Updateprogessbar is not working when Setting AssociatedUpdatePanelID?

Question

I have two updatepanel and a progressbar on the page. If I set the AssociatedUpdatePanelID, then progressbar is not working, without setting AssociatedUpdatePanelID progress bar is working but the problem is working for both updatepanel. But I need to work on one updatepanel.

 <asp:Button ID="Button1" runat="server" Text="Button1" OnClick="Button1_Click" />
    <asp:Button ID="Button2" runat="server" Text="Button2" OnClick="Button2_Click" />
    <asp:UpdateProgress runat="server" ID="prg" AssociatedUpdatePanelID="upnl">
        <ProgressTemplate>
            <img src="Images/progressbar.gif" />
        </ProgressTemplate>
    </asp:UpdateProgress>
    <br />
    <asp:UpdatePanel runat="server" ID="upnl" UpdateMode="Conditional">
        <ContentTemplate>
        </ContentTemplate>
        <Triggers>
            <asp:AsyncPostBackTrigger ControlID="Button1" EventName="Click" />
        </Triggers>
    </asp:UpdatePanel>
    <asp:UpdatePanel runat="server" ID="upnl1" UpdateMode="Conditional">
        <ContentTemplate>
        </ContentTemplate>
        <Triggers>
            <asp:AsyncPostBackTrigger ControlID="Button2" EventName="Click" />
        </Triggers>
    </asp:UpdatePanel>

Code Behind - C#

protected void Button1_Click(object sender, EventArgs e)
{
    System.Threading.Thread.Sleep(3000);
}
protected void Button2_Click(object sender, EventArgs e)
{
    System.Threading.Thread.Sleep(3000);
}

In this video, he also not set AssociatedUpdatePanelID http://www.asp.net/learn/ajax-videos/video-123.aspx

Answer 1

Excellant Article: Explaining every thing in very simple manner..........
http://www.codedigest.com/Articles/ASPNETAJAX/125_Using_UpdateProgress_Control_Effectively.aspx

By Design External triggers for an UpdatePanel do not fire an associated UpdateProgress, since the implementation of enabling the UpdateProgress control searches the control hierarchy for the calling control; an external trigger will not be present in the control hierarchy.