If you have an FE/BE (frontend/backend) Exchange configuration, you can use NLB to distribute traffic to the FE servers, but you don't want to think about using it for the back end servers. If you don't have an FE/BE configuration, you don't want to use NLB. NLB is used when you want to distribute traffic between systems that have the same data on them, like web servers who server dynamic data pulled from a separate database server, or a FrontEnd Exchange server that is pulling data from a BackEnd information store.
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