Network load balancing explained
NLB is also a high availability solution, but it works at the network level (as its name implies) rather than at the resource level (as we see in the Cluster Service). It also functions as a driver rather than as a service, as it did in NT 4.0. Up to 32 servers can be clustered in the same subnet, appearing to clients as one server and distributing the load between them. Each server in the network-load-balanced cluster is aware of each other, and when one server fails to respond, the rest converge and automatically redistribute the new connections among themselves. This offers true horizontal scaling because you can simply add another server to the cluster when network load increases beyond the existing servers' capabilities. When you reach the maximum of 32 servers, you can start another cluster and use DNS round robin to alternatively hand out the clustered (virtual) addresses. Check out this article for a clarification on the differences between NLB and DNS round robin. Note that there is no sharing of data between servers running NLB, so with Windows 2000 Advanced Server, you will have to independently configure each server and replicate data that's needed for the load balanced services. Windows 2000 Datacenter Server offers Application Center 2000 to help manage multiple NLB servers, but this setup is too expensive and complex for many corporate networks. NLB has a reputation for being used with Web servers, but you can also effectively use it on your corporate network with services such as file servers and printers, terminal servers, and even PPTP servers. However, IPSec cannot be load balanced, and this applies to straight IPSec in both transport and tunnel mode and L2TP/IPSec. You enable and configure NLB in the General tab of the Network Load Balancing properties for your network adapter. Here, you enter the parameters you want to use with the three tabs, Cluster Parameters, Host Parameters, and Port Rules. Monitoring and maintenance is done with a command-line utility called WLBS.exe, which you can run locally or remotely (if enabled). If you choose to run it remotely, you can also password protect it for obvious security reasons. Unlike Cluster Service, NLB requires no special hardware. The only stipulation is that two network cards are used to eliminate a single point of failure. And unlike Cluster Service, you don't have to run these servers in a domain. They can be in a workgroup outside your domain, thereby presenting a smaller security risk.