Exchange DAG Across Sites Configuration

For WAN link potential failure concern, please reference the Site Resilience Configuration part in article --

Exchange 2010 Mailbox Server Role Requirements Calculator

The three types of DAG -- Active/Passive User Distribution Model, Active/Active user distribution (Single DAG) and Active/Active user distribution (Multiple DAG). As the blog explained, the Active/Active(Single DAG) does does have a single point of failure (potentially), the WAN connection.  Loss of the WAN connection will result in the mailbox servers in one of the datacenters going into a failed state from a failover cluster perspective. But the Active/Active(Multiple DAG) remove single points of failure (e.g., the WAN). Hope it is good for understanding your second question.

For the first question, I am not quite clear on the scenario. If you can explain more?

Lu

Thanks Louis for your answering!

File Share Witness used to check for presence or absence of the mailbox servers, and then determine if the database should be mounted or not.  If the DAG is at one site, both Primary & Alternative FSW should be setup at the site.  If the DAG is accross to 2 site, should we set up both Primary & Alternative FSW at each site?

"As a best practice, in an environment where a DAG is extended across multiple datacenters (and Active Directory sites) and configured for site resilience, we recommend that you use a witness server in your primary datacenter (the datacenter containing the majority of your user population). If each datacenter has a similar number of users, the datacenter you choose to host the witness server will be considered to be the primary datacenter from the solution's perspective. If the witness server is in the datacenter with the majority of the client population, the majority of clients retain access after a failure."

http://technet.microsoft.com/en-us/library/dd298065(EXCHG.140).aspx

Requirements for the witness server:

• The witness server can't be a member of the DAG.
• The witness server must be in the same Active Directory forest as the DAG.
• The witness server must be running Windows Server 2008 R2, Windows Server 2008, Windows Server 2003 R2, or Windows Server 2003.
• A single server can serve as a witness for multiple DAGs; however, each DAG requires its own witness directory.

Thanks Bryan for your helpful answer!

It seems that a witness server in a third datacenter is not recommend as this configuration doesn't provide you with greater availability.

It's important that you examine the critical path factors if you use a witness server in a third datacenter. For example, if the WAN connection between the primary datacenter and the second and third datacenter fails, the solution in the primary datacenter will become unavailable.