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What do you need load balancing for?

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Kurt Tavares

City of Tracy

Load Balancing Microsoft Exchange

Microsoft Exchange Server, the mainstay of Microsoft’s Unified Communications solution has grown beyond being regarded as the standard in business email into a fully fledged communications tool.

Once difficult to manage at scale, the Microsoft Exchange platform has reached a level of maturity. With a simplified architecture, it has evolved to support high availability and scalability by design.

To implement highly available and scalable deployments of Microsoft Exchange Server, Microsoft recommends using a load balancer to distribute the traffic among multiple Exchange servers. Both current and legacy versions of Exchange support load balancing, with a different approach and recommendations dependant on the version you are running.

exchange-diagram

Example Exchange 2016 deployment using a minimal 2 server setup.

Exchange 2013 & 2016 Protocol Table

Protocol Role Ports Load balancing methods
TCP CAS 443 Used for Outlook on the Web, AutoDiscovery, Web Services, ActiveSync, Outlook Anywhere, Offline Address Book, Exchange Administration Center). Layer 4 DR (Direct Routing – Ultra-fast, local server based load balancing) Layer 7 SNAT
TCP CAS 25 Used for Inbound SMTP Layer 4 DR (Direct Routing – Ultra-fast, local server based load balancing) Layer 7 SNAT (Flexible, URL switching and cookie insertion capabilities)
TCP CAS 110, 995
143,993
Used for POP3 clients Used for IMAP4 clients Layer 4 DR (Direct Routing – Ultra-fast, local server based load balancing) Layer 7 SNAT (Flexible, URL switching and cookie insertion capabilities)

Exchange 2010 Protocol Table

Protocol Role Ports Load balancing methods
TCP CAS 80 Layer 7 SNAT
TCP CAS 443 Layer 7 SNAT
TCP HT 25 Used for the HT (Hub Transport) role Layer 4 DR (Direct Routing – Ultra-fast, local server based load balancing) Layer 4 NAT (Fast Load balancing throughput) Layer 7 SNAT (Flexible, URL switching and cookie insertion capabilities)
TCP CAS 110, 995,
143, 993,
135, 60201
Used for POP3 clients Used for IMAP4 clients RPC endpoint mapper Static port for Exchange address book service Layer 7 SNAT

FAQs

  • What parts of Microsoft Exchange do I need to load balance?

    In Exchange 2010, key functionality is split into 3 roles: Mailbox Server, Client Access Server and Hub Transport Server. In Exchange 2013, roles were consolidated into Client Access Server and Mailbox Server. In Exchange 2016, primary functionality was consolidated into a single role - the Mailbox Server. For all versions of Exchange, the load balancer is used to load balance all required client connection protocols, as well as inbound SMTP connections.

  • Do I need to enable persistence on the load balancer?

    For Exchange 2013 & 2016, all sessions to the CAS servers are stateless and therefore persistence/affinity is no longer required on the load balancer. For Exchange 2010, some protocols require affinity and others don't as detailed here.

  • Can I offload SSL on the load balancer?

    The load balancer fully supports SSL termination and backend server re-encryption. SSL offloading for Exchange 2013 and later is supported from 2013 SP1 as detailed here. However, for scalability and effective load sharing we recommend terminating SSL on the Exchange Servers rather than on the load balancer.

  • Can I deploy across Multiple Data Centers?

    For a detailed look at deploying across multiple data centers, please refer to this blog.

Surely you must have a question?