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As of Blackboard Learn 9.1 April 2014 release, IIS is optional.
Avoid limiting connection capability by throttling connection sizes. The number of connections to IIS is set by default to unlimited, so ideally an unlimited number of connections can be made to IIS and then passed to Tomcat without issue. Although this setting can be changed using the Internet Information Services (IIS) Manager, Blackboard recommends not changing it. Controlling the number of connections to IIS becomes important only when the management of resource consumption is the top priority.