MasterPage et UpdatePannel

Hello,

Je développe une Web App en VB.NET / ASP.NET. J'ai un petit soucis avec ma MasterPage. Losque je clique sur un bouton de mon menu, toute ma page est raffraichit, ce qui provoque la disparition / ré-apparition de toute ma page, ors j'ai mis un UpdatePannel qui contient mon ContentPlaceHolder. Je suppose que j'ai du louper quelque chose, mais quoi ?
Code:
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<%@ Master Language="VB" AutoEventWireup="true" CodeBehind="Site.master.vb" Inherits="IRE.Site" %>
 
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
<head runat="server">
    <title>IRE - IT RENT EQUIPEMENT</title>
    <link href="~/Styles/Site.css" rel="stylesheet" type="text/css" />
    <asp:ContentPlaceHolder ID="HeadContent" runat="server">
    </asp:ContentPlaceHolder>
</head>
<body>
    <form runat="server">
    <div class="page">
        <div class="header">
            <div style="background: #f0ab00; margin: 0px auto; height: 8px;">
            </div>
            <div style="float: left; margin: 10px 0px 0px; width: 350px;">
                <img id="logo" src="/imgs/_logo.png" alt="nothing" />
                <br />
                <label class="h2" id="labletitle">
                    IRE - IT RENT EQUIPEMENT</label>
            </div>
            <div id="metaNaviDiv">
                <div class="metaLink">
                    <asp:DropDownList ID="ddlCountry" runat="server" DataSourceID="dsCountry" 
                        DataTextField="CountryName" DataValueField="IdCountry" AutoPostBack="True">
                    </asp:DropDownList>
                    <asp:SqlDataSource ID="dsCountry" runat="server" 
                        ConnectionString="<%$ ConnectionStrings:IT_Rent_connection %>" 
                        SelectCommand="SELECT * FROM [tblCountry]"></asp:SqlDataSource></br>
                    <a id="HyperLinkWiki1" 
                        target="_new">Some like to the Wiki</a>
                    <br />
                    <a id="HyperLinkWiki2" href="https://wiki.wdf.sap.corp/wiki/display/browsers/Browsers+in+On-Premise+Suite"
                        target="_new">Some other link</a>
                    <br />
                    <a href="mailto:"><span>Contact us</span> </a>
                    <br />
                    Welcome <asp:Label ID="lblDisplayName" runat="server" Text=""></asp:Label>
                </div>
            </div>
        </div>
        <div class="left" style="height: 33px; width: 100%; background-image: url(imgs/toolbar_background.png);">
            <div class="navContent">
                <asp:Menu ID="NavigationMenu" runat="server" CssClass="menu" EnableViewState="false"
                    IncludeStyleBlock="false" Orientation="Horizontal">
                    <Items>
                        <asp:MenuItem Text="Booking Request" Value="Booking Request Page">
                            <asp:MenuItem NavigateUrl="~/MakeALoan.aspx" Text="Make a loan" Value="Make a loan">
                            </asp:MenuItem>
                            <asp:MenuItem Text="View My Booking" Value="View My Booking" 
                                NavigateUrl="~/MyBooking.aspx"></asp:MenuItem>
                        </asp:MenuItem>
                        <asp:MenuItem Text="IT Accessories Management" Value="IT Accessories Management">
                            <asp:MenuItem NavigateUrl="~/BookITAccessories.aspx" Text="Booking IT Accessories"
                                Value="Booking IT Accessories"></asp:MenuItem>
                            <asp:MenuItem Text="Return of IT Accessories" Value="Return of IT Accessories" NavigateUrl="~/ReturnITaccessories.aspx">
                            </asp:MenuItem>
                        </asp:MenuItem>
                        <asp:MenuItem Text="Booking Report" Value="Booking Report">
                            <asp:MenuItem Text="Search booking" Value="Search booking"></asp:MenuItem>
                            <asp:MenuItem Text="Booking Calendar " Value="Booking Calendar "></asp:MenuItem>
                        </asp:MenuItem>
                        <asp:MenuItem Text="Maintenance" Value="Maintenance">
                            <asp:MenuItem Text="Add Hardware Type" Value="Add Hardware Type" NavigateUrl="~/AddHardwareType.aspx">
                            </asp:MenuItem>
                            <asp:MenuItem Text="Edit Pool" Value="Edit Pool" NavigateUrl="~/EditPool.aspx"></asp:MenuItem>
                        </asp:MenuItem>
                        <asp:MenuItem Text="Administration" Value="Administration">
                            <asp:MenuItem Text="Site Configuration / User Administration" Value="Site Configuration / User Administration">
                            </asp:MenuItem>
                            <asp:MenuItem Text="Mail Configuration" Value="Mail Configuration" 
                                NavigateUrl="~/MailConfig.aspx">
                            </asp:MenuItem>
                        </asp:MenuItem>
                    </Items>
                </asp:Menu>
            </div>
        </div>
        <div style="background: #f0ab00; margin: 0px auto; height: 8px;">
        </div>
        <asp:ScriptManager ID="ScriptManager1" runat="server">
        </asp:ScriptManager>
        <asp:UpdatePanel ID="UpdatePanel1" runat="server">
            <ContentTemplate>
                <div class="main">
                    <asp:ContentPlaceHolder ID="MainContent" runat="server" />
                </div>
            </ContentTemplate>
        </asp:UpdatePanel>
        <div class="clear">
        </div>
        <div id="blockFooter">
            <div style="height: 100%; width: 100%; background-image: url(imgs/bg_footer_transparent.png);">
                <div class="footerText">
                    <div class="textLeft" style="width: 45px; color: white">
                        SAP 2011
                    </div>
                    <!--wSearch:noindex-->
                    <div class="link">
                    </div>
                </div>
            </div>
    </form>
</body>
</html>
Merci d'avance de vos réponses.

DeWaRs