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Switch DHCP Clients From One Server To Another

Posted on 2011-09-19
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Last Modified: 2012-05-12
We are planning on removing our DHCP server (Windows based) and replacing it with a Barracuda NG which will hand out the DHCP addresses instead.  What is the best way to have the clients disconnect from the Windows-based DHCP server and then reconnect to the NG DHCP server seamlessly and without major interruption?
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Question by:Steve215
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by:dumpsterdivingdave
ID: 36560803
How many clients do you have and what is the range of IP addresses in the DHCP scope?  

One thing you may be able to do is set your clients to obtain IP addresses from the beginning of the scope from the current server, and the end of the scope from the new server, and then once the new server is in place, re-expand the score.  For example:

current scope on current server is 10.1.1.1 - 10.1.1.100 with 50 clients
Change scope on current server to 10.1.1.1 - 10.1.1.50 and set scope on new server to 10.1.1.51 - 10.1.1.100.  Once all clients are located in the new scope, you should be able to bring down the old server.  Since the addresses are all still on the same subnet, there will be no disruption of service.  Once the lease on the address expires, they will obtain a new IP at the end of the scope from the new server.  Once all clients are on the new scope, you can extend the scope back to 10.1.1.1 - 10.1.1.100.
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by:Steve215
ID: 36560958
Address pool on the Windows server is 10.1.15.1 to 10.1.28.253.  Scope is the same on the NG device.  Figure 50 clients connecting.
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by:dumpsterdivingdave
ID: 36560975
What is the subnet mask?  255.255.0.0?
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by:Steve215
ID: 36560995
16-bit mask so yes, 255.255.0.0.
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by:dumpsterdivingdave
ID: 36561034
If you only have 50 clients, I would change the scope on the existing server to 10.1.15.1 - 10.1.15.75.  That should give you plenty of room to start.  On the new server, set the scope to 10.1.15.76 - 10.1.28.253.  Once all of the clients have IP addresses from the existing server, you should be able to take that server down.  Hook up a new client and ensure that it gets a IP from the new server obviously before you do anything with the old one.  After all of the leases have expired, the clients should pull new addresses from the new box.  Once all of the clients are on the new box, you can modify the range back to the 10.1.15.1 - 10.1.28.253.

This is assuming that there is no pattern to how the IP addresses are assigned.  If there is a pattern or reserved assignments, the process will be a little less straightforward, but still the same concept.
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by:Steve215
ID: 36562025
OK, so on the Windows server I set the IP Range to 10.1.15.1 - 10.1.15.75 and on the NG firewall the scope is now 10.1.15.76 - 10.1.28.253.  For some reason, clients are still pulling from the Windows DHCP server and not the NG. Is there a way to prevent this?
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by:dumpsterdivingdave
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You should be able to just take it off the network and they should start pulling from the NG server as long as everything is configured correctly on the NG server.
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yo_bee earned 500 total points
ID: 36562290
Or stop the DHCPServer service on the server.
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by:yo_bee
ID: 36562489
Just out of curiousity, why such a large scope for only 50 clients.
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by:yo_bee
ID: 36563716
note: make sure that the service is set to Manual or Disabled and still not Automatic or you will be scratching your head after a restart of the server.
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