I'm wondering why you wouldn't just add the "sub_site" part to your route? Route::get('sub_site/flag');
You could even enclose your routes in a route group with a 'sub_site' prefix.
Your use of "sub_site" also makes it appear that this might be a candidate for subdomain routing, have you looked into that? http://laravel.com/docs/5.1/routing#route-group-sub-domain-routing