CertiGuide to A+ (A+ 4 Real)  9  Chapter 16: Network Load Balancing and Clustering

Network Load Balancing (NLB) 1 How Clustering Works

Network Load Balancing provides for high availability and sharing of network load amongst multiple servers. By definition, these servers must each contain an identical copy of the data they are serving – for example, the Microsoft Internet Information Systems (IIS) sites must be identical on every server in the cluster. Because the server in the NLB cluster can service any request to the load balanced IP, the servers must be absolutely identical. As web sites are generally static web pages, that do not change. the actual data is stored on back-end database servers; they are perfect candidates for NLB. However, in some situations, it is necessary to provide high availability for data that cannot be duplicated as it changes regularly, and cannot be configured in the front end/back end model. An example of this would be a Microsoft Exchange installation. Even if multiple Exchange servers are installed in an infrastructure, only one server can use a storage group at any one time. If that server is offline, the storage group goes offline too. The same scenario is valid for many other examples, including SQL Servers and bespoke applications.

To resolve this type of issue, a cluster is used. There are three basic components to a cluster – 2 identical servers, 2 network cards per server and a number of shared hard drives. These shared disks can be either in a shared disk array, or a part of a SAN (Storage Area Network). The servers are connected to these shared disks usually via fibrechannel cards, and are connected to each other via a dedicated heartbeat network connection. The remaining network card is used to connect each server to the rest of the network. At least one shared disk must be available to the cluster, which stores the cluster registry hive and control the management of the shared disks. This special disk is called the “quorum”, and should only be used by Windows. Once the minimum components required for a cluster have been successfully configured, further cluster resources can be added.

Network Load Balancing (NLB) 1 How Clustering Works