the site default page issue

Hi All,
I have set the site default page is default.aspx and set that page allowed without user authentication
if we call http://www.mydomain.com/default.aspx, it is working without authentication
but if we are calling without filename (http://www.mydomain.com) , it is redirected to
http://www.mydomain.com/user/login.aspx?ReturnUrl=%2f.

the problem is, if we are calling without filename (http://www.mydomain.com), it should not redirect to login.aspx page
because the default.aspx page is not accecpted as the domain default page.

Notes : I have converted asp.net 2.0 to asp.net 4.0 web application. after conversion conly  this problem arises
 
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