Preparing the Network for NLB 2008

Windows Server 2008 is here, along with a new version of Network Load Balancing (NLB).  Just as in previous versions, NLB continues to provide an excellent option for scaling many kinds of applications and promoting higher availability.  And while the deployment and configuration of NLB is fairly straightforward, it’s important to ensure the network environment is ready for NLB. 


If you choose to deploy NLB using unicast, all of the NLB adapters will share a Cluster MAC address, in addition to the Virtual IP (VIP) address.  The idea behind the shared MAC is that when a host communicates with the MAC address for the NLB Cluster, all of the NLB nodes will respond, making it impossible for the switch to associate the MAC address to a particular port.  This in turn will cause the switch to simply flood the frames destined to the Cluster MAC out all of its ports, ensuring that all of the NLB nodes receive the frames.  Problems may arise when using multi-layer switches or virtual network environments if the switch does associate the Cluster MAC or the Virtual IP to a specific port.  In this case, only one NLB node will receive traffic destined to the Virtual IP address of the Cluster, preventing the remaining NLB nodes from sharing the load.  One way to get around this issue is to employ a hub.  By connecting all the NLB nodes into a hub, and then connecting the hub to a port on the switch, all of the NLB nodes will receive the traffic destined to the Cluster.  Another solution is to configure port mirroring on the switch to ensure traffic sent to one of the NLB ports is replicated to all of them.

As mentioned earlier, unicast NLB relies on switch “flooding” behavior to function properly.  If you want to limit the flooded traffic on your network, you  can create a separate VLAN encompassing only the ports the NLB nodes are connected to.


You can also opt to deploy NLB using multicast.  With multicast, each NLB node effectively has two MAC addresses: a physical MAC and a multicast MAC.  Switches typically do not associate ports with a multicast MAC address, so the traffic will be flooded out all ports.  The flooding of the multicast traffic may cause unintended network performance issues.  To resolve these issues, you can configure the switch with static mappings of the multicast MAC and the ports that the NLB nodes are connected to.

NLB Manager

One other point to keep in mind when deploying Windows Server 2008 Network Load Balancing is that the NLB Manager from Windows Server 2003 cannot be used to manage Windows Server 2008 NLB nodes.  You can manage the Windows Server 2008 nodes with the NLB Manager on a Windows Server 2008 server or with Windows Vista if you have the Remote Server Administration Tools (RSAT) installed.

For more information on deploying NLB, including upgrading from Windows Server 2003 NLB, check out the following article:

– Baruch Frost