<nav class="navbar navbar-expand-lg navbar-dark bg-dark fixed-top">
  <a class="navbar-brand" href="#">Navbar</a>
  <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
    <span class="navbar-toggler-icon"></span>
  </button>

  <?php
    wp_nav_menu( array(
      'theme_location'    => 'main_navigation',
      'depth'             => 2,
      'container'         => 'div',
      'container_class'   => 'collapse navbar-collapse',
      'container_id'      => 'navbarNav',
      'menu_class'        => 'navbar-nav mr-auto',
      'fallback_cb'       => 'WP_Bootstrap_Navwalker::fallback',
      'walker'            => new WP_Bootstrap_Navwalker())
    );
  ?>
</nav>