1. Planning and Implementing High Availability
Presentation: 105 minutes
Lab: 90 minutes
After completing this module, the students will be able to:
Understand high availability in Exchange Server 2013.
Understand highly available mailbox databases.
Manage highly available Client Access servers.
Required materials
To teach this module, you need the Microsoft® Office PowerPoint® file 20341B_06.pptx.
Important: We recommend that you use Microsoft Office PowerPoint 2007 or a newer version to display the slides for this course. If you use PowerPoint Viewer or an earlier version of Office PowerPoint, all of the features of the slides might not display correctly.
Preparation tasks
To prepare for this module:
Read all of the materials for this module.
Practice performing the demonstrations.
Practice performing the labs.
Work through the Module Review and Takeaways section, and determine how you will use this section to reinforce student learning and promote knowledge transfer to on-the-job performance.
As you prepare for this class, it is imperative that you complete the labs yourself so that you understand how they work and the concepts that are covered in each. This will allow you to provide meaningful hints to students who may get stuck in a lab, and it also will help guide your lecture to ensure that you cover the concepts that the labs cover.
Module 6
Planning and Implementing High Availability

2. Configuring Highly Available Client Access Servers
Module Overview
6: Planning and Implementing High Availability
Configuring Highly Available Client Access Servers

3. Lesson 1: High Availability on Exchange Server 2013
6: Planning and Implementing High Availability
Discussion: Virtualization High-Availability Technologies versus Exchange High-Availability Technologies for Mailbox Servers

4. Components of High Availability
6: Planning and Implementing High Availability
All parts of the application and infrastructure it relies on must be highly available
Data center infrastructure
Server hardware
Storage
Network infrastructure
Internet
Network services
When you consider high availability, in addition to the application, you need to consider other components. All of the components needed for communication must be highly available. Discuss each of these components with the students, and generally describe how to make the application highly available with redundant components.
Explore the options mentioned in the slide with the students. Some may have interesting examples from their own organizations.

5. What Is a Database Availability Group?
6: Planning and Implementing High Availability
A DAG is a collection of servers that provides the infrastructure for replicating and activating database copies. DAGs:
Require the failover clustering feature, although all installation and configuration is done with the Exchange Server management tools
Use Active Manager to control failover
Use an enhanced version of the continuous replication technology that Exchange Server 2007 introduced
Can be created after the Mailbox server is installed
Allow a single database to be activated on another server in the group without affecting other databases
Allow up to 16 copies of a single database on separate servers
Define the boundary for replication
Explain a database availability group (DAG) by describing all of the topics listed. You should mainly focus on the DAG’s 16-member limit, which means that you can have copies of a database located on up to 16 Mailbox servers. Also mention that you cannot replicate databases between DAGs, only within DAGs.

6. Understanding How Database Availability Groups Work
6: Planning and Implementing High Availability
Continuous replication protects databases across servers in the DAG
DB1
DB2
DB3
MBX1
MBX2
MBX3
The active database copy uses continuous replication to keep the passive copies in sync, based on their lag-time setting. A DAG leverages the Windows Server operating system failover-clustering feature. However, it relies on Active Manager to maintain the status of all of the databases that the DAG hosts.
Discuss the following DAG characteristics:
A single database can be switched over or failed over between servers within the DAG.
At any given time, a copy is either the replication source or the replication target, but not both.
A server may not host more than one copy of a given database.
Not all databases need to have the same number of copies. In a 16-node DAG, one database can have 16 copies, while another database is not redundant and contains only the one active copy.
Discuss the difference between a failover and a switchover.
DB1
DB2

7. Understanding How High Availability Works with Client Access Servers
6: Planning and Implementing High Availability
You make Client Access servers highly available with multiple Client Access servers and by configuring load balancing
You can achieve high availability and load balancing by using:
DNS round-robin
Network Load Balancing
Hardware-based load balancing
Discuss the important areas that should be considered when you plan for highly available Client Access servers by using Network Load Balancing (NLB). Mention that DNS round-robin is not a high-availability solution, and that it requires the client to identify when a server outage occurs.

