Windows NLB Unicast - NIC Setup?
I've read many articles regarding Windows NLB in Unicast mode that always seem to mandate (2) NIC's. However, I have not seen any detailed info on why this would be a requirement in a flat, single VLAN environment? Why would you "need 2 NICs in Unicast mode if you only have a single internal VLAN? Lets say you have 4 web servers in an NLB farm on a 10.10.10.x/24 network. Your backend SQL server is also on 10.10.10.x/24. You have a simple firewall that NATs from the NLB VIP = 10.10.10.100 to/from the Internet. External users hit the site using the NATd public IP, which maps internally to 10.10.10.100 and the web servers then talk to the SQL server at 10.10.10.200. What would be the purpose of multiple NICs per web server in this Unicast NLB scenario?
February 21st, 2012 7:48am

When you use unicast mode, it changes the cluster adapters MAC address to the cluster MAC address. This cluster address is the same MAC address that is used on all cluster hosts. When this change is made, clients can no longer address the cluster adapters by their original MAC addresses. In Unicast mode, the NLB-bound interfaces for all nodes in a cluster share the same MAC address (the NICs' actual MAC addresses are pretty much just overwritten), which means that when a cluster node ARPs for the other node's IP address and gets its own MAC back in return, no traffic ends up hitting the wire. For example, if you have 2 nodes A & B with MAC address 00-11-aa-bb-cc-dd & 00-22-aa-bb-cc-dd respectively & when you add these 2 nodes in unicast mode, the virtual MAC address created suppresses the NICs original MAC address & both the nodes will have a common MAC address 02-bf-xx-yy-zz thats the NLB MAC. Hence communication on the dedicated MAC of the NIC becomes impossible, due to which the need of the second NIC arose. However after Win Server 2003 SP1 there was a workaround to this, you can set the registry key InterhostCommSupport & set the value to 1 which makes the communication between the 2 nodes possible without having the need to add the second NIC while using Unicast Mode.Shweta - MSFT
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February 21st, 2012 9:08am

With 2008 and above UnicastInterhostCommSupport is enabled by default.Shweta - MSFT
February 21st, 2012 9:14am

Hi dkraut, Thanks for posting here. Just FYI, in some cases if we are going to manage individual node host in NLB cluster via protocol such like RDP then wed better make it to have a second NIC for such remote management tasks otherwise the connection might will be broken randomly if manage it via the VIP address we set in NLB and will take a long time to recovery the connection cos in Unicast mode both nodes will have same MAC address for communication and the switch which they connect with might will not transfer the packet to the correct port that node connect with. I think this is why they suggested to have second NIC when in unicast mode. Regards, Tiger Li TechNet Subscriber Support in forum If you have any feedback on our support, please contact tnmff@microsoft.com.Tiger Li TechNet Community Support
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February 21st, 2012 9:50pm

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