var req;

function processReqChange() {
  if (req.readyState == 4) {
    if (req.status == 200) {
	  document.getElementById("lista_agencias").innerHTML=req.responseText;
    } else {
      alert("Error al mostrar la lista de agencias\n");
    }
  }
}

function get_data(tabla) {
	url="listado_agencias.php";
	tabla="tabla="+tabla;

  if (window.XMLHttpRequest) {
    req = new XMLHttpRequest();
    req.onreadystatechange = processReqChange;
    req.open("POST", url, true);
	req.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
	req.send(tabla); 

  } else if (window.ActiveXObject) {
    req = new ActiveXObject("Microsoft.XMLHTTP");
    if (req) {
      req.onreadystatechange = processReqChange;
      req.open("POST", url, true);
		req.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
    	req.send(tabla); 
 
    }
  }
}