实现步骤如下，仅供参考学习：
实现了R1与R5之间的PPP pap认证
[r1]int serial 0/0/0
[r1-Serial0/0/0]ip add 15.1.1.1 24
[r1-Serial0/0/0]ppp pap local-user panxi password cipher 123456
[Huawei]sysname isp
[isp]int loopback 0
[isp-LoopBack0]ip add 5.5.5.5 24
[isp-LoopBack0]q
[isp]aaa
[isp-aaa]local-user panxi password cipher 123456
[isp-aaa]int serial 0/0/0
[isp-Serial0/0/0]ip add 15.1.1.2