Friday, 10 January 2014

Network Load Balancing (NLB) 2008 Dual NIC

Today i was attempting to configure a new 2008 R2 Network Load Balanced IIS servers for a highly available Moodle environment. I hit an issue that meant for some reason my NLB IP address wouldn't respond to pings, even after implementing my Cisco fix here. My PC was in a different subnet so i tried to ping from the same subnet, and this worked. 


In my scenario i have two nodes each with 2 NIC's, one for main network communication and the other dedicated for Load balancing, both on the same subnet. So all communications to the cluster would go to NIC 2. NIC 1 is in this case only used to RDP to the server.

Similar to the config in the image below.

NIC 1 on each NLB Node has the default gateway and DNS settings, as per MS best practices, and NIC 2 simply has an IP address and a mask.


What is happening in this scenario is that when the NLB cluster NIC receives my ping packets on NIC 2, it has no way of getting back to my PC because NIC 2 has no default gateway. 

In 2003 server the default was that packets received on one NIC if they had no DG or route then they would automatically forward out of the NIC that does have one (the only route to the network).

The default in 2008 onwards is for this to be disabled. So my ping packet was reaching the NIC correctly, but was unable to respond to it because it has no route back.

The fix is very simple.

1. Load an Administrative command prompt.

2.  netsh interface ipv4 set interface "NIC 2" forwarding=enabled 

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