As of today, with latest @angular/router 3.0.0-rc.1, here are a couple of references on how to do that through CanActivate guards on routes:

1. angular 2 reference
2. Two answers to this SO question, by Nilz11 and by Jason

The main gist of logic looks like:

// ...
canActivate(route: ActivatedRouteSnapshot, state: RouterStateSnapshot) {
  if (this.authService.isLoggedIn) {
    // all ok, proceed navigation to routed component
    return true;
  }
  else {
    // start a new navigation to redirect to login page
    this.router.navigate(['/login']);
    // abort current navigation
    return false;
  }
}

Your guard can easily just be an injectable which, as such, can include its own injectables. So we can simply inject the router, in order to redirect. Don't forget to add the service as a provider in your app module.

@Injectable()
export class AuthGuard implements CanActivate {

  constructor(private router: Router, private authService: AuthService) {}

  canActivate(route: ActivatedRouteSnapshot, state: RouterStateSnapshot): Observable<boolean> | boolean {
    if (!authService.isAuthenticated()) {
      this.router.navigate(['/login']);
      return false;
    }
    return true;
  }
}

export const ROUTES: Routes = [
  {path: 'login', component: LoginComponent},
  {path: 'protected', loadChildren: 'DashboardComponent', canActivate: [AuthGuard]}
];

Yes, you can! Doing this will prevent your component from instantiating for nothing.

First, make a new file src/app-injector.ts

let appInjectorRef;

export const appInjector:any = (injector = false) => {
    if (!injector) {
        return appInjectorRef;
    }

    appInjectorRef = injector;

    return appInjectorRef;
};

Then, get the reference from bootstrap

// ...
import {appInjector} from './app-injector';
// ...


bootstrap(App, [
  ROUTER_PROVIDERS
]).then((appRef) => appInjector(appRef.injector));

Finally in your function

// ...
import {appInjector} from './app-injector';
// ...

@CanActivate((next, prev) => {
  let injector = appInjector();
  let router = injector.get(Router);

  if (42 != 4 + 2) {
    router.navigate(['You', 'Shall', 'Not', 'Pass']);
    return false;
  }

  return true;
})

Et voilà !

It was discussed here https://github.com/angular/angular/issues/4112

You can find a complete example here http://plnkr.co/edit/siMNH53PCuvUBRLk6suc?p=preview by @brandonroberts