What is Network Load Balancing (NLB)?

A. NLB is a core part of all editions of Windows Server 2003 (including Web Edition), which, as the name suggests, load balances incoming TCP/IP traffic across members of the NLB cluster. Unlike the cluster service, which is available only in Windows 2003 Enterprise Edition and Windows 2003 Datacenter Edition, NLB requires no shared disk subsystem and is primarily designed to distribute traffic to the members of the NLB cluster in which the individual members effectively offer their own self-contained service or instance of an application (e.g., a group of Web servers, streaming servers, VPN servers, Microsoft SharePoint servers or terminal servers).

You can have up to 32 of any of the aforementioned server types, each essentially running on their own, but rather than users deciding which one to use and selecting an alternate if one is unavailable, NLB assigns a new IP address, which any of the NLB cluster members can respond to. NLB distributes the incoming requests to one of its members via a distributed algorithm, and if a member of the NLB cluster becomes unavailable, the NLB quickly responds and stops directing clients to that server (usually in less than 10 seconds).

As an example, imagine I have five Web servers with IP address 10.1.1.1 through 10.1.1.5. Instead of letting clients pick one to go to, I create an NLB cluster with IP address of 10.1.1.10, which will decide which of the five possible Web servers the request will be directed to, as the figureshows. All the clients use IP address 10.1.1.10 (or a DNS entry which resolves to 10.1.1.10) and are then seamlessly redirected to one of the NLB members. It's also important to note that NLB doesn't run on a separate server; the NLB service runs on all the members of the NLB cluster. NLB is a core part of the Windows 2003 OS--you simply need to configure it.

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