Load Balancing Microsoft Exchange Server 2013 CAS Server

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Eustace Richard
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1 Load Balancing Microsoft Exchange Server 2013 CAS Server Author : Shuvro Load Balancing Exchange 2013 CAS Exchange 2013 has been designed for simplicity of scale, hardware utilization, and failure isolation. This has greatly simplified both the deployment process and the implementation of a load balancer. Exchange 2013 has reached its enterprise grading with DLP services also, but today in this article I will focus on the load balancing Exchange Server With Exchange 2013, now we have several new options for load balancing the Client Access Servers (CAS) and I would quickly cover those areas. Exchange 2013 Architecture Exchange 2013 has been designed with some architectural changes from the previous versions of Exchange Now exchange 2013 has only two roles, as you can see in the following pictures. Those roles are the Client Access Server and the Mailbox Server. Apart from previous exchange; it has CAS, MBX, UM, HT and Edge roles. The functionality of the Hub Transport and Unified Communications roles from the previous version has been added into these two roles (CAS & MBX). CAS now serves the role as a proxy server and the work horse is MBX server. And Microsoft did a tremendous job conjuring a small footprint over hardware requirements, as this redesign also gets reflected on the High Availability designs and as well as the DAG considerations. Speaking of hardware s on Exchange 2010 you would use a Layer-7 load balancers; but now you can use a Layer-4 load balancers which would do the trick of balancing Exchange 2013 traffic in a large organization. Something you should know about MBX: A server that hosts all the components that processes, renders and stores the data. Clients do not connect directly to MBX2013 servers; connectivity is through CAS2013; Evolution of E2010 DAG. Collection of servers that form a HA unit Databases are replicated between servers in a given DAG (Database Availability Group). Servers can be in different locations for site resiliency, with a Maximum of 16 Mailbox servers and 100 database copies per server. [caption id="" align="aligncenter" width="820"] 1 / 6

2 New Exchange Roles[/caption] [caption id="" align="aligncenter" width="1019"] 2 / 6

Exchange 2010 Roles[/caption] Hardware Load Balancers Microsoft Best Practices always have recommended against using a Network Load Balanced cluster for the CAS role for the last two versions of

One of Microsoft s objectives with this version of Exchange 2013 was to promote and support lower cost hardware load balancer options; eventually the cost gets lower for your load balanced traffic

3 Exchange 2010 Roles[/caption] Hardware Load Balancers Microsoft Best Practices always have recommended against using a Network Load Balanced cluster for the CAS role for the last two versions of Exchange. That recommendation remains the same for Exchange One of Microsoft s objectives with this version of Exchange 2013 was to promote and support lower cost hardware load balancer options; eventually the cost gets lower for your load balanced traffic throughput. The CAS Array I bet that most of you folks out there who would read this article would get their hands on Exchange 2010, probably know that you need to create a CAS Array to balance out your CAS server(s). Also, by creating the CAS arrays you are conforming to MS best practices. The CAS array did not provide any load balancing or high availability features whatsoever, but instead it just populated the name of the CAS array to the RpcClientAccessServer property of the mailbox databases. This value was in turn populated to the clients during the AutoDiscover process to tell all clients where to connect to get their mails. 3 / 6

4 An architecture change in Exchange 2013 now means that clients point to a unique GUID for each mailbox. The CAS server is able to perform an Active Directory lookup based on this GUID, and then find the correct mailbox server to target. Which eventually leads to the thought that the CAS array object is no longer used in Exchange The Load Balancing Options Another benefit of the architecture of Exchange 2013 is that we now have two better load balancing options. Those options are the less complex layer 4 load balancing, or the more resilient (more resilient=high cost) layer 7 load balancing. Exchange Server 2013 also comes with Thin, stateless (protocol session) servers organized in a load balanced configuration which DOES NOT require session affinity (aka, layer 7 LB), sounds providence right? Right it is. [caption id="" align="alignnone" width="880"] Protocol Lineup[/caption] Layer 4 Load Balancing One of the best known Layer 4 load balancers is Microsoft Network Load Balancer or NLB, this is a core network load balancer software that is available to users of mission critical Microsoft applications including Microsoft Exchange and Microsoft Lync the unified communications platform. Like other Layer 4 load balancer solutions it is fairly simple. Most layer 4 load balancers perform the following functions: Load balance occurs at a simple level and directs traffic based on server response time 4 / 6

5 Each server shares the same IP address The least busy server responds to the content demand Round Robin DNS is extensively used Layer 4 load balancing refers to load balancing Exchange at the transport level. Layer 4 load balancing is easy to configure, and only requires two namespaces in most situations (A legacy name space is still needed for coexistence with Exchange 2007.) The downside to layer 4 load balancing is that the load balancers are not aware of individual service failures (no heartbeat check), they are only able to see service availability on a server-by-server basis. This means that if one of your CAS were to have a failure with just OWA, the entire server would be taken out of service, routing all traffic to other servers. Layer 7 Load Balancing Layer-7 Load balancing is complex and far more efficient. Layer 7 load balancing is effectively ruled by directing the traffic according to the application that is being used. Layer 7 load balancing refers to load balancing at the application level. A load balancer in this configuration will be able to detect individual service failures, and route traffic accordingly. So if a single server were to have a failure in the IIS directory that runs OWA, the load balancer would be able to route OWA traffic away from that server while still leaving it available for EAS, Outlook, and all other types of traffic. A layer 7 load balancing configuration does require more complex hardware load balancers, along with the configuration of SSL offloading. This means that the deployment is going to be more time consuming, and will require more expensive hardware load balancers. Additionally it is necessary to more closely monitor the application of patches to servers in this configuration. Historically, patches have changed settings that can affect the operations of a server with SSL offloading configured. There is NO Third Option The architecture of Exchange 2013 does provide for a third option, which I believe combines the best aspects of both of the previous options. The key to this third option is using layer 4 load balancing, but with multiple name spaces. Above I said that layer 4 load balancing only requires two namespaces, and that is correct. However, if additional name spaces (each with its own public IP address) are configured, then the hardware load balancers can be configured to recognize individual service failures without the added complexity of layer 7 load balancing. References: Microsoft Exchange Server improved: Client Access Server: CAS Load-Balancing Best Practices (Part 1) (Great Post): 5 / 6

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What to Know About Exchange 2013 and Load Balancing

What to Know About Exchange 2013 and Load Balancing What are the major differences between Exchange 2010 and Exchange 2013? To answer that let s discuss the main architectural differences between Exchange