How DAC Mode Works In Exchange 2010 SP1…

MS Exchange

Exchange admins are a bit confused when it comes to DAC mode. Though most of the them know that it has to be used in most scenarios when deploying Exchange 2010 SP1, they are not sure as to what the function of DAC mode is.

DAC stands for Datacentre Activation Coordination and it works using a new protocol named Datacentre Activation Coordination Protocol (DACP). With Exchange 2010 SP1, all kind of DAG deployments can use DAC mode, even one that has just two DAG members. DAC mode was designed to avoid the “split brain syndrome” that will occur in one particular scenario of datacentre switchover. Let me explain the scenario with the help of an example. (The images used are from TechNet video presented by Scott Schnoll). I have two datacentres with two DAG members in each datacentre. The primary datacentre has the file share witness (hub server) and everything works as normal.

Two DCs in normal operation

My primary datacentre goes down, let’s say due to a power outage. Both my DAG members and the hub server hosting FSW goes offline. Along with that, my WAN connection goes down as well. This will cause my entire DAG to fail, as there are only 2 DAG members online out of the five in the cluster (4 mailbox servers and one HUB server hosting FSW).

Primary DC fails

In this scenario, I try to restore the DAG in the secondary datacentre by using an alternate witness server (hub server in 2nd datacentre). Both mailbox servers in the 2nd datacentre comes online as there are now 3 members online (out of 5). Everything works as expected and I am happy.

Let’s say that the power in my primary datacentre gets restored and both my mailbox servers and primary witness server comes online. The servers will come online before the network devices (which is how it is usually). Now, my WAN connection is still down and Exchange servers in both datacentres are up and running. My secondary datacentre has already been activated to serve the clients manually. But, since there are three servers online (out of the 5) in the primary datacentre as well (2 mailbox and 1 hub), the mailbox servers will try to activate databases as well (or will it?). This will lead to both DCs hosting same databases, which cannot and should not happen. DAC mode was developed to make sure that this doesn’t happen.

Split brain syndrome

Now, how does DAC mode make sure that the servers in my primary datacentre doesn’t mount databases when the power is restored? When DAC mode is enabled, it changes the way Active Manager works. When a DAG member gets restarted or when the Exchange Replication service is restarted (when DAC mode is enabled), it sets the DACP bit (in MSExchangeRepl.exe) to zero (which means that it can’t mount any database). Active Manager can only mount a database in only two conditions.

  1. If it can communicate with all other DAG members, in both datacentres (which won’t be possible in our case as the WAN link is down).
  2. If it can communicate with atleast one DAG member which has DACP bit set to one (which is a DAG member that is serving clients). In our case, these are the servers in the secondary DC. But, as the WAN connection is down, this won’t be possible.

Hence, our mailbox servers in the primary datacentre won’t mount databases, as the DACP bit will be set to zero, while the servers in the secondary datacentre will continue to work.

DACP bits

This is the only scenario for which DAC mode was designed – to make sure that DAG members won’t mount databases when power gets restored in the primary datacentre, while secondary datacentre is “active” and WAN connection is down.

Once the WAN connection is restored, DAG members in primary datacentre can communicate with all other DAG members and see that the databases are already mounted in the secondary DC & they won’t mount any database, which is the normal behaviour.

Hope this clear the confusion around DAC mode.

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  1. Thanks for the explanation!! It has been very useful to understand what happened last weekend as we had to shutdown all our Exchange 2010 SP1 environment (2 sites with 3 mailbox server in each site).

    Reply
  2. Hi,
    I want to study all the possible scenarios for DAG. Can you Assist i just name the types. i will study it myself, if its easy to share its links i will highly appreciate.
    You did a great work its appreciable.
    Thanks in Advance.

    Reply
  3. Hi Rajit, Thanks for this article. I have small clarity here. Initially the primary centre having active mailboxes and due to the power failure the secondary mailboxes act on active node on the secondary data centre.
    Now the question is, when the power failure happened, DACP set to zero (0) from 1 due to replication failed to another DAG member @ secondary data centre ? If i misunderstood, please clarify…

    Reply
  4. Very clear explanation. when i read about DAC mode in Technet, its very confusing.
    Now i am clear about DAC mode. Thanks very much for the good article !!!

    Reply
  5. Hi Rajith,

    Please suggest how can I configure dag in mulitple sites with users hosted in both location.

    Site A
    1. CAS/HT1
    2 MBX1
    Site B
    1. CAS/HT1
    2. MBX2

    Reply
  6. Hi Rajith. Good Explanation however had one doubt. As you said

    “Active Manager can only mount a database in only two conditions.”. The second condition

    If it can communicate with at least one DAG member which has DACP bit set to one (which is a DAG member that is serving clients)

    Now once the WAN link is restored, DAG members in primary site will be able to contact the DAG member which has a DACP bit set to one.
    So in that case, Would the DAG member on the primary site mount the database?

