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Problem with Exchange mail flow when DC is down

Posted on 2009-04-09
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Last Modified: 2013-12-23
Hello,
I currently have two W2K Domain controllers, DC1 (physical)  and DC2 (virtual).  DC1 is the primary DNS server, DC2 the secondary.  I have an Exchange 2K server that is configured to use DC1 as primary DNS and DC2 as secondary.  We're planning on decommissioning DC1 and replacing it with a new virtual DC (different name, hopefully the same IP if all goes well).  We'd like to keep the same IP so we don't have to change all of our static DNS configs.

I've moved all the FSMO roles off of DC1 onto DC2.  I've pointed the Exchange RUS services from DC1 to DC2.  In the past, I was told that there was always a problem with taking down DC1 and having mail flow correctly.  To test, I turned off DC1.  I rebooted our Exchange server and even though all services came up OK, mail would not flow.  As soon as I turned DC1 back on again, mail flowed successfully.

I didn't want to tinker with the DNS settings since it's still a production environment, but to me it seems like there is more going on.  It should just fail over to the secondary DNS if the primary is down, so I don't know if that's the issue.  Is there something I'm missing with Exchange and it's relationship to a "primary" DC?  Outside of the RUS services, I've found no reference to DC1 in the ESM.  

Any help would be greatly appreciated,

Thanks!
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Question by:mcascio
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Author Comment

by:mcascio
ID: 24108837
also, both DC1 and DC2 are Global Catalogs.
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Accepted Solution

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Mestha earned 250 total points
ID: 24109478
Rebooting the Exchange server should have forced Exchange to look for another server, which it would appear that it did. Otherwise the services wouldn't start. Exchange only works with Global Catalog servers.

Have you checked that there are no DNS servers set on the SMTP virtual server which are hard coded? Was it all email that stopped or just external email?

You haven't split the domain controllers up using sites and services, anything like that?
If you look at the domain controller config in the properties of the server in ESM, does it show both servers as global catalogs?

Simon.

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Author Comment

by:mcascio
ID: 24110423
Hello Simon,

I do not see any hardcoded DNS server entries in the SMTP virtual server.  When DC1 was down, all email stopped, both sending and receiving.

While I do have domain controllers in other sites, these two in question are in the same site.  Both servers are listed as Global Catalog servers.
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Expert Comment

by:Mestha
ID: 24111536
The messages should have queued, what did they say when they queued?
There is always a chance that there is something wrong with the second domain controller and Exchange cannot use it for some reason.

Simon.
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Expert Comment

by:ABLComputers
ID: 24115250
ON DC 2 check your DNS Settings. also take a look at your port forwarding on your firewall. The problem that you're having is a DNS problem. I would set DC2 as primary DNS and DC1 as Secondary. Most importantly you'll need to set a MX record pointing to DC2 (your Exchange server).
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Author Comment

by:mcascio
ID: 24129178
ABL,

Thanks for your comment.  However, DC2 is not my Exchange server.  DC1 and DC2 are domain controllers and my Exchange server is a seperate server (so 3 all together).

I'm going to try changing the primary and secondary DNS on the domain controllers, however that, to me, doesn't seem like enough to stop mail flow completely.  It should fail to the secondary DNS, but I could be wrong.  I'll be able to test this on Wednesday when we're able to take down all servers.
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Author Comment

by:mcascio
ID: 24129209
Mestha,

The messages did seem to queue as I received the messages I sent when DC1 was down (after I brought it back up).  Interestingly enough, when I looked in the queue I didn't see anything that alerted me to a problem.  I also looked in Event Viewer to see if there was anything there, and nothing.

I'll be able to investigate more this Wednesday and will let you know.

thanks
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Assisted Solution

by:ABLComputers
ABLComputers earned 250 total points
ID: 24129245
That's normally a DNS problem. Your Exchange server is probably using DC1 as it's DNS Server. Look into your DNS Setting on your Exchange Box. Try using nslookup command on that box and make sure that it can resolve with DC1 offline. You can start with disabling the NIC card on DC1 to avoid shutting down and restarting.

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Author Comment

by:mcascio
ID: 24129258
ABL,

Thanks.  I will try that and update on how it goes.
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Author Comment

by:mcascio
ID: 24218113
After I swapped the primary DNS with the secondary DNS, and after waiting a couple of minutes, mail started to flow normally.  Seems like that is all that needed to be done.  Thanks for everybody's replies.
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