layout.phtml

<?php
            echo $this->navigation('default-navigation')
                ->menu()
                ->setPartial('default/navigation/sub_menu')
                ->render(
                null,
                array(
                     'indent'           => 4,
                     'ulClass'          => '',
                     'minDepth'         => 1,
                     'maxDepth'         => null,
                     'onlyActiveBranch' => true,
                     'renderParents'    => false
                )
            );
            ?>