Network Load Balancing Clusters

This section presents a brief overview of Network Load Balancing concepts.

Network Load Balancing enables all cluster nodes on a single subnet to concurrently detect incoming network traffic for one or more IP address.

The Network Load Balancing Driver

On each cluster node, a Network Load Balancing driver acts as a filter between the network driver and the higher layers of the TCP/IP stack to allow a portion of the incoming network traffic to be received by the node. How the traffic is filtered depends on the current set of traffic handling rules.

Host Priority

Every node (also called host) in the cluster must be assigned a host priority, a unique integer that identifies the node and determines the order in which traffic is handled by default. The node with the lowest priority handles all traffic not otherwise handled by the current set of rules.


The cluster nodes exchange periodic messages and, in the event of a node failure, participate in a process called convergence.

Port Rules

Port rules determine how incoming connections are distributed among nodes. A port rule consists of the following parameters:

  • A range of port numbers to which the port rule applies.
  • The protocols (TCP, UDP, or both) to which the port rule applies.
  • The filtering mode for the port rule. The filtering mode determines how traffic is handled by the port rule and can be set to one of the following values.
Filtering modeDescription

Single-host filtering mode

One node (the node with the highest handling priority) handles all traffic. This mode load balances multiple, single-host applications.

Multiple-host filtering mode

Traffic is distributed across all cluster nodes according to the load percentage set for each node. This mode load balances a single, cluster-wide application.


No traffic is accepted by the ports in the port range. When all of a node's ports are disabled, the node is said to be draining.


A port rule applies the filtering mode to all traffic of the specified protocol flowing into all ports in the specified range.