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I am looking into SharePoint but have some problems to understand how to plan the servers in order to get good availability at the same time as using load balancing. I have basically planned for the most basic setup which is

Two Front end servers

One Application server

Two SQL servers

I want two front servers in order to get good up time so if one server craches the system should still work, the same applies for the SQL servers. I will have some sites that always needs to be up while some others are less important and can be down for a while. Therefor I want to have load balancing between the servers. If a server crashes I can afford to close some sites and that the other sites are a slower.

The problem is that I can understand how the load balancing is working. So I have looked into NLB in windows 2008. Lets say that I install that on my front end servers. Would that mean that my system will work even if one of my two front end server crashes?

This is how I understand it is working, I have a domain called Foo. The users tries to access Foo which routes to front end server 1, The NLB will then dependent on the load route the user to front end server 2 or keep the user at front end server 1.

The thing I can't understand is this, the DNS needs to route the user to a server that then can route the user dependent on the load. But What if front end server 1 crashes that are running the NLB. Will the user then still be able to access Foo thorugh front end server 2?

The same question applies to the databases? How does that work? Will I need to purchase some extra hardware or will SharePoint or SQL take care of both availability and load balancing?

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With NLB what happens is that there is one additional Virtual IP that you need to do the load balancing. Each server has it's own IP. When you setup NLB an additional IP is created with it's own MAC address.

When you use NLB with Unicast, recommended with servers with 2 NIC's, both servers listen for thraffic on the Virtual IP's MAC address. Each server will respond with the virtual IP depending on which machines acesses the Virtual IP.

An example would be client A on subnet a.b.c.d/24 accesses the Virtual IP and machine 2 answers. The client is now bound to machine 2, although it still communicates with the Virtual IP.

When client B on subnet w.x.y.z/24 connects, machine 1 answers and client B will now get it's data from machine 1. NB: For sharepoint, which stores all is databases in the same instance, your SQL cluster will have to be in Active-Passive mode.

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SQL does support Active-Active, but not shared instances. – Jeff Jan 20 '10 at 3:30

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