UpdatePanel 和 Timer 实现局部刷新

我想用<asp:Timer ID="Timer1" runat="server" Interval="30000" OnTick="Timer1_Tick">
30秒刷新UpdatePanel2，但是 每次都是把所有的页面刷新一次，请问 怎么设置能实现刷新UpdatePanel2部分？<%@ Page Language="C#" AutoEventWireup="true" CodeFile="index.aspx.cs" Inherits="index" %><html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>WWWWWW</title>    <script language="JavaScript" type="text/javascript" src="JS/Prototype.js"></script>    <script language="JavaScript" type="text/javascript" src="JS/Public.js"></script>    <script language="JavaScript" type="text/javascript" src="JS/jspublic.js"></script>    <script language="JavaScript" type="text/javascript" src="JS/AdminIndex.js"></script>    <style type="text/css">
        .style1
        {
            width: 721px;
        }
    </style>
</head>
<body id="Indexbody">
    <form runat="server" id="form1">
    <asp:ScriptManager ID="Scriptmanager1" runat="server">
    </asp:ScriptManager>
    &nbsp;<asp:UpdatePanel ID="UpdatePanel1" runat="server">
        <ContentTemplate>
            <table cellspacing="0" cellpadding="0" border="0">
                <tbody>
                    <tr>
                        <td>
                            <%-- 身份：<asp:Label ID="Label1" runat="server" Text="Label"></asp:Label><br>
                            名称：<asp:Label ID="Label2" runat="server" Text="Label"></asp:Label><br>
                            消息:
                            <asp:Label ID="Label3" runat="server"></asp:Label>
                            <asp:Image ID="Image1" runat="server" ImageUrl="~/images/sms1.gif" Visible="False"
                                alt="你有未读消息" />--%>
                            <asp:UpdatePanel ID="UpdatePanel2" runat="server" UpdateMode="Conditional">
                                <ContentTemplate>
                                    <asp:Timer ID="Timer1" runat="server" Interval="30000" OnTick="Timer1_Tick">
                                    </asp:Timer>
                                    <table>
                                        <tr>
                                            <td>
                                                身份：<asp:Label ID="Label1" runat="server" Text="Label"></asp:Label>
                                                <br>
                                                名称：<asp:Label ID="Label2" runat="server" Text="Label"></asp:Label>
                                                <br>
                                                消息:
                                                <asp:Label ID="Label3" runat="server"></asp:Label>
                                                <asp:Image ID="Image1" runat="server" ImageUrl="~/images/sms1.gif" Visible="False"
                                                    alt="未读消息" />
                                            </td>
                                        </tr>
                                    </table>
                                </ContentTemplate>
                                <Triggers>
                                    <asp:AsyncPostBackTrigger ControlID="Timer1" EventName="Tick" />
                                </Triggers>
                            </asp:UpdatePanel>
                        </td>
                        <td colspan="3">
                            <div id="sys_top" name="sys_top" style="height: 66px; background-color: #DAE9F5;
                                width: 1009px;">
                                <center style="width: 916px">
                                    <font size="6" color="">WWWWWW</font></center>
                            </div>
                        </td>
                    </tr>
                    <tr style="vertical-align: top">
                        <td id="frmTitle" align="left">
                            <iframe id="left" style="z-index: 2; visibility: inherit; width: 180px; height: 794px"
                                name="left" src="menua.aspx?type=0" frameborder="0" scrolling="no"></iframe>
                        </td>
                        <td class="but" onclick="switchSysBar()">
                            <div style="border-right: 0px; border-top: 0px; border-left: 0px; width: 12px; height: 700px;
                                border-bottom: 0px">
                                <center>
                                    <table id="Divb" cellpadding="0" cellspacing="0" style="width: 12px; height: 720px;">
                                        <tr>
                                            <td style="background: url(Images/butBG.gif);">
                                                <img id="switchPoint" alt="关闭左栏" onmousemove="switchSysBarb()" onmouseout="switchSysBarc()"
                                                    src="Images/close.gif">
                                            </td>
                                        </tr>
                                    </table>
                                </center>
                            </div>
                        </td>
                        <td class="style1">
                            <iframe id="sys_main" name="sys_main" style="z-index: 2; width: 991px; height: 798px;"
                                frameborder="0 " scrolling="auto"></iframe>
                        </td>
                    </tr>
                </tbody>
            </table>
        </ContentTemplate>
    </asp:UpdatePanel>
    </form>
</body>
</html>
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using Entity;public partial class index : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if(!Page.IsPostBack){        if (Session["User"] == null)
        {
            Response.Redirect("Login.aspx");
        }
        else
        {            //UserEntity user = (UserEntity)Session["User"];
            SessionClass se = new SessionClass();
            UserEntity user = se.getUserEntity();
            this.Label2.Text = user.name;
            this.Label1.Text =user.isadmin=="1"?"管理员":"普通用户";        }
        }    }
    protected void Timer1_Tick(object sender, EventArgs e)
    {
        UserEntity u = Session["User"] as UserEntity;        if(null!=u){
           
            int Count = new DAO.Dao().Search_Mess("","0", u.isadmin, u.name).Rows.Count;
            if(Count>0){
                this.Label3.Text = "有" + Count + "新消息！";
                this.Image1.Visible = true;
            }
                   }    }
}