Microsoft Exchange Server 2013 High Availability: Design a highly available Exchange 2013 messaging environment using real-world examples.

As we saw in Chapter 1, Getting Started, Exchange 2013 does not require session affinity any longer. To understand why this is the case, let us look at how the new CAS role works from a protocol perspective:

Another major change in Exchange 2013 is in the role of Microsoft's RPC-based Messaging API (MAPI). Previously, the Outlook clients used two transport options for RPC traffic: HTTP or TCP. With Exchange 2013, RPC/TCP is no longer supported as a connectivity solution; only RPC/HTTP (also known as Outlook Anywhere) is supported.

Changing the protocols used for communication between the CAS and Mailbox server roles from RPC/TCP to a protocol that is more tolerant to latency and throughput over Internet and WAN connections allows these roles to not be as tied together from a geographical or user affinity perspective as they were before. This change allows CASs in one Active Directory site to authenticate and proxy requests to a Mailbox server located in a different site.

Exchange 2010 introduced the new shared RPC Client Access namespace that Outlook clients use to connect. Exchange 2013, instead of using an FQDN for the RPC endpoint, it uses a mailbox GUID together with...

In order to achieve a high level of availability for the CAS role, there are two requirements: deploy more than one CAS and use a load balancing solution. Both these requirements have the purpose of decreasing the downtime for users by allowing failover of connections in case of an issue with one or more CASs. This failover is often automatic, depending on the load balancing solution implemented.

Load balancing is the method by which incoming client connections are distributed over a variety of endpoints (in our case, CASs) to ensure the load across all endpoints is similar. In Exchange, from an availability perspective, its main purpose is to provide failover redundancy if one or more CASs fail, ensuring that users continue to be able to access their mailboxes, thus reducing the impact of a CAS failure.

Furthermore, both these advantages are often achieved while presenting a single hostname for clients.

In Exchange 2010, the CAS role handled and processed client...

In the last few versions of Exchange, the Autodiscover service has been available. This service greatly simplifies deployments by automatically configuring user profile settings for clients running Outlook as well as supported mobile phones.

With earlier versions of Outlook (Outlook 2003 or earlier versions) and Exchange (Exchange 2003 SP2 or earlier versions), administrators had to manually configure user profiles to access Exchange. If the messaging environment was changed, more work was required on these profiles, or Outlook would stop functioning properly.

In order to achieve a highly available scenario, Autodiscover is a crucial component. Before detailing how it should be configured, we need to first understand how Autodiscover works.

Using Autodiscover, Outlook locates the connection point made up of the user's mailbox GUID, plus the @ sign, plus the domain of the user's primary SMTP address. The following information is also returned to the client:

Achieving high availability for Client Access servers is a two-step process. First, you need to deploy multiple CASs, and second, you need to use a load balancing solution that best meets your organization's needs and requirements. This will significantly decrease, or even fully prevent, downtime and avoid single point of failures for this role.

With all the architectural changes made to Exchange 2013, Layer 4 load balancing is now a viable solution, and one that is more than adequate for most organizations. However, for some specific scenarios, Layer 7 might still be required due to its capabilities such as to analyze traffic as it passes through the load balancer.

Finally, it is crucial that certificates and Exchange services such as Autodiscover are correctly configured to take full advantage of the load balancer capabilities and Exchange availability features.

In the next chapter, we will explore high availability for the Mailbox server role, including all the changes and improvements...