Windows 2008 NLB with 2 NICs

I ran into a problem with our standard configuration for web servers, and couldn’t find the real solution documented anywhere, so here it does.

We run our ColdFusion servers on dual node Windows Network Load Balancing (NLB) servers running in IGMP multicast mode. We run it on machines with two network cards. The cluster address is on one NIC and the nodes answer on another. It’s the configuration we’ve come to like after years of working with NLB and port flooding and other anomalies.

I’m installing a new production NLB cluster for Knowledge@Wharton. To future proof it, and avoid upgrades down the road, I’m going with ColdFusion 8 64 bit on Windows 2008 64 bit. I ran through the configuration steps that I always take setting up an NLB cluster, and it worked… sort of. See the cluster address answered if you called it from another host on the subnet that the cluster was installed on. However, if you were off subnet it didn’t answer. This is suboptimal for a web server.

I worked with our networking team, and they figured out (from this post: ) that if you added a gateway to the cluster NIC, it would work. This is counter to the way NLB has worked before, and generally not best practice. So we opened a support case with Microsoft. After a few tries, I finally got an engineer that was an expert on NLB in 2008, he had the exact cause and solution for this problem: by default IP Forwarding is not enable in Windows 2008. 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’s fixed by using one specific command line option.

(Make sure you are using a command prompt with administrative privlidges)

netsh interface ipv4 set int “[name of the NIC]” forwarding=enabled

That’s it.

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