.dashed-box{width:500px;height:100px;overflow:hidden;position:relative;}
.dashed-top{width:2000px;height:0px;border-bottom:2px #ccc dashed;position:absolute;top:0;left:-1400px;}
.dashed-left{width:0px;height:2000px;border-left:2px #ccc dashed;position:absolute;left:0;top:-1600px;}
.dashed-bottom{width:2000px;height:0px;border-bottom:2px #ccc dashed;position:absolute;left:0px;bottom:0;}
.dashed-right{width:0px;height:2000px;border-left:2px #ccc dashed;position:absolute;right:0;top:-1600px;}

<div class="dashed-box">
<div class="dashed-top"></div>
<div class="dashed-left"></div>
<div class="dashed-right"></div>
<div class="dashed-bottom"></div>
</div>

setInterval(function(){
 var $left=$(".dashed-top").css("left");
 var $top=$(".dashed-bottom").css("left");
 $left=parseInt($left);
 $top=parseInt($top);
 if($left<0){
  $left+=2;
 }else{
  $left=-1400;
 }
  
 if($top>-1000){
  $top-=2;
 }else{
  $top=0;
 }
 $(".dashed-top").css("left",$left+"px");
 $(".dashed-right").css("top",$left+"px");
 $(".dashed-bottom").css("left",$top+"px");
 $(".dashed-left").css("top",$top+"px");
},60);