set default controller in codeigniter 3

The default controller is meant to identify the controller and method to use if one isn't specified in a URI ... http://www.codeigniter.com/user_guide/ge...ved-routes

It is *not* meant to identify a subfolder!

In your case,

Code:

$route['default_controller'] = 'login';

tells CodeIgniter to use the Login controller in any subfolder accessed without specifying a controller, eg '/backend' or '/frontend'.

There has been lots of discussion about this on the forum, found by searching it...
http://forum.codeigniter.com/thread-6435...controller
http://forum.codeigniter.com/thread-6404...controller
http://forum.codeigniter.com/thread-6350...controller
http://forum.codeigniter.com/thread-6339...controller
and many more.