\routing.yml
login:
    pattern: /login
    defaults: { _controller : MyAppBundle:Security:login }

login_check:
    pattern: /login_check

logout:
    pattern: /logout

// SecurityController.php
...
class SecurityController extends Controller{

    public function loginAction(){
       if($test_if_credentials_present){
           $response = $this->forward('login_check',
              array('_username' => $username, '_password' => $password);
           return $response;
       }
    }
}

php app/console router:debug login_check

[router] Route "login_check"
Name         login_check
Path         /login_check
Host         ANY
Scheme       ANY
Method       ANY
Class        SymfonyComponentRoutingRoute
Defaults
Requirements NO CUSTOM
Options      compiler_class: SymfonyComponentRoutingRouteCompiler
Path-Regex   #^/login_check$#s

$router = $this->get('router');

$uri = $router->generate('login_check'); // /login_check

$attributes = $router->match($uri);

Array
(
    [_controller] => AcmeFooBundleControllerLoginController::checkAction
    [_route] => login_check
)

return $this->forward($attributes['_controller'], array(
    '_username' => $username,
    '_password' => $password
));