The above network design is no good for DAG. If DC is down, EX2 does not work.

When EX2 is down, inside message delivery has trouble.

I implemented two DCs with EXCHANGE installed and configured with DAG. It does not work.

Get-MailboxDatabase | Sort Server | FT Name,Server

New-DatabaseAvailabilityGroup -Name DAG1 -DatabaseAvailabilityGroupIpAddresses 191.121.3.3 -WitnessDirectory c:\myWitness -WitnessServer FW.1ASK2.COM
Add-DatabaseAvailabilityGroupServer -Identity DAG1 -MailboxServer Ex1
Add-DatabaseAvailabilityGroupServer -Identity DAG1 -MailboxServer Ex2

Add-MailboxDatabaseCopy -Identity 'Shaw' -MailboxServer 'EX2' -ActivationPreference '2'
Add-MailboxDatabaseCopy -Identity 'Telus' -MailboxServer 'EX1' -ActivationPreference '2'

After the Database Availability Group is populated with member servers, you can add a new database to it, too.

new-mailboxdatabase -Server 'EX2' -Name 'test2' -EdbFilePath 'C:\Program Files\Microsoft\Exchange Server\V14\Mailbox\test2\test2.edb' -LogFolderPath 'C:\Program Files\Microsoft\Exchange Server\V14\Mailbox\test2'

mount-database -Identity 'test2'

Create two new mailboxes in Telus database
$password=Read-Host "Enter password please" -asSecureString
New-Mailbox -Name 'Jeff' -Alias 'Jeff' -UserPrincipalName 'Jeff@1ask2.com' -SamAccountName 'Jeff' -FirstName 'Chris' -Initials '' -LastName '' -Password $password -Database 'Telus'
New-Mailbox -Name 'Chris' -Alias 'Chris' -UserPrincipalName 'Chris@1ask2.com' -SamAccountName 'Chris' -FirstName 'Chris' -Initials '' -LastName '' -Password $password -Database 'Telus'

Note: RPCClientAccessServer

To access mailboxes in Telus database, the clients (outlook) must go through EX2.

Set up outlook clients.
Vista workstation, configuring Outlook 2003 for Jeff@1ask2.com

Vanarts-PC.1ask2.com,configuring Outlook 2010 for Chris@1ask2.com

Test Database switchover

Move-ActiveMailboxDatabase Telus -ActivateOnServer Ex1 -MountDialOverride:None

After the switchover, shutdown the ex2 computer. Even though the Telus database is mounted on Ex1, both Outlook clients cannot connect to their mailboxes. Why?

The RpcClientAccessServer is still set to EX2.1ask2.com.

Move-ActiveMailboxDatabase Telus -ActivateOnServer Ex2 -MountDialOverride:None
Set-MailboxDatabase telus -RpcClientAccessServer fw.1ask2.com
Reconfiguring both outlook clients,

Move-ActiveMailboxDatabase Telus -ActivateOnServer Ex1 -MountDialOverride:None
Shutdown Ex2.1ask2.com
Both outlook clients work.

For Database Availability Group, even though it is not necessary to have the exchange server to be installed on Witness server, it is better to have it installed as CAS. If just two exchange servers, one server is down, you have to redirect the RpcClientAccessServer.

For example,

Set-MailboxDatabase telus -RpcClientAccessServer ex1.1ask2.com

We can use the network load balancing technology to protect Client Access Server (FW.1ask2.com).

Aftert Exchange 2010 (all roles) is uninstalled from fw.1ask2.com, the SwitchOver still works.

Move-ActiveMailboxDatabase Telus -ActivateOnServer Ex1 -MountDialOverride:None

Move-ActiveMailboxDatabase Telus -ActivateOnServer Ex2 -MountDialOverride:None

Problem in this network

EX1 is the DC. When EX2 is down, mailbox users within the DAG cannot send messages among themselves. They can send messages out to Internet users. In the queue viewer, "SMTP Relay in an Active Directory Site" shows. DNS domain cannot be resolved. More test is needed.