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I am working in a business that is planning to switch ISPs really soon. We will need to change a lot of information in our DNS, WAN and Wan-WiFi.

I have some questions for you guys that can help me, and maybe, other people, to plan correctly the change from our current ISP to a new one.

First of all, DNS...

What should I know, when we will receive the new IPs from the ISP? What are the major changes that I need to take in consideration?

We have an Exchange server (OWA), VPN, two sites linked with a Wireless connection (10km away) Citrix servers, VMWare, Exchange, Domain Controller (That are linked from each place) and other important servers.

What are the crucial steps or information that I need to know to avoid any downtime here?

The business is 24/7.

Regards, David.

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UPDATE: Thank you very much to everyone that answered this question. Basicaly, I would need to know the steps, step by step that you guys would do or plan when moving from an ISP to another one. What would be the first step to check, the second one and so on. Thanks a bunch! David. – r0ca Aug 27 '09 at 0:33
up vote 6 down vote accepted

This is one of the reasons why people get portable IP addresses -- so they can avoid the pain of reIPing all your hosts.

You hopefully have everything behind a firewall so you can just add / change NATs on the firewall.

Then just run both ISPs in tandem and connect them both to your firewall, update the DNS records and the default route of your firewall, and everything should be good to go. Anticipate taking 2-3 weeks doing this so you have time to test and verify everything one step at a time. You want to avoid a flag day if at all possible.

You'll want to shrink the TTLs of your DNS records well in advance of times you plan on making these changes.

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+1 for shrinking TTL. In my experience most network operators do actually honor the TTL given by the name servers. – wolfgangsz Aug 26 '09 at 18:10
+1 likewise, DNS migration can be the single biggest pain in this exercise. – 21st Century Moose Aug 26 '09 at 18:36
Not sure whether I got more enjoyment out of flag day or TTL shrinking. ;) – Kara Marfia Aug 26 '09 at 20:23
Perfect! I will make sure DNS and TTL shrinking are ok. Kind regards! David. – r0ca Aug 27 '09 at 0:24
I have added the update. If you guys want to help me out, please feel free to! Always appreciated :) – r0ca Aug 27 '09 at 0:37

The closest you'd be able to get to avoiding all downtime would be to have both ISPs set up in tandem, and managing the transition over time. Otherwise, you need to prepare management for the certainty that you'll be down while the new DNS records propagate, and probably for additional time while you figure out what's been missed.

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Thank you for this answer. We decided with the ISP to have a 30 days grace after migrating to the new ISP. – r0ca Aug 27 '09 at 0:23
May your switch be extremely uneventful. Boring, even. ;) – Kara Marfia Aug 27 '09 at 13:42

first of all as chris suggested above, lower TTL values for the domain/s you have in your dns zones

second, it is a must you have both isp's for a while (in case you have problems, revert to the current working settings)

third, there is no such thing as "no downtime", but it can be done transparently for the users (they won't sense the change in settings) if this is done during non-working hours; I would suggest the weekend nights (you have 3 windows - friday-saturday, saturday-sunday, sunday-monday - when you can play with the settings); those work periods are to be announced to anyone interested, or at leas high level management

fourth, there is no shame in asking for somebody's help in the remote locations, at least until the connection is made

in my opinion, point 2 is the most important, if anything goes bad, revert to the working state


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