8. Understanding How Transport High Availability Works
6: Planning and Implementing High Availability
SMTP Server
MBX01
MBX02
(Shadow Server)
There are three servers in this animation: the SMTP Server, the MBX01 server, and the MBX02 server (the shadow server).
Explain how the SMTP server first sends the message to the MBX01 server. The connection between the SMTP server and MBX01 stays active. The MBX01 server then opens another connection to the MBX02 server and sends the message to the shadow queue on the MBX02 server. The MBX02 server then confirms the receipt to the MBX01 server, and closes the connection. The MBX01 server confirms the receipt to the SMTP server, and then closes that connection.
The Transport service on the MBX01 server processes the primary message, and the mailbox Transport service delivers the message to the local mailbox database. The MBX01 server then moves the message into the local primary Safety Net queue. After the message is moved to the Safety Net queue, the MBX02 server polls the MBX01 server for the discard status of the primary message. Lastly, it moves the message from the shadow queue to the shadow Safety Net.
Active Queue
Shadow Queue
Primary Queue
Transport Service
Shadow Safety Net
Mailbox Transport Service
Transport Service
Mailbox Database

9. Understanding How High Availability Works with Edge Transport Servers
6: Planning and Implementing High Availability
Mailbox Server
Edge Transport Server 1
Edge Transport Server
Site A
Site B
Edge Transport Server 2
First mention that Edge Transport is not available in the release version of Exchange Server 2013.
Discuss how the requirements for high availability for external message delivery differ from those for message reception. For external message delivery:
Messages are not rerouted based on availability.
Multiple sites do not automatically provide redundancy.
They use an Edge Transport server.
For message reception:
You can use multiple Internet connections, in addition to Edge Transport servers.
Multiple MX resource records can provide load balancing or failover from a primary mail server to a secondary mail server.

10. What Is Site Resilience?
20341B
What Is Site Resilience?
6: Planning and Implementing High Availability
Site resilience is the ability of the messaging system to survive the failure of a site
An alternate data center can be located at another company location that has sufficient capacity
A DAG can span Active Directory sites
Other roles and services must already exist in the site
Site resilience is the messaging system’s ability to survive a site failure, and continue functioning through the use of an alternate data center. The alternate data center is in a different location.
Unlike in Exchange Server 2007, site resilience in Exchange Server 2013 does not require an Active Directory site to be stretched across a WAN link because DAGs support use across multiple subnets.
DAG is used for the Mailbox servers. The Client Access server role must be present in each site, but it does not require special configuration. Other services, such as domain controllers, global catalogs, and DNS must also be available in the alternate data center.

11. 20341B
Discussion: Virtualization High-Availability Technologies versus Exchange High-Availability Technologies for Mailbox Servers
6: Planning and Implementing High Availability
Do you currently use virtualization for maintaining high availability of Exchange Server 2010 Mailbox Servers such as Hyper-V in Windows Server 2012 clustering?
What are the advantages and disadvantages of using virtualization versus DAGs?
Which of these approaches would you recommend: virtualizing mailbox servers on multiple hosts, or using multiple physical mailbox servers with DAGs? Why do you make this recommendation?
This discussion should focus on the advantages and disadvantages of using Hyper-V-based high availability versus DAGs. The decision is if you should make mailbox servers highly available by implementing host clustering (one MBX server virtualized, but two or more Hyper-V hosts), or if you should instead implement DAGs with two or more MBX servers. If you virtualize the mailbox role, you use your physical hardware in the most effective manner. However, if you do not virtualize the mailbox role, you need to make sure that the Exchange Server handles the mailbox communication in the most reliable way possible.
For example, when you use DAGs, live migration is as effective as using DAGs and database copies. However, you can also explain that DAGs remove the logic from the physical machine, so that you only have to install a new server if another one fails, and then join that server to the DAG.
Stress to the students that these are two different approaches that utilize different technologies.

