Clustering is different than Federated Servers.
Clustering is a Windows and SQL feature, while you can configure what is referred to as an Active-Active cluster what that really means in Node 1 runs an instance of SQL Server supporting Applications A and B and Node 2 runs an instance of SQL Server supporting Applications C and D. However in the event of a failure of Node 1 or Node 2 the other node can take over all of the resources for the failed Node. Eg. if Node 1 fails, Node 2 takes over its instance of SQL Server (it starts it up as its own process), its virtual Name and virtual IP address, it also has to take over the disk resource (on a shared SCSI bus) where the databases for Apps A and B reside.
Windows does provide Network Load Balancing (NLB). However, NLB is used for middle tier business logic in COM+ components, and can't be used for SQL Server.
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