replace these four older servers.  I would like to use DHCP failover, instead of this high/low thing they have going on.  The problem is how to do that.

How is the best way to do this?  THAT is my question.

Option 1 is to bring up one new DC at a time and do a one to one swap.  Turn off DHCP on server 1, back it up and restore it to its replacement and start DHCP on the new server.  That way I get my current leases, and stand the least likely chance of problems.  Of course, when I'm done, I still have four DCs with DHCP and the whole high/low scope going on, but I could deal with that as a new problem.

Option 2 is to take my lease times down to five mins or something REALLY low.  After I spin up my new DCs, I could install DHCP, set up failover and then activate the new servers moments after killing the service on the four old servers.  For some reason, that seems problematic.  

At any given moment, I have 600 + active users in play so I'm trying to mimimize down time, duplicate IPs, etc.

Thoughts, suggestions, recommendations on how best to migrate all of this?

Thanks

Cliff
crp0499CEOAsked:
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Cliff GaliherCommented:
IF you really want to minimize downtime, *don't* lower your lease time but follow the rest of your second plan.

1) Get your new 2012 DCs online.
2) Install the DHCP role on one.
----
begin maintenance window (only a few minutes with proper planning.)
3) Disable *both* existing DHCP servers.
4) Enable the new DHCP server and configure options.
Thus ends maintenance window
---
Because of how DHCP works, when machines attempt to renew a lease, they'll just get a new one. And because you shut down your old DHCP servers, they won't be trying to renew existing leases either.  And the resiliency of the Windows and DHCP stack will take care of address conflict resolution preventing the new DHCP server from handing out conflicting addresses. Most machines will even end up with the same address they had before.

Then set up your second DHCP server, run the failover wizard, and you have redundancy without the 80/20.  Cranking down the lease time doesn't really provide much benefit. It won't hurt either, but is unnecessary and may create added network traffic.

Of course the "  For some reason, that seems problematic. "  comment is interesting, as you didn't comment on why you saw it as problematic or what issues you've read about, heard about, or thought about to make the approach undesirable.

Given that you are wanting to go away from the split DHCP scenario, this would be the better approach as you aren't doing a straight migration anymore.
crp0499CEOAuthor Commented:
Taking the lease times down seems problematic to me.  Someone suggested that to me.  

In my head, if I take my lease times down to say five mins, then I have a five minute window to make the switch.  If I leave it at three hours (that's what it is now), I have a three hour window to make the switch, which is more than enough time.
Cliff GaliherCommented:
Keep in mind that machines will try to renew their leases BEFORE they expire. And even at that, unless you got every machine to request their lease at exactly the same time *and* started your maintenance one second after the renewal period, you won't get three hours from a three hour lease.  

A maintenance window is a maintenance window, no matter how many knobs you tweak.  Plan on making the change during off-peak hours.  Communicate the change to the parties who might be impacted. And plan on a minor disruption.  I see no reason to crank down the lease. But don't plan on a transparent, error free move with a full three hours to do so. That just isn't how DHCP works.

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crp0499CEOAuthor Commented:
Thanks Cliff
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Windows Server 2012

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