The load balancer setup plays a vital role in client connection to Exchange Server. It helps establish the connection from the client to the Exchange Client Access Server (CAS) servers, which runs the client connection service, and distributes the client connection load across the multiple backend Exchange servers. Load balancers have various mechanism to check the health and availability of target CAS servers and establishes the connection to the most suitable one at any given time.

Configuring the Load Balancer when migrating from Exchange 2010 to Exchange 2013

When organizations are migrating from Exchange 2010 to Exchange 2013, it may take some time to complete the project. As a result, it is essential to configure the previous and new Exchange systems to coexist. A correctly configured load balancer setup can make this possible.

There are various protocols that clients use to connect to the Exchange server. They are:

  1. Outlook Web App (OWA)
  2. Autodiscover
  3. Exchange Web services (EWS)
  4. Exchange Active Sync(EAS)
  5. Offline Address book (OAB)
  6. Outlook Anywhere
  7. MAPI
  8. SMTP Protocol

Figure 1 shows a typical Exchange 2010 deployment with a load balancer.

Figure 1. Current Exchange 2010 organizations before introduction Exchange 2013

The table below shows the Exchange 2010 Internal and External URL configuration that will be used on the Exchange 2010 CAS Servers.

Exchange Organizations after introducing Exchange 2013 Server

As servers running Exchange 2013 Server are introduced into an organization the same CAS URLs and name space from Exchange 2010 can be used, with one addition for Outlook Anywhere as discussed below. Additionally, the same security certificates used in Exchange 2010 server can be used on Exchange 2013 Server as well. The Table below includes the additional Outlook Anywhere URL.

Outlook Anywhere is the default protocol for client connection on Exchange 2013. To allow co-existence with Exchange 2010 it should also be enabled to use Outlook Anywhere. All the Outlook clients should be using Outlook Anywhere as the default connection. The PowerShell script below can be used to enable Outlook Anywhere on Exchange 2010 Server:

Get-ExchangeServer | Where {($_.AdminDisplayVersion -Like “Version 14*”) -And ($_.ServerRole -Like “*ClientAccess*”)} | Get-ClientAccessServer | Where {$_.OutlookAnywhereEnabled -Eq $False} | Enable-OutlookAnywhere -ClientAuthenticationMethod Basic -SSLOffloading $False -ExternalHostName -IISAuthenticationMethods NTLM, Basic

Figure 2 shows a typical deployment model for Exchange 2013 and Exchange 2010 co-existence. With Outlook Anywhere configuration in place the load balancers need configured to work with Exchange 2013 Server to use the and URLs shown in the table. This allows the clients to connect and if the target mailbox is on an Exchange 2013 Server it directly connects to the user’s mailbox. If the target mailbox is on a legacy Exchange 2010 Server then the Exchange 2013 CAS server proxies the connection to the Exchange 2010 CAS server, and this sets up the connection to the mailbox. The proxying of this connection is entirely transparent to the users.

DNS changes will also be required on the load balancer that is handling requests to the CAS servers. This is necessary to ensure that the correct requests are sent to the right Exchange 2013 and Exchange 2010 servers during the co- existence. The same will be true for load balancers directing SMTP requests if in use. The DNS changes required will be unique to each Exchange organization.

Figure 2. Exchange 2013 and Exchange 2010 Co-existence.

The points below outline how various protocols are proxied from Exchange 2013 to Exchange 2010:

OWA Protocol

OWA Client -> Exchange 2013 CAS - > Proxies Connection -> Exchange 2010 CAS -> Exchange 2010 mailbox

Active Sync Protocol

Active Sync Client -> Exchange 2013 CAS - > Proxies Connection -> Exchange 2010 CAS -> Exchange 2010 mailbox

Exchange Web Service

Active Sync Client -> Exchange 2013 CAS - > Proxies Connection -> Exchange 2010 CAS -> Exchange 2010 mailbox

Outlook Anywhere

Active Sync Client -> Exchange 2013 CAS - > Proxies Connection -> Exchange 2010 CAS -> Exchange 2010 mailbox

Hopefully this article series will help you to understand how a load balancer plays a role in configuring coexistence between Exchange 2010, 2013, and 2016. The next two articles will cover coexistence when migrating from Exchange Server 2010 to Exchange Server 2016, and from Exchange Server 2013 to Exchange server 2016.