如图：

1,在R1-R2之间为HDLC封装

[r1]interface Serial 0/0/0

[r1-Serial0/0/0]link-protocol hdlc

[r2]interface Serial 0/0/0

[r2-Serial0/0/0]link-protocol hdlc

2.R2-R3之间为ppp封装，pap认证，R2为主认证方，R2-R4之间为ppp封装，chap认证，R2为主认证方

[r2]aaa  

[r2-aaa]local-user huawei password cipher huawei123

[r2-aaa]local-user huawei service-type ppp

[r2]interface Serial 0/0/1   

[r2-Serial 0/0/1]link-protocol ppp

[r2-Serial 0/0/1]ppp authentication-mode pap

[r2-Serial 0/0/1]ip address 23.1.1.1 24

[r3]interface Serial 0/0/0      

[r3-Serial0/0/0]link-protocol ppp

[r3-Serial0/0/0]ppp pap local-user huawei password cipher huawei123

[r3-Serial0/0/0]ip address 23.1.1.2 24

3.做完之后在写缺省路由指向r2的环回，使得所有的PC互通。

4.构建GRE环境

interface Tunnel0/0/0 

ip address 10.1.1.1 255.255.255.0

tunnel-protocol gre p2mp

source 12.1.1.1

nhrp entry multicast dynamic

nhrp network-id 100

5.实现全网可达。