In Exchange 2007, a common approach for achieving site resiliency was through the use of an active/passive or active/active datacenter model where first local high availability within a datacenter was achieved by using cluster continuous replication (CCR) based clustered mailbox servers which meant deploying both cluster nodes in a CCR cluster in the same primary datacenter and a fail or switchover did not force clients to attempt a connection to the other datacenter.
A method called standby continuous replication (SCR) was then used to make the solution site resilient by replicating log files from the (CCR) based clustered mailbox server (SCR source servers) to one or more clustered SCR target servers deployed in the other active or passive datacenter. When a disaster took down the primary datacenter, the clustered mailbox servers (CMS) was then brought online on the standby clusters (SCR targets) in the other datacenter.
Load balancing and high availability for the Client Access server (CAS) role within a datacenter was usually achieved by using either Windows Network Load Balancing or a load balancer solution from a 3rd party vendor however with Exchange 2007 most went with WNLB including Microsoft IT. So if Exchange 2007 was deployed across multiple datacenters each with their own Internet connection, the recommended approach in regards to the Client Access server role was to use a unique namespace for each datacenter (ex: mail.contoso.com and mail.standby.contoso.com). In case of a disaster that results in the primary datacenter to become unavailable, the external and internal DNS records for client access was pointed to CAS servers in the failover datacenter.
Resiliency was designed into the Hub Transport role so that Hub Transport server to Hub Transport server communication inside an organization automatically or “automagically” load balances between available Hub Transport servers in an Active Directory site. However in order to load balance inbound SMTP connections from external SMTP servers/clients, internal LOB application, and miscellaneous network devices, using WNLB or a load balancer solution is recommended.
If some organizations did not have the budget needed for local high availability based on CCR combined with site level mailbox resiliency via SCR, a multi-site CCR cluster was deployed. Others simply did not go with local mailbox resiliency but instead deployed a multi-role (Client Access, Mailbox, Hub Transport) Exchange 2007 server in each datacenter. Then SCR was enabled so that databases were replicated to the other Exchange 2007 server.
Although this solution was ideal when it came to keeping the number of Exchange servers required down to a minimum, it had no real high availability on the mailbox level (other than RAID used for storage protection) so it was really more of a disaster recovery solution more than it was high availability. This is because several manual steps to switch or failover to the secondary datacenter were required.
With enhanced design in Exchange 2010, high availability not only locally but site to site was more easily achieved through the use of new features such as RPC Client Access arrays, database availability groups (DAG) and the added necessity for implementing hardware load balancing as part of the Exchange 2010 site resilient solution.
Like Exchange 2007, there are multiple ways to establish site resiliency with Exchange 2010. One common way is by an active/active datacenter model. With that said, in an active/active deployment, LoadMaster DR can be used to Round Robin DNS requests based on FQDN’s (Fully Qualified Domain Name) you have set up either on your internal or external DNS server or both. For more information on LoadMaster DR, please access the following links here and here.