关键配置： 

R3的配置

!
interface Loopback0
 ip address 1.1.1.1 255.255.255.0
!
interface Serial2/0
 ip address 192.168.12.3 255.255.255.0
 clock rate 64000
ip route 2.2.2.0 255.255.255.0 Serial2/0 
ip route 3.3.3.0 255.255.255.0 Serial2/0 
ip route 192.168.13.0 255.255.255.0 192.168.12.4 
 
R4的配置：
interface Loopback0
 ip address 2.2.2.2 255.255.255.0
 
 interface Serial2/0
 ip address 192.168.12.4 255.255.255.0
 
 ip route 1.1.1.0 255.255.255.0 192.168.12.3 
ip route 3.3.3.0 255.255.255.0 192.168.13.5 
!
R5的配置：
interface Loopback0
 ip address 3.3.3.3 255.255.255.0
 !
interface Serial3/0
 ip address 192.168.13.5 255.255.255.0
!
ip route 1.1.1.0 255.255.255.0 Serial3/0 
ip route 2.2.2.0 255.255.255.0 Serial3/0 
ip route 192.168.12.0 255.255.255.0 192.168.13.4 
!