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New DHCP Server

Posted on 2014-11-19
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Last Modified: 2014-11-20
Hello all and thanks for having a great resource.

I have a complicated question to ask and hope someone has experience that can help.

Current Situation: My network has a W2K3 server running Active Directory for a no longer used AD Domain. I would like to get rid of this server but it is also running DHCP for my campus network. The previous engineer used the entire class B address as the scope and then excluded addresses that he wanted to use for static addressing. I would like to change this as I do not believe it is the correct way to do it.

New Situation: I have a W2K8R2 server running as a Domain controller for the AD domain that we are currently using. The subnet for the campus is the same. I have created a DHCP scope using 10.50.7.1 - 10.50.8.254 from the range 10.50.0.0 as the scope of addresses. I have also updated the DNS and other options to match what is currently needed versus what was needed in the past. I have not yet authorized the new DHCP server. The servers are on the same subnet.

Question: Is it just as simple as turning off the DHCP server service on the old server and Authorizing the new DHCP server and all is well? Will the clients get an address from the new DHCP server when they cannot reach the old one?

If not, what would be the proper way of moving to a new DHCP server without migrating the current settings from the old one?

Thanks for any input.
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Question by:juggalo1
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by:lruiz52
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You should be fine just switching to the new server. here what i would do;

1. On the old server, shrink the lease time to 30 minutes.
2  Export/import the database
3. Reset the lease time on the new server.
4. Remove leases on the old server then deauthorize the server.
5. Authorize the new server
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by:Hypercat (Deb)
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Assuming the IP address ranges are the same, you would need to first make sure that you've created address reservations for any devices that were excluded from the old scope.  That way they can retain their old IP addresses (either static or reserved) without any potential conflicts.  Then I would set the lease time on the old server to a shorter period, say one or two days, and let that change take effect across the network. This ensures that the existing addresses will be renewed quickly by any workstations that aren't manually restarted when the new server is brought up. Your new server will, of course, have a standard lease time of 7 or 8 days that will take effect when the lease is renewed for the first time on the new server. Lastly, I would make the changeover at the time of least traffic on your network, and when most workstations are likely to be idle or shut down (shut down being the best of all possible worlds).
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by:juggalo1
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Thank you for the responses. I would like to change servers and not move the old database to the new server. I have accounted for the reservations but I would like to start with a clean slate with a new scope and options. My fear is that once I take the old DHCP server offline the clients will no longer be able to see it and may not be able to obtain a new address from the new server because they are looking to renew an address from the old server. None of the clients will have a current address from the new scope. Is this something that will fix itself when the clients can no longer see the old server?
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Hypercat (Deb) earned 500 total points
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The clients will find the new server if they have been restarted or were shut down during the transition from the old to the new server. However, clients that are up and running may keep trying to contact the old server, so that may cause problems.  They would need to be restarted or have "ipconfig /release" and "ipconfig /renew" run to find the new DHCP server.  That's why I said that "shut down" would be the best of all possible worlds.  I don't know how large your network is but since you have a fairly large subnet I would have to assume that you have more than a couple of hundred devices on your network. How much control do you have over the behavior of your users and whether or when their workstations are shut down or restarted?
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by:Hypercat (Deb)
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This diagram of the DHCP address process indicates that the client will re-initialize if it can't find the DHCP server it has previously used when it's trying to renew. Read from Figure 4.4 down:

http://technet.microsoft.com/en-us/library/cc958935.aspx

The problem I see is that users may experience some noticeable lack of network connectivity when their workstation cannot immediately renew its address and has to reinitialize to find the new DHCP server.
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by:juggalo1
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Thanks for the input. I will make the changes over the Christmas holiday and ask everyone to shut down their machines before leaving for the weekend.
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