After configuring network load balancing (NLB) are you experiencing the following with your Server 2008 boxes?
- Communication with the cluster VIP from any computer on the same subnet is successful.
- Communication with the cluster VIP from any computer on a different subnet fails.
- If you apply the default gateway to the cluster NIC, everything works.
The problem? IP forwarding for any network interface on a Server 2008 box is disabled by default (as opposed to Server 2003 where forwarding was enabled by default). This leads to the behaviors exhibited above as any packet entering the inbound NIC (your NLB VIP address), has no way to exit the subnet and is subsequently dropped.
Fortunately for us, enabling IP forwarding on the cluster network interface is easy enough with a quick trip to the command line or registry (I prefer the command line/netsh route as a reboot is not required).
The following steps will enable IP forwarding on your cluster NIC:
NETSH
- Determine the name of the clustered interface:
netsh interface show int
- Enable IP forwarding on the NIC:
netsh interface ipv4 set interface "Clustered Interface" forwarding=enabled
- Verify that forwarding has been enabled on the interface:
netsh interface ipv4 show interface "Clustered Interface"
Registry
- Add/Edit the following key:
Key name: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters Value Name: IpEnableRouter Data Type: REG_DWORD Value: 1
- Reboot server.
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