Iframe fully reloaded freshly when user click

I have the following pages
1. FrmMain.aspx
2. FrmProduct.aspx
3. FrmCustomer.aspx
4. FrmCustomerListing.aspx

FrmMain.aspx

<table>
<tr>
<td>
<asp:linkbutton id="lnkbtnMasters" runat="server" text="Masters"/>|
<asp:linkbutton id="lnkbtnServiceLocation" runat="server" text="Service Location"/>|
<asp:linkbutton id="lnkbtnCustomers" runat="server" text="Customer"/>|
</td>
</tr>
</table>
<table>
<tr>
<td>
<div id="divMasters" runat="server">
 <asp:linkbutton id="lnkbtnNewProduct" runat="server" text="New Product"/></br>
 <asp:linkbutton id="lnkbtnProductList" runat="server" text="Product Listing"/ ...