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Exchange Server 2013 supports roles across multiple physical server nodes. This is common with the Client Access Server, for example, which allows Exchange to scale and service extremely large and busy email environments. However, admins must include some amount of Exchange load balancing in the multi-node deployment to spread out workloads and prevent certain nodes from being overtaxed.
Using round-robin traffic distribution can help spread inbound client traffic across multiple Client Access Server (CAS) setups. Secure Socket Layers (SSL) can also encrypt Exchange traffic in flight, but the processing overhead involved can affect CAS performance. Some Exchange admins use SSL offloading, which shifts SSL encryption and decryption from CAS nodes to the load balancer. This frees up computing resources on each CAS node. SSL offloading demands that every CAS role is configured identically; it also must be enabled for every service, such as Outlook Web App, Exchange Admin Center or Outlook Anywhere.
Finally, admins can use Windows Network Load Balancing (WNLB) in the Exchange deployment, but there are some possible restrictions with this Exchange load balancing option. WNLB works by IP addresses rather than by services, so it can respond to the loss of an IP address but not necessarily the loss of a service such as Exchange ActiveSync. In addition, WNLB is incompatible with Windows failover clustering, so it won't work on Exchange Servers with database availability groups (DAGs) for high-availability deployments.
In these scenarios, it's usually best to use a hardware load-balancing product, though virtualization can allow WNLB to support CAS and Mailbox VMs in a DAG on the same physical server. Exchange 2013 supports a wide range of hardware- and software-based load balancers from vendors like A10 Networks, F5, Riverbed and others.
This is part four in a series on ways to improve Exchange 2013. Part one highlighted some free tools to help admins assess the state of their deployments. Part two covered Outlook areas to optimize. Part three highlighted ways to increase security.
Stay tuned for part five, which covers possible virtualization options.
Five ways to avoid trouble when upgrading to Exchange 2013.