Exchange Server 2010 : Availability Planning for Client Access Servers (part 3) - Global Server Load Balancing

1.2.5. Global Server Load Balancing

Global server load balancing (GSLB), or wide-area load balancing, is a more sophisticated version of DNS round robin available from some hardware load balancer vendors. This solution is typically deployed as a hardware device or even as a feature of a hardware load balancer. This type of load balancing uses DNS to load-balance client connectivity between sites based on a number of factors such as location of the client, response time of the servers, availability of the servers, custom weights, and more. GSLB is typically used in multiple site configurations to provide load balancing between sites. To provide full site redundancy the GSLB device should be located outside of either of the load-balanced sites or deployed in multiple sites. One way to use the GSLB is to load-balance Autodiscover to ensure that it is available even during a single site outage. In Figure 1, Autodiscover.constoso.com is set up for GSLB—all traffic will be sent to the IP address for the Denver Autodiscover service. In the event of a failure of Denver, the GSLB device can send all traffic for Autodiscover.contoso.com to the second site.

The GSLB device will accept DNS requests from the client and then return the appropriate IP address based on the rules defined. The TTL for the returned IP address is set low to ensure that changes are received by the client as quickly as possible. As with DNS round robin, because GSLB relies on DNS client resolution, its functionality is limited when the client DNS resolution is uncontrolled.

1.2.6. Load Balancing Summary

As you can see, you have a variety of solutions to choose from, depending on business requirements and budget. Table 1 combines affinity, load balancing, and other considerations when choosing a solution for load balancing.

Table 2 summarizes the configuration needed to support all of the Client Access Server protocols. If the load balancer is used to terminate the SSL certificates, the traffic between the load balancer and the Client Access server will be unencrypted; thus, the unencrypted port is used. Each of the services can be provided with separate load-balanced IP addresses to apply different load-balancing policies to each.

By default the Outlook client will make a connection to the RPC Endpoint Mapping Service on TCP/IP port 135 on the server to negotiate a dynamic RPC port above TCP 1024 for usage. If no firewalls or load balancers are between the clients and servers this is usually not an issue. You can reduce the number ports that need to be load balanced by modifying the Client Access servers to scope down the ports that are required. You must make three modifications:

After the load balancer is configured, certificates need to be applied and the internal and external URLs need to be set on each of the Client Access servers.