We had a Windows 2008 Server Network Load balancing setup consisting of two Windows 2008 Servers wich had two NIC’s each. The Load Balancing was configured to use the one of the two Interfaces for dedicated Load balancing, the other interfaces were configured for normal network traffic as shown in the picture below.
The virtual cluster IP address responded if you send it a ping from another host on the same subnet that the cluster was installed on. However, if you were on another segment of the network it didn’t respond.
Originally we were send off track due to an issue with the vendor’s NIC Teaming solution that was enabled by default and had proved difficult to remove.
This however proved not be the cause of our problem with the NLB configuration not responding to off-subnet network locations.
What did prove to be the case is that in Windows 2008 Server NLB by default IP Forwarding is not enabled. This is the feature of Windows networking that, in the context of NLB, allows responses to requests sent to one NIC to be routed out the other. It can be enabled by using a netsh.exe command.
“netsh interface ipv4 set int “[name of the NIC]” forwarding=enabled”
The command will respond with an “Ok.” and no reboot is required for the configuration to apply. This command has to be run on both machines and only for the interface configured for normal network traffic.
In this case the NLB became active immediately after running the commands on both machines. This little command is something that is good to remember when installing Windows Server 2008 NLB solutions and I for one will add it to my checklist.