拓扑图
R2为外网，R1为边界路由器。R3,R4为内部路由器
基本配置
R2配置：
R2#sh run
Building configuration...
Current configuration : 794 bytes
!
version 12.4
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname R2
!
boot-start-marker
boot-end-marker
!
enable password cisco
!
no aaa new-model
memory-size iomem 5
!
!
ip cef
no ip domain lookup
!
interface Loopback0
ip address 2.2.2.2 255.255.255.0
!
interface Serial0/0
ip address 12.1.1.2 255.255.255.0
serial restart-delay 0
!
interface Serial0/1
no ip address
shutdown
serial restart-delay 0
!
interface Serial0/2
no ip address
shutdown
serial restart-delay 0
!
interface Serial0/3
no ip address
shutdown
serial restart-delay 0
!
ip http server
!
control-plane
!
line con 0
exec-timeout 0 0
logging synchronous
line aux 0
line vty 0 4
!
!
end