【问题1】
在由L2TP构建的VPN中，主要由(1)和(2)两种类型的服务器构成。
1．将图2-7中(1)和(2)处空缺名称填写在相应位置。
2．简要说明两种服务器的主要作用。
参考答案：1．(1)LAC(L2TP Access Concentrator或L2TP访问集中器)
(2)LNS(L2TP Network Server或L2TP网络服务器)
2．LAC是附属在网络上的具有PPP端系统和L2TP协议处理能力的设备，LAC一般就是一个网络接入服务器，用于为用户提供网络接入服务。
LNS PPP端系统上用于处理L2TP协议服务器端部分的软件。
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