基于安全的考量，很多公司可能采用广域网的技术进行内网通信

    为了实现同步传输，需要由DCE提供一个时钟频率

               ① DTE：数据通信设备，一般在用户端

               ② DCE：数据通信设备，一般在运营商

    PPP协议作广域网连接时被广泛使用

    PPP（点到点协议）能够支持点到点认证功能，其认证的方式有PAP，CHAP

    现实生活主要是PAP，只要认证一次，下次就可以直接使用，免去了带宽的占用，流量的浪费

    以下是PAP的配置案例：

    网络拓扑图：

    PAP客户端：

R1#ping 10.10.10.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.10.10.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 16/40/84 ms

    PAP服务器配置：

R2#en
R2#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R2(config)#us
R2(config)#username  oracle pass
R2(config)#username  oracle password oracle
R2(config)#int s0/0
R2(config-if)#en
R2(config-if)#encapsulation  ppp
R2(config-if)#ppp
*Mar  1 00:04:14.835: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0, changed state to down
R2(config-if)#ppp authen
R2(config-if)#ppp authentication  pap
R2(config-if)#no sh
R2(config-if)#end

    PAP客户端配置：

R1#ping 10.10.10.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.10.10.2, timeout is 2 seconds:
...
*Mar  1 00:04:43.303: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0, changed state to down..
Success rate is 0 percent (0/5)

    PAP客户端未配置之前，无法ping通

R1#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)#int s0/0
R1(config-if)#enc
R1(config-if)#encapsulation  ppp
R1(config-if)#ppp pap se
R1(config-if)#ppp pap sent-username  oracle pa
R1(config-if)#ppp pap sent-username  oracle password  oracle
R1(config-if)#no
*Mar  1 00:07:20.415: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0 changed state to up
R1(config-if)#no sh
R1(config-if)#end
R1#ping
*Mar  1 00:07:29.311: %SYS-5-CONFIG_I: Configured from console by console
R1#ping 10.10.10.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.10.10.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 20/64/132 ms