asp:menu populated from asp:dropdown using ajax.net problem

I have a dropdown that when selected, chooses which sitemap to bind my menu to. This is because each sitemap has a different set of permissions using the security model.  I'm trying to use ajax.net so that when the dropdown selected index is changed, the menu populates without reloading the page. Its simple not doing it and the page refreshes every time. Anyone have experience with this?? Thanks so much

<asp:DropDownList ID="ddNavigation" AutoPostBack="true" runat="server" BackColor="White" Font-Names="verdana" Font-Size="X-Small" ForeColor="RoyalBlue" OnSelectedIndexChanged="ddNavigation_SelectedIndexChanged"
							Width="170px">
							<asp:ListItem Value="0">Home</asp:ListItem>
							<asp:ListItem>Administration</asp:ListItem>
							<asp:ListItem>Employees</asp:ListItem>
							<asp:ListItem>Requirements/Attendance</asp:ListItem>
							<asp:ListItem>Reports</asp:ListItem>
						</asp:DropDownList>
						<asp:UpdatePanel ID="UpdatePanel1" runat="server">
							<ContentTemplate>
								<asp:Menu ID="Menu1" runat="server" BackColor="RoyalBlue" Width="170px" DynamicHorizontalOffset="2" Font-Names="Verdana" Font-Size="X-Small" ForeColor="White" StaticSubMenuIndent="5px" StaticDisplayLevels="2"
									Font-Underline="False" CssClass="menu">
									<DynamicHoverStyle BackColor="MidnightBlue" ForeColor="White" />
									<DynamicMenuStyle BackColor="RoyalBlue" CssClass="sublevel1" />
									<DynamicMenuItemStyle HorizontalPadding="5px" VerticalPadding="2px" />
									<StaticHoverStyle BackColor="MidnightBlue" />
								</asp:Menu>
							</ContentTemplate>
							<Triggers>
								<asp:AsyncPostBackTrigger ControlID="ddNavigation" EventName="SelectedIndexChanged" />
							</Triggers>
						</asp:UpdatePanel>

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