For this verison of windows, it requires both NICs to have a Gateway.
This is because Windows 2008 R2 uses something called a Strong Host Model.
Someone else says:
Now I tested web site access from an external IP and it worked perfectly. My conclusion? You have to configure the default gateway on an NLB NIC if using Network Load Balancing on Windows Server 2008 R2. Otherwise it will not route correctly to other networks; it should pick up the default gateway from the management NIC but it does not.
Windows 2008 introduces a “strong host model” that doesn’t allow the
different NICs to talk to each other. For example, if a request comes in on
the 2nd NIC and there’s no default gateway setup, then the NIC will not use
the 1st NIC to reply to the requests. (even though there’s a default gateway
setup on that 1st NIC).
In order to change that behaviour and go back to a 2003 model, you go to the
command prompt and then you type:
netsh interface ipv4 set interface NLB weakhostreceive=enable
netsh interface ipv4 set interface NLB weakhostsend=enable
(where NLB is the name of the network interface… default is Local Area
As an alternative, you can set a default gateway on the 2nd NIC but that can
introduce more problems where the system doesn’t know which way to send
traffic. MS said that I could set the metric to 2 on the 2nd NIC and that
way it will only be used if the 1st NIC is unavailable.