    Thanks

    Vilas

    Reply
    • No.. if DAG knows that there is bit 1 enabled for other DAG members then it understands.. Databases are mounted there. So it will not mount database in primary DC and this is how DAG works. Problem only occur when split brain situation happen due to power outage + WAN network and after sometime Power restore but WAN takes time.

      Reply
  7. One confusion, at one place you said : “Active Manager can only mount a database in only two conditions.”
    1.If it can communicate with all other DAG members, in both datacentres (which won’t be possible in our case as the WAN link is down).
    2.If it can communicate with atleast one DAG member which has DACP bit set to one (which is a DAG member that is serving clients). In our case, these are the servers in the secondary DC. But, as the WAN connection is down, this won’t be possible.

    and at last you said :
    Once the WAN connection is restored, DAG members in primary datacentre can communicate with all other DAG members and see that the databases are already mounted in the secondary DC & they won’t mount any database.

    Please understand me.

    Reply
  8. Outstanding Sir. I understand it, but I’ve been searching for a way to explain it to the “smarter” people who sit a few IT seats higher. This should bring it down to their level. Thanks a million. :)

    Reply

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Disable Windows Copilot Using Intune

Windows Copilot is Microsoft’s take on making life easier for Windows users using the power of AI. This article explains how to disable the feature using Intune, if your organization is not ready yet to walk into the AI world.

Disable Windows Copilot Using Intune

We need to create a Configuration Profile for Windows devices in the Intune portal to disable Windows Copilot. Below are the steps that we need to create the profile.

Launch the Intune Portal and login as a Global Admin or Intune Admin.

Navigate to Devices -> Windows -> Configuration Profiles.

Windows Configuration Profile Intune

Click on Create -> New Policy.

Select Windows 10 & later as the platform and Settings Catalog as the profile type & click on the Create button.

Settings Catalog Intune CoPilot

Give the policy a meaningful name & description and click Next.

Policy Name Disable CoPilot

Within the configuration settings, click on the Add Settings option.

Add Settings Disable CoPilot

Search for ‘copilot’, Windows AI will come up as the category. Click on Windows AI and the Turn off Copilot in Windows (User) setting will come up. Check the box and click Next.

Turn off CoPilot Setting Intune
Turn off CoPilot Setting Intune Summary 1

Specify scope tags if required and click Next.

Select tags CoPilot Intune

Select who this policy should apply to in the Assignments section. I have selected to add all users. If you want to test the setting, you can create a test group and select that group here.

Similarly, you can also exclude certain group from disabling AI (say IT team) if required.

Add all users disable copilot intune

A summary of selected settings will be displayed. Click on the Create button to setup the policy to disable Windows Copilot.

Create configuration policy disable CoPilot Intune 1

Wait for the replication to complete in the cloud backend and login to your machine. Your chatty Copilot should now be disabled.

Disable Windows Copilot On Windows 11 Pro

Follow the steps below to disable Copilot on a personal Windows 11 Pro machine (say your own laptop).

Search for ‘group’ in Windows 11 and click on Edit Group Policy option.

Group Policy Windows 11 Disable Copilot

Navigate to User Configuration -> Administrative Templates -> Windows Components -> Windows Copilot.

Windows Copilot GPO setting

Double click on Turn off Windows Copilot setting on the right pane.

Select Enabled and click OK.

Turn off copilot gpo Windows 11 Pro 1

Close the Group Policy Editor. This will disable Windows Copilot on a Windows 11 Pro machine.

Summary

We have learned to disable Windows Copilot using Intune and Group Policy on Windows 11 machines.

Please let me know if you have any questions in the comments section.

Promote Windows Server 2025 To Domain Controller

Domain controllers are the backbone of any Active Directory domains in the Microsoft world. Any Windows server can be promoted to be a domain controller. In this article, we will go through the steps of promoting a Windows 2025 Server to be a domain controller.

Windows Server 2025

The latest version of the server operating system has been named Windows Server 2025. You can start with a 2025 Server & create an AD domain or you can promote a member server that is already a part of a domain.

The Windows Server 2025 needs to be installed on a machine before it can be promoted to be a domain controller.

Promote Windows Server 2025 To Domain Controller

If you have been working with Windows servers long enough, everything starts with the Server Manager app. Promoting a server to a domain controller is no different.

Launch ‘Server Manager’ & click on Add roles and features.

Windows 2025 Server Manager

You land on the summary page that explains what is required to run this wizard successfully. Click Next.

Windows Server 2025 Add Remove Roles

Select Role-based or Feature-based installation and click Next.

Windows Server 2025 Role Based Install

Select the server that needs to be promoted and click Next.

Windows Server 2025 Destination Server

Select Active Directory Domain Services (second option) and click on Add Features.

