项目登录后cas没能实现单点登录的问题

我的模拟场景是这样的：cas服务器3.1.2  两个测试项目A、B都是使用了cas-client2.1。配置完成后单个系统登录认证都没问题。但当A登录成功后，点B的链接不能直接进入系统，还需要再次登录认证（应该是点B链接能直接进B系统的，不需要认证了）。反过来也一样（B成功认证，点A不能直接进入系统）。下面是测试项目的web.xml（A，B都一样得区别就是项目名不一样）：<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.4" xmlns="http://java.sun.com/xml/ns/j2ee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee 
    http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">

<!-- SingleSignOutHttpSessionListener LISTENER -->
<listener>
<listener-class>
org.jasig.cas.client.session.SingleSignOutHttpSessionListener
</listener-class>
</listener>

<!-- CAS SINGLE SIGN OUT FILTER -->
<filter>
<filter-name>CAS Single Sign Out Filter</filter-name>
<filter-class>
org.jasig.cas.client.session.SingleSignOutFilter
</filter-class>
</filter> <filter>
<filter-name>struts2</filter-name>
<filter-class>
org.apache.struts2.dispatcher.FilterDispatcher
</filter-class>
</filter> <filter>
<filter-name>CASFilter</filter-name>
<filter-class>
edu.yale.its.tp.cas.client.filter.CASFilter
</filter-class>
<init-param>
<param-name>
edu.yale.its.tp.cas.client.filter.loginUrl
</param-name>
<param-value>
http://A(或B):8080/cas/login
</param-value>
</init-param>
<init-param>
<param-name>
edu.yale.its.tp.cas.client.filter.validateUrl
</param-name>
<param-value>
http://A（或B）:8080/cas/proxyValidate
</param-value>
</init-param>
<init-param>
<param-name>
edu.yale.its.tp.cas.client.filter.serverName
</param-name>
<param-value>A（或B）:8080</param-value>
</init-param> </filter>

<filter-mapping>
<filter-name>CAS Single Sign Out Filter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping> <filter-mapping>
<filter-name>CASFilter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
<filter-mapping>
<filter-name>struts2</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
</web-app> 请知道的兄弟们讲解下。