12. Lesson 2: Configuring Highly Available Mailbox Databases
6: Planning and Implementing High Availability
Demonstration: How to Monitor Replication Health

13. Exchange Server 2013 DAG quorums:
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What Is a Quorum?
6: Planning and Implementing High Availability
A quorum uses a consensus of voters to ensure that enough cluster members are available to provide services
Exchange Server 2013 DAG quorums:
Are based on votes in Windows Server 2012
Allow nodes, file shares, and shared disks to have votes, depending on the quorum mode
Use node majority with a witness server for quorum:
DAGs with an even number of Mailbox servers use the witness server
DAGs with an odd number of Mailbox servers use node majority
You can have non-voting nodes, if needed
Explain the concept of a quorum and how a cluster requires a quorum before the cluster can start. Mention that, by default, every member of a DAG is one vote, and an additional vote is available for the witness server. To achieve a majority, at least half of the votes must be available.

14. General configuration
20341B
Planning Software and Hardware Components for Database Availability Groups
6: Planning and Implementing High Availability
General configuration
Nodes in the same domain
Not domain controllers
Windows Server 2012 Datacenter, or Windows Server 2008 R2 Datacenter or Enterprise
Network configuration
Two adapters
Less than 250 ms latency
DAG must have one IP on the MAPI network
Witness server
Provide an overview of the planning aspects for DAGs. Make sure that the students understand the concept of a DAG member server, and what is required to add a server to a DAG.

15. What Is Active Manager? Active Manager:
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What Is Active Manager?
6: Planning and Implementing High Availability
Active Manager:
Runs a process on each server in the DAG
The PAM is one node
The remaining nodes are SAM
Manages which database copies are active and which are passive
Stores database state information
Manages database switchover and failover processes
Does not require direct administration configuration
Explain the role of the Active Manager when you decide which database will be mounted if the active one fails. Mention that it is not the failover cluster service that includes the Active Manager, and explain that Exchange Server 2013 incorporates this functionality.

16. What Is Continuous Replication?
20341B
What Is Continuous Replication?
6: Planning and Implementing High Availability
Database Availability Group
DB1
DB1
DB1
Explain the scenario first. In the slide, there is a DAG that contains three servers, and each has a database copy of DB1. The left DB1 (blue) is the active database, while the other two databases are passive copies.
Then discuss the continuous replication – file mode process using the slide, as follows:
Exchange Server writes, and then closes the active log.
The replication service replicates the closed log to the servers that host the passive databases.
Because each copy of the database is identical, Exchange Server inspects, and then replays or applies, the transaction logs to the database copies. The databases remain in sync.
Mention that you must seed the databases for this process. Seeding a database ensures that a good copy of the database is available on each Exchange server, so that you can replay transaction logs as appropriate.
Next, describe continuous replication – block mode, which was introduced with Exchange Server Service SP1. Explain that this mode is automatically triggered when Exchange Server 2013 recognizes that all log files have been copied correctly to the Exchange servers that host the passive databases. Then talk through the block mode process:
Once in block mode, any block of data written to the ESE log buffer on the Exchange server that hosts the active database is automatically copied to the replication log buffer of all passive database copies.
When the ESE log buffer is full, the final block is sent to the passive databases, and a transactional log file is written on the Exchange server that hosts the active database. Then the ESE log buffer is emptied.
When the Exchange servers that host the passive databases receive the final block that fills up their replication log buffer, they also save the buffer to a transactional log file. Then the buffer is emptied, and the process starts again.
When the server with the active database fails, but the replication log buffer is not full, the buffer is saved to a new transactional log file.
MBX1
MBX2
MBX3
File Mode
Block Mode
ESE Log Buffer
Replication Log Buffer
Replication Log Buffer

17. Configuring a Database Availability Group
6: Planning and Implementing High Availability
To configure DAGs, you must define the following:
Witness Server – server used to store witness information
Witness Directory – directory used on the witness server to store witness information
Database availability group IP addresses – IP address(es) used by a DAG
Also consider these settings for larger or multi-site implementations:
DAG networks including replication
DAG network compression
DAG network encryption
Third-party replication mode
Alternative witness server
Alternative Witness Directory
This topic discusses all of the relevant DAG settings that are available to configure a DAG. You should explain the details of each setting and mention when to use it. For example, mention that you need to configure DAG Networks only if you have multiple subnets and the automatic discovery of these subnets does not work correctly.

