How to force user to re-login after my session variable expires in ASP.Net?

ASPNet_Developer used Ask the Experts™
I am user FORMS AUTHENTICATION in my web application.
  <authentication mode="Forms">
    <forms loginUrl="Login.aspx"
           cookieless="false" />

I have a session variable sessionID. When my session variable expires of is set to less than zero. I would like to force the user to re-login again. Right now even when after I set  timeout="3000". I still able to user some of the webpages. What did I miss? Can any please suggest what I should do.
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Top Expert 2011

look here: Implementing Auto-Logout functionality

You need to check if the session variable is not nothing on page load.

if isnothing(SessionID) Then
end if
Most Valuable Expert 2012
Top Expert 2014
You do realize that timeout is in minutes?
Top Expert 2011

3000 minutes?

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