Role based Authorization

Hi,
My application comes from a portal. When we click a link in portal, my application opens. From the portal, i get userID and then
i get user role from DB.
If i need to do authorization, how to do it?
ex: for user.userid = 20 i got user.role="admin" from DB
If i keep allow roles in web.config as 
allow roles="admin". will it do authorization check?


how web.config knows that this user is in this role?
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