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Controlling the Distribution of Client Traffic Within the Cluster

Updated: March 28, 2003

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

One of the intended purposes of Network Load Balancing is to distribute incoming client traffic within the cluster. You can control the distribution of client traffic within the cluster by using Network Load Balancing port rules. Port rules are criteria-based policies that allow you to direct client requests to specific cluster hosts, based on TCP and UDP port numbers.

A default port rule is created during the installation of Network Load Balancing. In many instances, the default port rule is sufficient for some of the applications and services that use Network Load Balancing. When the default port rule is insufficient, you can create custom port rules. For more information about the default port rule, see "Identifying the Behavior of the Default Port Rule" later in this chapter.

The default port rule is sufficient for the following applications and services:

  • VPN remote access farms with Routing and Remote Access

  • Load-balancing application hosting with Terminal Services


  • The ISA Server setup process defines the port rules that are necessary for ISA Server. No custom port rules are necessary for ISA Server.

If the default port rule is sufficient for your solution, and creating custom port rules is unnecessary, see "Specifying Cluster Network Connectivity" later in this chapter.

For IIS 6.0 Web farms or for custom applications, custom port rules might be required.

In some instances, the applications and services might require that the same client traffic be handled differently for the same or different applications and services. The virtual IP address assigned to the cluster can handle client traffic only in one way. However, you can specify a virtual cluster for each of the applications, allowing each application to have its own load-balancing behavior. Virtual clusters are a logical construct within the cluster, and they require no additional hardware.

For example, two Web applications might require different load-balancing behavior for HTTP (TCP port 80). You can create a virtual cluster for each Web application that allows different load-balancing behavior for the HTTP client traffic.

You can create a virtual cluster by specifying a virtual IP address in a Network Load Balancing port rule. The virtual IP address that is assigned in the port rule is associated with the application that requires the different load-balancing behavior.

Figure 8.6 illustrates the relationship between a Network Load Balancing cluster and the virtual clusters specified for the cluster. Each of the applications — Web applications A, B, and C — requires different load-balancing behavior. A virtual IP address is assigned to each virtual cluster and associated with each application. A DNS entry associates the virtual IP address with a URL for the corresponding application.

Figure 8.6   Relationship Between an NLB Cluster and Virtual Clusters

Relationship of NLB Cluster and Virtual Clusters

For more information about including Network Load Balancing port rules in your design, see "Identifying Applications or Services That Require Custom Port Rules" later in this chapter.

Control the distribution of client traffic within a cluster by completing the following steps:

  1. Identify the behavior of the default port rule.

  2. Identify applications or services that require custom port rules.

  3. Specify the client traffic to be affected by the custom port rule.

  4. Specify the affinity and load-balancing behavior of the custom port rule.


  • The port rules applied to each cluster host must be identical, with the exception of the load weight (in the multiple hosts filter mode) and the handling priority (in the single hosts filter mode). If there is a discrepancy between port rules on existing cluster hosts, the cluster will not converge.

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