Why IDP Prompts for login every time?

[Vishal K. Gandhi]

Hi,

I am participating in deployment of a new IDP server (v2.3.5) and 
experiencing a strange problem.
When user requests resources on SP side, the SP redirects user to IDP 
for authentication.  User gets access to the resources through SP after 
successful authentication which I believe expected behaviour.  The 
interesting part is that the IDP asks for authentication (username and 
password page) again if local logout is performed on SP side only.  SP 
is not configured to logout from IDP yet.  It is configured to perform 
only LOCAL logout.

I've set up a test IDP server with exactly same version which appears to 
be working fine.  It recognizes existing session with browser and 
doesn't ask to re-authenticate.
I compared all the configuration files from this test IDP server with 
those on new IDP server and both the sides look identical to me.  This 
leaves me very curious here and I can't seem to find what's that 
different between both of these servers.

Has anyone experience similar sort of problem.  Any ideas to find and 
resolve the issues will be really appreciated.

Thanks.