Hi KatieNgoc,
You need to put bootstrap cdn only masterpage head section.
Refer below sample.
HTML
Master.master
<%@ Master Language="C#" AutoEventWireup="true" CodeFile="MasterPage.master.cs" Inherits="MasterPage" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
<script type="text/javascript" src='https://ajax.aspnetcdn.com/ajax/jQuery/jquery-1.8.3.min.js'></script>
<script type="text/javascript" src='https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.0.3/js/bootstrap.min.js'></script>
<link rel="stylesheet" href='https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.0.3/css/bootstrap.min.css'
media="screen" />
<asp:ContentPlaceHolder ID="head" runat="server">
</asp:ContentPlaceHolder>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:ContentPlaceHolder ID="ContentPlaceHolder1" runat="server">
</asp:ContentPlaceHolder>
</div>
</form>
</body>
</html>
Login.aspx
<%@ Page Title="" Language="C#" MasterPageFile="~/MasterPage.master" AutoEventWireup="true"
CodeFile="Login.aspx.cs" Inherits="Login" %>
<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="Server">
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="Server">
<div style="max-width: 400px;">
<h2 class="form-signin-heading">
Login</h2>
<label for="txtUsername">
Username</label>
<asp:TextBox ID="txtUsername" runat="server" CssClass="form-control" placeholder="Enter Username"
required />
<br />
<label for="txtPassword">
Password</label>
<asp:TextBox ID="txtPassword" runat="server" TextMode="Password" CssClass="form-control"
placeholder="Enter Password" required />
<div class="checkbox">
<asp:CheckBox ID="chkRememberMe" Text="Remember Me" runat="server" />
</div>
<asp:Button ID="btnLogin" Text="Login" runat="server" OnClick="ValidateUser" Class="btn btn-primary" />
<br />
<br />
<div id="dvMessage" runat="server" visible="false" class="alert alert-danger">
<strong>Error!</strong>
<asp:Label ID="lblMessage" runat="server" />
</div>
</div>
</asp:Content>
Logout.aspx
<%@ Page Title="" Language="C#" MasterPageFile="~/MasterPage.master" AutoEventWireup="true"
CodeFile="Logout.aspx.cs" Inherits="Logout" %>
<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="Server">
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="Server">
<div>
<h2>
Welcome</h2>
<hr />
<h4>
<asp:LoginName ID="LoginName1" runat="server" Font-Bold="true" />
</h4>
<br />
<asp:LoginStatus ID="LoginStatus1" runat="server" CssClass="btn btn-warning" />
</div>
</asp:Content>
Screenshot
