R1部分：
R1(config)#interface fastEthernet 0/0
R1(config-if)#ip addr 172.17.10.254 255.255.255.0
R1(config-if)#no sh
R1(config)#interface fastEthernet 0/1
R1(config-if)#ip addr 172.17.20.1 255.255.255.0
R1(config)#ip route 10.255.255.188 255.255.255.252 172.17.20.250
R1(config)#ip route 10.255.255.176 255.255.255.252 172.17.20.250
R2部分：
R2(config)#interface fastEthernet 2/1
R2(config-if)#ip addr 172.17.20.250 255.255.255.0
R2(config-if)#no sh
R2(config)#interface fastEthernet 2/0
R2(config-if)#ip addr 10.255.255.189 255.255.255.252
R2(config)#interface fastEthernet 1/0
R2(config-if)#ip addr 10.255.255.177 255.255.255.252
R2(config)#ip route 172.17.10.0 255.255.255.0 172.17.20.1
R3部分：
R3(config)#interface fastEthernet 2/0
R3(config-if)#ip addr 10.255.255.190 255.255.255.252
R3(config-if)#ip nat outside
R3(config-if)#no sh
R3(config)#interface fastEthernet 1/0
R3(config-if)#ip addr 10.255.255.178 255.255.255.252
R3(config-if)#interface fastEthernet 0/0
R3(config-if)#ip addr 172.17.20.250 255.255.255.0
R3(config-if)#ip nat inside
R3(config)#ip route 172.17.10.0 255.255.255.0 10.255.255.189
R3(config)#ip route 172.17.10.0 255.255.255.0 10.255.255.177 20
R3(config)#ip route 1.1.1.0 255.255.255.0 172.17.20.1
R3(config)#access-list 100 permit ip 1.1.1.0 0.0.0.255 any
R3(config)#route-map main_outside permit 10
R3(config-route-map)#match ip address 100
R3(config-route-map)#match interface fastEthernet 2/0
R3(config)#route-map backup_outside permit 10
R3(config-route-map)#match interface fastEthernet 1/0
R3(config)#ip nat pool main 10.255.255.190 10.255.255.190 netmask 255.255.255.0
R3(config)#ip nat pool backup 10.255.255.178 10.255.255.178 netmask 255.255.255.0
R3(config)#ip nat inside source route-map main_outside pool main overload
R3(config)#ip nat inside source route-map backup_outside pool backup overload
R3(config)#ip nat inside source static tcp 1.1.1.2 80 10.255.255.190 80 route-map main_outside extendable
R3(config)#ip nat inside source static tcp 1.1.1.2 80 10.255.255.178 80 route-map backup_outside extendable
R4部分：
R4(config)#interface fastEthernet 0/0
R4(config-if)#ip addr 172.17.20.1 255.255.255.0
R4(config-if)#no sh
R4(config)#interface fastEthernet 0/1
R4(config-if)#ip addr 1.1.1.1 255.255.255.0
R4(config)#ip route 172.17.10.0 255.255.255.0 172.17.20.250
測試結果：
