_sub_menu.twig

 {% for item in menu %}

    <li class="{% if item.submenu is defined %}dropdown{% else %} index-{{ loop.index }}{% if loop.first %} first{% endif %}{% if loop.last %} last{% endif %}{% endif %}">

        <a href="{% if item.submenu is defined %}#{% else %}{{ item.link }}{% endif %}" {% if item.title is defined %}title='{{ item.title|escape }}'{% endif %}
           class='{% if item.submenu is defined %}dropdown-toggle{% else %}{% if item.class is defined %}{{item.class}}{% endif %}{% endif %}' {% if item.submenu is defined %}data-toggle="dropdown"{% endif %}>
            {{item.label}} {% if item.submenu is defined %}<b class="caret"></b>{% endif %}
        </a>

        {% if item.submenu is defined %}
            <ul class="dropdown-menu">
                {% for item in item.submenu %}
                    <li class="{{ item.class }}">
                        <a href="{{ item.link }}" {% if item.title is defined %}title='{{ item.title|escape }}'{% endif %}>
                            {{item.label}}
                        </a>
                    </li>
                {% endfor %}
            </ul>
        {% endif %}
        
    </li>

  {% endfor %}