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Exchange 2010 DAG Question

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Exchange 2010 DAG Question - 2.Sep.2010 10:43:25 AM   


Posts: 16
Joined: 2.Sep.2010
Status: offline

Let me start by explaing the scenario.

I have a customer that has 2 sites in different geographical locations connected site-to-site via a VPN (Router to Router)

Site 1 - 192.168.1.x (houses EX1)
Site 2 - 192.168.2.x (houses EX2)

To increase resilience they have decided to opt for a redundant Exchange Solution, so if one sites internet goes down. They are still able to send / receive email at the other site.

My plan is to have an exchange server at each site with all the rolls installed with a DAG (both servers members holding the Mailbox Databases)

I would have two Mailbox databases named MBX-Site-1 & MBX-Site-2, EX1 would hold the active copy of the MBX-Site-1 Mailbox and EX2 would hold a passive copy.

Now I have a few questions:

Letís say the internet connection went down at Site 1. So the Exchange server at Site 2 cannot connect to the Mailbox on site 1, so would this mark the copy on the Exchange Server at the second site as the Active node ? Thinking about this how would EX1 ever know that EX2 now holds the Active copy of the database? Surely when the internet connection comes back online they will both think that they hold the active copy of MBX-Site-1?

Letís say that during this time users have sent some emails at site 1. Even though they are stuck in the Exchange queue as their internet connection is down, and as the secondary MX record for this company points to EX2 some emails have been delivered to Mailboxes on this server. Now when the internet connection comes back online at Site 1 are the Mailboxes merged as both Exchange Server think they hold the active copy?

Sorry I am asking a lot.

I need to know before I go ahead and implement and also find this very interesting as this is our first DAG


Post #: 1
RE: Exchange 2010 DAG Question - 29.Oct.2010 3:11:36 PM   


Posts: 2335
Joined: 12.Apr.2008
From: The Netherlands
Status: offline

Let's start with the last point: database can't be merged because there can only be one active copy, else you got the split-brain issue.

It depends which database will be active. This will be the site which has quorum, in most cases the primary site, this is also best practice. In your situation this will be the site which contains a DAG member and the file-witness share.

Starting from SP1 you can define a second file-share witness. If not installing SP1 you will need to manually active/define a file-share witness in the second site. In your situation it is advisable to enable DataCenter Activation Coordination (DAC) Mode to prevent split brain.

For more information have a look at this article:


Best regards,

Johan Veldhuis

Visit my Exchange blog

(in reply to online_pc_support)
Post #: 2

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