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ISP Changeover w/ internally hosted Exchange and Web Services

Posted on 2007-12-04
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Last Modified: 2013-12-24
I am looking for expert advice or recommended steps for changing ISP's (and IP addresses) to minimize interruption in services.  Please take into consideration that there is an exchange server and a number of web servers hosted internally.
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Question by:rymattcart
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by:weareit
ID: 20403748
Internally nothing should really have to change (with the exception of the settings on your router/firewall).

There are two basic methods to this madness. ;)

First Method:
Quick, dirty and cheap.

Modify your DNS records on your domain host so that any host, cname/alias and or mx records point to the new IP assigned by the new ISP.

Downtime will occur but if you plan on a weekend/latenight when network usage is at its lowest, you should be able to make the changeover as smooth as possible.


Second Method:
You can minimize downtime by using this method but it is a little more costly.

First have your new ISP run the new connection to your building and activate it.  Add a second router/firewall to your network and connect the new ISP connection to the WAN interface on this device [Leave your current ISP and router in place].  At the same time access your DNS records for the external domain and modify the TTL to it's lowest setting (generally 15 - 30 minutes).  Make sure you do this for all records.

After you have the new router/firewall in place, make sure you reimplement any pinholes/port forwards in the new firewall.

Modify your external DNS domain host, cname/alias and mx records to point towards the new ip address.  This way as the records update or propogate accross DNS servers the clients will all be pointed towards the new IP address that your ISP has assigned.

After about 2 - 3 days you should be able to safely remove the old router and ISP from your system without any blow back.

One final thought, in either case you may have to contend with SSL certificates that are assigned to the previous IP (they shouldn't have been but just in case) and reverse DNS records.  In either case I would recommend checking both to ensure that your bases are covered.

-saige-
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weareit earned 250 total points
ID: 20403783
Obvious changes I failed to mention.

On the server, you will need to modify the following:
At least, the DNS entries on the NIC(s) so that they point to the DNS of the new ISP
If you use forwarders on your DNS server, these will need to be modified
If you use the second method, DHCP options to include the second router
If you are using your ISP as a smart host on your Exchange server, you will need to modify the smart host depending upon if your new ISP uses one.  If they do not you will need to remove it all together when you retire the old ISP.

-saige-
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