MVC Button passing value to handler

The entire view is 2 buttons and 2 div tags shown the values of 2 session variables.  The controller code (also given below) is just the GET and POST for the ActionResult Index.

My questions is this.  Each button has a value, 100 and 200 respectively, yet when the handler is called the value of "iValue" is always null.

Can I not pass the value of a button?  If not how can I determine which button called the controller?



index.cshtml - view code
--------------------------------
@{
    ViewBag.Title = "Index";
}

<h2>Index Switch test with button value pass back</h2>

<div style="color:red"> (THIS IS NOT WORKING) Value passed by button handler = @Session["ValuePassedToButtonHandler"]</div>
<div>(THIS IS WORKING) The switch changes with each button click = @Session["SwitchVar"]</div>
 
<form method="post">
<button name="CallIndexPost" type="submit" value="100">Call Home Controller ActionResult Index giving value 100</button>    
</form>

<form method="post">
<button name="CallIndexPost" type="submit" value="200">Call Home Controller ActionResult Index giving value 200</button>    
</form>


HomeController.cs - code
--------------------------------
 public class HomeController : Controller
    {
        static bool bSwitch = false;
        //
        // GET: /Home/
        [HttpGet]
        public ActionResult Index()
        {

            Session["ValuePassedToButtonHandler"] = "Not Set";
            Session["SwitchVar"] = bSwitch.ToString();
            return View();
        }

        [HttpPost]
        public ActionResult Index(int? iValue)
        {
            bSwitch = !bSwitch;
            if (iValue != null)  // this value is always null, never 100 or 200??????
            {
                Session["ValuePassedToButtonHandler"] = iValue.ToString();
            }
            return this.RedirectToAction("Index");
          }

    }