When you provision a web application in SharePoint 2010 you get the option to enable Claims based authentication. However, after the provisioning there’s no option in the GUI to turn it on. PowerShell saves the day again with the option to change from classic to claims based authentication using the lines below.
$WebApp = Get-SPWebApplication "http://site:80" $WebApp.UserClaimsAuthentication = "True" $WebApp.Update()
The user running these command should be a member of the SharePoint_Shell_Access role on the config DB, and a member of the WSS_ADMIN_WPG local group.
There are some cases where the web.config wasn’t updated automatically with the appropriate entries to enable claims based authentication. This will then have to be done manually. To do so read step 5 from this blog post.