18. Configuring Databases for High Availability
6: Planning and Implementing High Availability
After creating a DAG, adding Mailbox servers to the DAG, and configuring the DAG, you must still complete the following steps:
Create database copies
Set activation preference number
Set replay lag time
Set truncation lag time
Discuss the process of configuring a database to be highly available within a DAG. This includes describing and discussing the importance of configuring truncation, replay lag times, and preferred list sequence numbers.
Question: How do you plan to use the preferred list sequence number?
Answer: Answers may vary. However, many of the students will prefer to spread the activity over multiple servers. Rotating the preference for the databases through all available servers allows each server to serve client requests actively.

19. What Are Lagged Mailbox Database Copies?
6: Planning and Implementing High Availability
A lagged database copy is a database that uses a delayed replay lag time to commit the log files to the database
You create a lagged database to prevent:
Database Logical Corruption
Store Logical Corruption
Rogue Admin Protection
Lagged database copy enhancements in Exchange Server 2013:
Automatic log play down
Simpler activation with Safety Net
Lagged copies can now be configured in EAC
Explain the functionality of a lagged database copy and situations in which lagged copies are used. In most scenarios, lagged copies are used when you do not perform regular backups to address issues such as database logical corruption. Also mention the enhancements to lagged database copies in Exchange Server 2013, specifically automatic log play down and simpler activation with Safety Net.

20. 20341B
Demonstration: How to Create and Configure a Database Availability Group
6: Planning and Implementing High Availability
In this demonstration, you will see how to create a new database availability group
Leave the virtual machines running for subsequent demonstrations.
Preparation Steps
Before performing the steps, you must have the 20341B-LON-DC1, 20341B-LON-MBX1, 20341B-LON- MBX2, 20341B-LON-MBX2, and 20431B-LON-CAS2 virtual machines running for this demonstration. Because of the configuration changes required before you can start this demonstration, follow these detailed steps to start the required virtual machines:
On the host computer, click Start, point to Administrative Tools, and then click Hyper-V Manager.
In Hyper-V Manager, click 20341B-LON-DC1, and in the Actions pane, click Start.
In the Actions pane, click Connect. Wait until the virtual machine starts.
Sign in using the following credentials:
User name: Adatum\Administrator
Password: Pa$$w0rd
You must now move the subnet object currently associated with the Swindon site to the London site before starting the Exchange Servers:
On LON-DC1, click Server Manager.
In Server Manager, click Tools and then click Active Directory Sites and Services.
In Active Directory Sites and Services, click Subnets.
Right-click /25 and then click Properties.
In the /25 Properties dialog box, in the Site list, click London and then c lick OK.
Close Active Directory Sites and Services.
Repeat steps 2 to 4 for 20341B-LON-MBX1, 20341B-LON-MBX2, 20341B-LON-CAS1, and B-LON-CAS2.
(More notes on the next slide)

