Bootstrap Glyphicons not displaying in angular2

Boostrap 4 does not support Glyphicons anymore, you can use Font Awesome instead:

npm install --save fortawesome/fontawesome-free

and add the css File to your .angular-cli.json

"apps": [
          {
             ....
             "styles": [
                "styles.css",
                "../node_modules/bootstrap/dist/css/bootstrap.css",
                "../node_modules/font-awesome/css/font-awesome.css"
             ],
             ...
         }
       ]

],

Replace CSS class with the Font Awesome classes:

<i class="navbar-toggler-icon fa fa-bars"> </i>

recompile app: ng serve

Update:
Since Font Font-Awesome 5 and Angular 5 there is an offical angular-fontawesome package available:

https://github.com/FortAwesome/angular-fontawesome#installation