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Error detection and handling

Updated: January 21, 2005

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

Error detection and handling

Network Load Balancing error-handling mechanisms are designed to minimize the possibility of disrupting the cluster's service to client requests, while allowing the cluster's parameters and member hosts to be dynamically changed. For example, you can add hosts to the cluster, remove them for maintenance, add port rules, and modify rule Network Load Balancing parameters, all without interrupting service.

The Network Load Balancing driver checks its configuration parameters entered in the Network Load Balancing Properties dialog box for consistency and validity. Network Load Balancing will not start cluster operations if a problem, such as an invalid IP address, is detected. All detected errors are reported in the Windows event log. For more information, see Event logging.

When a local host joins the cluster, it exchanges its primary parameter values, such as its host priority and port rules, with the other hosts in the cluster. If an inconsistency is detected among the hosts, Network Load Balancing recovers from the error according to the type of inconsistency detected, as follows:

 

Inconsistency Network Load Balancing recovery method

Duplicate host priorities

The host joining the cluster handles no traffic until convergence is complete. The cluster does not converge to a consistent state until duplicate hosts are no longer detected.

Different number of port rules

The host joining the cluster handles no traffic until convergence is complete. The cluster does not converge to a consistent state until all hosts have the same number of rules.

Inconsistent port rule

The host joining the cluster handles no traffic until convergence is complete. The cluster does not converge to a consistent state until the rule is consistent across all hosts in the cluster.

If inconsistencies are discovered when a host attempts to join the cluster, the host should be taken out of the cluster using the nlb.exe stop command. Locate and correct the problem using the Network Load Balancing Properties dialog box on each host prior to returning the host to the cluster using the nlb.exe start command.

You can add port rules or update parameters by taking each host out of the cluster in turn, updating its parameters, and then returning it to the cluster. During this process, other cluster hosts detects inconsistencies in the rules and handles them as described previously. This permits the cluster to maintain service for client requests while the changes are made. For example, if a rule is added, it does not take effect until all hosts have been updated and have rejoined the cluster. If a rule is modified, the rule is not used until it is made consistent on all hosts.

Note

  • WLBS stands for Windows NT Load Balancing Service, the former name of Network Load Balancing in Windows NT 4.0. For reasons of backward compatibility, WLBS continues to be used in certain instances.

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