21. Understanding the Failover Process
20341B
Understanding the Failover Process
6: Planning and Implementing High Availability
If a failure occurs, the following steps occur for the failed database:
Active Manager determines the best copy to activate
The replication service on the target server attempts to copy missing log files from the best “source”:
If successful, the database mounts with zero data loss
If unsuccessful (failover), the database mounts based on the AutoDatabaseMountDial setting
The mounted database generates new log files (using the same log generation sequence)
Safety Net requests are initiated for the mounted database to recover lost messages
When the original server or database recovers, it determines if any logs are missing or corrupt, and fixes them if possible
Discuss what would happen if a failure occurred and all of the logs could not be copied to the passive copy before you need to make it active. Discuss how to use the transport dumpster when failover occurs.
Active manager selects the best copy to activate, as follows:
Enumerates the available copies.
Ignores all servers that are unreachable.
Sorts available copies by how up to date they are (content index, copy queue length, and replay queue length).
Breaks ties based on activation preference.
Activity
Draw a simple Exchange Server messaging environment on the board. Be sure to include one Client Access server and two mailbox servers configured in a DAG. Guide the students through a scenario in which the Transport service delivers a message to the active database on the first mailbox server. Immediately after Exchange Server delivers the message, the server fails, and no data is recovered. Next, guide the students through the steps on the slide to discuss how this database failover process works.
Discuss the AutoDatabaseMountDial setting and when to use it. The three possible values for the AutoDatabaseMountDial setting are:
BestAvailability. If you specify this value, the database automatically mounts if the copy queue length is less than or equal to 12. The copy queue length is the number of logs that the passive copies recognize as needing replication. If the copy queue length is more than12, the database will not automatically mount. When the copy queue length is less than or equal to12, Exchange Server attempts to replicate the remaining logs to the passive copies and mount the database.
GoodAvailability. If you specify this value, the databases automatically mount immediately after a failover, if the copy queue length is less than or equal to six. If the copy queue length is more than six, the databases do not automatically mount. When the copy queue length is less than or equal to six, Exchange Server attempts to replicate the remaining logs to the passive copy and mount the database.
Lossless. If you specify this value, the databases do not automatically mount until all logs that were generated on the active copy are copied to the passive copy.

22. Planning, Monitoring, and Managing a Database Availability Group
6: Planning and Implementing High Availability
Allocate the necessary permissions for managing a DAG
Organization Management
DAGs
Database copies
Failure may not be noticed
Exchange Server 2013 includes several scripts and commands for DAG monitoring and management
Consider using System Center Operations Manager 2012
Explain to the students that you should allocate permissions only to those who are knowledgeable about DAGs, and are responsible for their maintenance. The ability to manage DAGs and database copies might not be required by most Exchange Server administrators.
A unique problem in managing DAGs is that you might not notice the failure of a DAG member immediately, because users are not affected. To actively monitor DAG members, consider using Microsoft Systems Center Operations Manager 2012 or another monitoring tool.
Question: Which users in your organization will have permission to manage DAGs?
Answer: Answers will vary. In general, large organizations may have a small, dedicated group for managing the DAG-level components. Mid-sized organizations will likely allow Exchange Server administrators to manage the DAG.

23. Demonstration: How to Monitor Replication Health
20341B
Demonstration: How to Monitor Replication Health
6: Planning and Implementing High Availability
In the demonstration, you will see how to view the health status of database copies in the Exchange Administration Center or Exchange Management Shell
Leave the virtual machines running for subsequent demonstrations.
Preparation Steps
Demonstration Steps
On LON-CAS1, in the EAC Feature pane, click Servers.
On tabs, click databases, and then in the list view, click Mailbox Database 1.
In the Details pane, under Mailbox Database 1\LON-MBX1, click View details.
In the Edit database copy window, explain Status, which could be Healthy, Initializing, Failed, Mounted, Dismounted, Disconnected, Suspended and Failed, Suspended, Resynchronizing, or Seeding. Also describe the Content index state, copy queue length (logs), and replay queue length (logs). Then click cancel.
Click to the Start screen, and then click Exchange Management Shell.
At the Exchange Management Shell prompt, type Test-ReplicationHealth, and then press Enter.
This cmdlet performs a variety of tests, and reports back the status for the replication components. Mention that the *FAILED* result occurs because one database is not replicated to other servers.
At the Exchange Management Shell prompt, type Get-MailboxDatabaseCopyStatus –Server LON- MBX1, and then press Enter.
This cmdlet shows status information about all database copies on the server defined.
At the Exchange Management Shell prompt, type CD “C:\Program Files\Microsoft\Exchange Server\V15\Scripts”, and then press Enter.
At the Exchange Management Shell prompt, type .\CheckDatabaseRedundancy.ps1 – MailboxDatabaseName “Mailbox Database 1”, and then press Enter.
Note that the Current Status should be green to indicate that all databases are healthy and that the database is mounted on the server with the lowest activation preference number (APN).

