<?php
class My_Plugins_Auth extends Zend_Controller_Plugin_Abstract {
   public function dispatchLoopStartup(Zend_Controller_Request_Abstract $request) {
       if (!Zend_Auth::getInstance()->setStorage(new Zend_Auth_Storage_Session("suasession"))->hasIdentity()) {
           $request->setControllerName('auth')
                   ->setActionName('login');
       }
   }
}