Windows Server 2025 AD Domain Services

Go with the default options for features that need to be installed.

Windows Server 2025 AD Domain Services Features

A summary of AD DS pops up next, click next to continue.

Windows Server 2025 AD DS

Select Restart the server automatically if required and click Install.

Windows Server 2025 AD Restart Server

You get to keep an eye on the progress of the installation.

Windows Server 2025 AD Install Progress

Once the role has been installed, you will find an exclamation mark on the top right corner of the Server Manager. Click on that and select Promote this server to be a domain controller.

Windows Server 2025 AD Install Continue

You get an error straight away (which you have never seen before) – Error determining whether the target server is already a domain controller. Role change is in progress or this computer needs a restart.

Windows Server 2025 Domain Controller Setup Error

We never needed to restart the server after installing the role in the DC promotion process. Given that it is an insider build of Server 2025, I am hoping that this will get fixed before the public release.

Restart the server, launch Server Manager and click on the Promote this server to be a domain controller option again.

Windows Server 2025 AD Install Continue 1

I am setting up a brand new AD forest and hence I select the third option (Add a new forest) and enter my root domain name.

Windows Server 2025 Add a forest

Next window brings the option to set your forest & domain functional level and the DSRM password. In the insider build, it shows what looks like a variable (the Windows server version on which the you are working).

Windows Server 2025 Forest Functional Level

You can leave the default options in the DNS options wizard and click next.

Windows Server 2025 DNS Options

Enter the netbios name of the domain in the next window and click next.

Windows Server 2025 Netbios Domain Name

You can stick with the default paths for the AD database, log & sysvol folder or pick a location of your choice.

Windows Server 2025 AD Paths

Review the selections that you have made so far and click next.

Windows Server 2025 Options Review

Wait for the green check mark on the prerequisites page and click next.

Windows Server 2025 Pre reqs Check

Click Install in the final window & wait for the magic to happen. Once the machine gets restarted (which it will do automatically), you will have a brand new domain controller based on Windows Server 2025.

Windows Server 2025 AD Snap In

Summary

Promoting a Server 2025 to a domain controller follows pretty much the same steps as previous operating systems. The Insider build has few errors that needs to be fixed, but hey, it is an insider build!

Please let me know if you have any questions in the comments section.

Install Windows Server 2025 – Full Guide

Microsoft has released an insider preview of it’s next server operating system named Windows Server 2025. We will have a look at the installation steps involved in setting up a 2025 server.

Windows Server 2025

Microsoft has gone with the same look and feel of Windows 11 operating system in it’s current server operating system – Windows Server 2025. As the product is in insider preview, there might be slight changes before it hits the public shelves.

It is refreshing to see a ‘modern’ feel in the installation process of a server operating system. Gone are the days where the installation of a consumer based OS felt much better compared to it’s server counterpart.

Installing Windows Server 2025

Let’s take a look at the steps involved in setting up a Windows Server 2025 machine. First step is to download the ISO from the Windows Insider portal.

Next step is to boot the virtual / physical machine from the ISO which will kick off the installation of Server 2025.

First option to select is the language settings. Pick the one based which relates to you and click next.

Windows Server 2025 language settings

Select the keyboard settings in the next screen and click next.

Windows Server 2025 keyboard

You get the option to select whether you want to Install Windows Server or Repair the installation. The bottom left corner also has the option to go to the previous version of setup.

Windows Server 2025 setup option

You are asked to enter the product key, which is available in the Windows Insider portal.

Windows Server 2025 product key

Next option to choose is the type of image you want to install – Windows 2025 core or full blown desktop experience.

Windows Server 2025 Desktop

You need to agree to the licensing terms to move forward in the next step.

Windows Server 2025 Agreement License

Select the partition on which the server OS should be installed and click next. You also have the option to slice the partitions the way you see fit in the same screen.

Windows Server 2025 Disk Partitions

The Ready to Install window comes up, click the install button.

Windows Server 2025 Install 1

Installation of Server 2025 is underway and you get to see the progress.

Windows Server 2025 Install Progress

Once the installation is complete, you need to enter an administrator password of your choice to finalize the setup.

Windows Server 2025 Password

And there you go! You see a Windows 11 login screen staring at you ;-)

Windows Server 2025 Login Screen

After logging in, you get to set the options around sending diagnostic data to Microsoft, which I always set as ‘required only’.

Windows Server 2025 Diagnostic Data

The Windows Server 2025 desktop looks similar, doesn’t it? ;-)

Windows Server 2025 Desktop Feel scaled

Now that the server is up and running, you can promote it to be a domain controller.

Summary

The Windows Server 2025 has the same look and feel as a Windows 11 operating system. The installation options also provide that modern ‘feel’ and makes it a bit soothing to the eyes!

Please let me know if you have any questions in the comments section.