24. Lesson 3: Configuring Highly Available Client Access Servers
6: Planning and Implementing High Availability
Demonstration: Configuring Options for Highly Available Client Access Servers

25. Client Access servers used by all clients
Planning Software and Hardware Components for Highly Available Client Access Servers
6: Planning and Implementing High Availability
Client Access servers used by all clients
Implement load balancing for Client Access servers
Sites performing a proxy are most important
Same SSL certificate is used on all nodes
Consider redundant Internet connections for external users
Client Access servers are used by all clients to access mailboxes. In previous versions of Exchange Server, you might have multiple mailbox servers with MAPI clients accessing them directly. Now, even MAPI clients access the Client Access servers. Therefore, the Client Access servers should be highly available, except for those in small locations or those considered not important enough to justify the cost.
To implement high availability for Client Access servers, you need to have some method of load balancing between them. You can select the method with which your organization is most comfortable. NLB is included as part of Windows Server However, hardware-based load balancing should be used in larger environments.
If a site proxies a Client Access server to another site, implementing high availability is even more important because an outage affects multiple sites. Whether or not a proxy is done depends on the external URLs’ configuration.
The same SSL certificate is used on all nodes in the Client Access server array, because access to the array is gotten by using a single URL.
For external users that require high availability, you need to consider the use of redundant Internet connections from different providers, and Internet access for multiple sites.

26. What Is Network Load Balancing?
6: Planning and Implementing High Availability
NLB enables high availability and scalability for server applications, typically web based
NLB enables access to two or more servers through a single IP address
NLB requires all servers in the NLB cluster to be running the same server applications with the same configuration
Client requests are distributed across available servers in the NLB cluster, based on a common algorithm
All servers monitor each other through heartbeat messages
If a server fails to send heartbeat messages for five seconds, the other servers automatically converge and redistribute the client connections across the available servers
When NLB is installed as a network driver on each of the member servers or hosts in a cluster, the cluster presents a virtual IP address to client requests. The client requests are broadcast to all the hosts in the cluster, but only the host that the given client request is mapped to accepts and handles the request. All the other hosts drop the request. Depending on the configuration of each host in the cluster, the statistical mapping algorithm, which is present on all the cluster hosts, maps the client requests to particular hosts for processing.

27. Considerations for Implementing Highly Available Client Access Servers
6: Planning and Implementing High Availability
When implementing highly available Client Access servers, consider the following:
All Client Access servers should have a certificate to match their address space
All Client Access servers should have the same protocols enabled
Use a hardware or software network load balancer
Use layer 4 versus layer 7 load balancing
Deploy servers with similar hardware, memory and performance
Describe the key considerations for implementing highly available Client Access servers. Make sure that the one considered is a network load balancer. For a service-aware load balance, we recommend a hardware network load balancer. Also explain the difference between layer 4 and layer 7 load balancing. Layer 7 load balancing is service-aware and therefore checks if Outlook Web Access (OWA) is running. Layer 4 load balancing is only server-aware and therefore checks if the server is responding. Exchange Server 2013 supports both load balancers.

