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170 Eleventh Hour Security : Exam SY0-201 Study Guide If the failover server doesn't receive a response during one of these checks, it will then takeover the role of the primary server and provides services to users. Many operating systems provide the ability to cluster servers together. Server clusters are groups of independent servers that are connected together, so that if one fails the other will continue to provide services. Each server handles its own local resources and has a copy of the services and applications that run on other servers in the cluster. In many clusters, the servers share a single disk sys- tem, and appear on the network as a single entity. When a user makes a request for a resource, it is sent to the cluster. If one of these servers failed, the others in the cluster would still function and be able to take over processing requests from the network and providing services. There are two forms of server clusters that can be used on a network: active/active and active/passive. An active/active cluster has all of the servers actively responding to requests, so that if one server fails, all of the other servers in the cluster can con- tinue processing requests. An active/passive cluster has servers that are only used if the active server fails. In this type of cluster, the active server process requests while the other only becomes active if the first one fails. EXAM WARNING Remember that server clusters provide fault tolerance and redundancy, allowing users to continue making requests to servers even if one of the servers in the cluster fails. An active/active cluster provides high availability because all of the servers are regularly responding to requests. This isn't true in an active/passive cluster, where the passive server only becomes active when the primary server fails. Connections Redundancy is often found in networks, such as when multiple links are used to connect sites on a wide area network (WAN). Network lines may be used to connect two sites, with a separate network line set up in case the first goes down. If this first link fails, the network can be switched over to use the second link. In other instances, additional lines may be set up in other ways to provide redundancy. ISP Many companies depend on Internet connectivity almost as much as network connectivity. In some cases, such as e-commerce businesses, they depend on it more. A redundant Internet Service Provider (ISP) can be used to provide con- nectivity when an organization's primary ISP's service becomes unavailable. The