Unfortunately, ISA Server 2006 Standard edition does not natively support Network
Load Balancing (NLB). This has to do with the limitations of ISA Standard in properly
matching load-balanced traffic sent between two separate networks (also known as
bi-directional affinity).
NOTE
Because the limitations of ISA Server 2006 Standard with Network Load Balancing are
related to issues of bi-directional affinity, it is theoretically possible to use NLB
between different unihomed ISA Standard servers. Although this is possible, it is not a
supported configuration and is not recommended.
Larger organizations, when deploying unihomed ISA servers, often turn to the Enterprise
version to provide failover of services by using NLB. This enables the reverse-proxy functionality
to remain up and running upon the failure of a single server. For these organizations,
downtime of an OWA site or a company website is simply not acceptable, and the
Enterprise version of the software supports improving the overall uptime of the solution.
For more information on ISA Server 2006 Enterprise Edition, see Chapter 6, ???Deploying
ISA Server Arrays with ISA Server 2006 Enterprise Edition.
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