28. In this demonstration, you will see how to configure a DNS round-robin
Demonstration: Configuring Options for Highly Available Client Access Servers
6: Planning and Implementing High Availability
In this demonstration, you will see how to configure a DNS round-robin
After you have concluded the demonstration, please revert the virtual machines.
Preparation Steps
In this demonstration, you will see how to configure a DNS round-robin for the two Client Access servers LON-CAS1 and LON-CAS2.
Demonstration Steps
On LON-DC1, in Server Manager, click Tools, and then click DNS.
In DNS Manager, in the left pane, expand LON-DC1, expand Forward Lookup Zones, click Adatum.com and then right-click Adatum.com, and then click New Host (A or AAAA).
In the New Host dialog box, in the Name field, type webmail. In the IP address field, type , and then click Add Host. Click OK.
In the New Host dialog box, in the Name field, type webmail, then in the IP address field, type , and then click Add Host. Click OK and then click Done.

29. Lab: Implementing High Availability
6: Planning and Implementing High Availability
Exercise 3: Testing the High-Availability Configuration
Ensure that your students perform the startup instructions correctly otherwise the lab may fail.
Exercise 1: Creating and Configuring a Database Availability Group
To complete the Mailbox server high-availability configuration, create a database availability group (DAG), and make the Mailbox Database 1 database highly available.
Exercise 2: Deploying Highly Available Client Access Servers
You decide to implement software Network Load Balancing (NLB) to load balance LON-CAS1 and LON- CAS2 for Client Access server connections. You will use the IP address as the virtual IP address that handles the mail.adatum.com namespace for your client server connections. Now you must complete the configuration to achieve this.
Exercise 3: Testing the High-Availability Configuration
To verify that your high-availability configuration works as expected, you will check Client Access server and DAG failover.
Virtual Machines B-LON-DC1
20341B-LON-CAS1
20341B-LON-CAS2
20341B-LON-MBX1
20341B-LON-MBX2
User Name Adatum\Administrator
Password Pa$$w0rd
Logon Information
Estimated time: 90 minutes

30. 20341B
Lab Scenario
6: Planning and Implementing High Availability
You are the messaging administrator for A. Datum Corporation. You have completed the basic installation for four Exchange Server servers. Now you must complete the configuration so that they are highly available. This basically requires you to configure your mailbox databases as well as your Client Access servers to be highly available, and to test if an automatic failover works.

31. 20341B
Lab Review
6: Planning and Implementing High Availability
In the lab, one mailbox server failed. How did the other mailbox server achieve a quorum?
Question
When do you need to pre-stage the cluster network object for a database availability group?
Answer
This is required for mailbox servers that are running on Windows Server 2012, but not for those running Windows Server 2008 R2. It is also required if you need to install an Exchange Server 2013 server, but do not have the appropriate AD DS permissions required to create computer objects.
In the lab, one mailbox server failed. How did the other mailbox server achieve a quorum?
Exchange Server 2013 DAGs use the node and file share majority quorum mode. This requires that more than half of the votes be available. In the lab, when one mailbox server failed, the witness server and one mailbox server was still available, thus two of three nodes were available. For that reason, the cluster achieved quorum and took over the failure.

32. Module Review and Takeaways
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Module Review and Takeaways
6: Planning and Implementing High Availability
Common Issues and Troubleshooting Tips
Review Question
Question
Your DAG has two mailbox servers (nodes) and one witness server. When will you lose quorum and be unable to mount the databases automatically?
Answer
You would lose quorum if you lose one member server and the witness server. If just the witness server fails, the databases can still mount. If just one node fails, the other node still reaches quorum because the witness server vote will allow quorum.
Best Practice
To decide for a witness server for a DAG, you should prefer a Client Access server over a file server.
Common Issues and Troubleshooting Tips
Common Issue:
You cannot add an Exchange server to a DAG.
Troubleshooting Tip:
Make sure that all Exchange servers that you want to add to a DAG run on the same Windows Server operating system version. You cannot combine different Windows Server operating systems, such as Windows Server 2012 and Windows Server 2008 R2.
When you add a server to a DAG, replication of the database fails.
Make sure that all servers in a DAG have the same drive letters available, because all database copies are stored on the exact same drive and directory as the members in the database participating in replication.
Instructor Note: Ensure that you cover the common issues and the corresponding troubleshooting tips listed in this section. Encourage students to share tips from their own work environments.