routes.MapRoute("Default", "{controller}/{action}/{id}", 
    new { controller = "Home", action = "Index", id = "" });
	
protected override void HandleUnknownAction(string actionName) {
  //your code here.
}
	
routes.MapRoute("default-action", "{controller}/{actionName}/{id}", new {action="DefaultAction"});
	
public ActionResult DefaultAction(string actionName, string id) {
  //handle default action
}