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NLB: The Importance Of Interface Metrics

More on Network Load Balancing… If you’re setting up a garden-variety NLB (WLBS / “Wibbles“) cluster, there are a couple of things that will possibly make life easier:


For multi-NIC configurations where one adapter is for incoming cluster traffic and the other is for back-end stuff (say, Terminal Services/Website with a database connection or similar):

  • configure the NLB adapter with an IP interface metric higher (eg, higher cost) than the dedicated adapter

    • this causes traffic from that node (host-initiated outbound traffic) to be sourced from the dedicated adapter rather than the NLB adapter

  • and/or, ensure both NICs are in different IP subnets and that the routing’s configured to use the correct adapter for the target

    • for example, in an Internet-facing scenario, the NLB NIC will probably have the default gateway, and you’d use a static route for the back-end

For single-NIC configurations using multicast mode, no need to adjust metrics, but:

  • make sure the “dedicated IP address” (as defined in NLB properties) is the default IP address for the adapter

    • same sort of reasoning as the above – the first-bound IP is first to be used for outbound communications

This also applies to situations in which you might need to make outbound connections from the NLB adapter in a multi-NIC scenario (web services, perhaps), so the golden rule is: if you need a dedicated IP address (a question we’ve gone into before), you need to make it the first IP bound to the clustered NIC.