Asp.net mvc 2 为同一页面创建两个路由选项（MVC）

我希望能够以两种不同的方式访问同一页面：

首先，使用参数来显示一些特定信息

        routes.MapRoute(
            "About",
            "About/{id}",
            new { controller = "About", action = "Index" }
        );

第二，无参数，显示一般事物

        routes.MapRoute(
            "About",
            "About",
            new { controller = "About", action = "Index" }
        );  

如何构建路线以接受这两个选项

routes.MapRoute(
    "About",
    "About/{id}",
    new { controller = "About", action = "Index", id = UrlParameter.Optional }
);

然后：

public ActionResult Index(string id)
{
    // if id = null => /About was requested
    // if id != null => /About/abc was requested
    ...
}