How does Microsoft’s Network Load Balance Services work (also also how to setup your SharePoint Server to do load balance out of the box):

How does Microsoft’s Network Load Balance Services (NLBS) work (how to setup your SharePoint Server to do load balance out of the box):

–          each servers’ have its own IP (the usual IP addresses)

–          a cluster is then setup using NLBS (by enabling NLBS on all three servers).

–          When NLBS is enable, you need to assign a IP to the cluster (virtual IP, in this case its 10.0.0.1)

–          Once setup, the three servers will then take turn on accepting inbound 10.0.0.1 request.

–          NLBS is not the same as round robin DNS, once the Cluster is setup, the three servers communicate with each other on the its current traffic and load and if one of the servers in the cluster is down another server then takes over.

–          The NLBS works the same way as a regular hardware load balancing (where in hardware load balancing you would assign the IP to the hardware)

  • Because NLBS is a windows service it utilizes the same TCP/IP stack on the server.
  • After NLBS is setup, two IP addresses appear on the NIC, both the server’s real IP and the cluster’s IP.  This is a NORMAL behavior of NLBS.
    • One issue arise, because of this dual IP behavior, the network itself must support multicast
      • Both the switch and the router.
      • My prior email send to the hosted network: 
        • In order for us to setup software load balancing on the network card needs to operate in multicast mode, when operating in multicast model, it will appear to have two MAC address on one single NIC (one virtual MAC and one original MAC).  My understanding is that some routers/switch reject the ARP replies, as the ARP request contains a unicast IP address with a multicast MAC address (and the router/switch considers this invalid and rejects the update to the ARP table). 
    • Multicast is needed mainly due to this dual IP behavior after NLBS is setup and configure.
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