Bootstrap-3 fixed menu?

Question

Bootstrap

Hello, there was a difficulty with a fixed menu on boostrap - 3. Please tell me how to make the navbar class navbar-default navbar-fixed-top

<nav class="navbar navbar-default navbar-fixed-top" role="navigation">
  <div class="container">
    ...
  </div>
</nav>

began to be fixed in the same way as on this site .