Classic mode is where IIS only works with ISAPI
extensions and ISAPI filters directly. This is how IIS 6 and below
behaved. Using classic mode, ASP.NET is simply an ISAPI extension
(aspnet_isapi.dll) and an ISAPI filter (aspnet_filter.dll). When using
classic mode the server uses two piplines to handle requests, one for
native code and the other for managed code. In this mode the application
doesn't take full advantage of everything IIS 7.X has to offer.
Integrated mode handles all requests through a unified pipeline for
IIS and is tightly integrated with ASP.NET through that same pipeline.
ASP.NET sees every relevant request and manipulates things along the way
rather than acting as an external plugin. With integrated mode ASP.NET
runs much more efficiently in IIS and will yield greater performance for
Some legacy code may require that you run in classic mode to execute
without error. To take full advantage of IIS we strongly encourage you
to review your code if your application throws an error in Integrated
but runs fine in Classic.