使bootstrap下拉菜单只向下移动？

<div class="custom-dropdown">
  <button class="btn btn-primary dropdown-toggle" type="button" data-toggle="dropdown">
    下拉菜单
    <span class="caret"></span>
  </button>
  <ul class="dropdown-menu">
    <li><a href="#">选项1</a></li>
    <li><a href="#">选项2</a></li>
    <li><a href="#">选项3</a></li>
  </ul>
</div>

.custom-dropdown .dropdown-menu {
  top: 100% !important;
  bottom: auto !important;
}