拓扑图：

switch配置：

interface Ethernet0/0
 switchport access vlan 10
 duplex auto
!
interface Ethernet0/1
 switchport access vlan 20
 duplex auto
!
interface Ethernet0/2
 switchport trunk encapsulation dot1q
 switchport mode trunk
 duplex auto
!
interface Ethernet0/3
 switchport access vlan 30
 duplex auto
!
interface Vlan1
 no ip address
 shutdown
!
!
ip forward-protocol nd
no ip http server

R1配置：

interface Ethernet0/0
 no ip address
!
interface Ethernet0/0.1
 encapsulation dot1Q 10
 ip address 192.168.1.1 255.255.255.0
 ip nat inside                                        //注意，ip nat inside要应用于子接口上
 ip virtual-reassembly in
!
interface Ethernet0/0.2
 encapsulation dot1Q 20
 ip address 192.168.2.1 255.255.255.0
 ip nat inside
 ip virtual-reassembly in
!
interface Ethernet0/0.3
 encapsulation dot1Q 30
 ip address 192.168.3.1 255.255.255.0
!
interface Ethernet0/1
 ip address 8.8.8.8 255.255.255.0
 ip nat outside
 ip virtual-reassembly in
!
interface Ethernet0/2
 no ip address
 shutdown
!
interface Ethernet0/3
 no ip address
 shutdown
!
ip forward-protocol nd
!
!         
no ip http server
no ip http secure-server
ip nat inside source list 100 interface Ethernet0/1 overload
ip route 0.0.0.0 0.0.0.0 8.8.8.9
!
access-list 100 permit ip 192.168.1.0 0.0.0.255 any
access-list 100 permit ip 192.168.2.0 0.0.0.255 any
!
!
!
control-plane
!
!
!
!
!
!
!
line con 0
 logging synchronous
line aux 0
line vty 0 4
 login    
 transport input all
!
!
end

R2配置：

interface Ethernet0/0
 ip address 8.8.8.9 255.255.255.0
!
interface Ethernet0/1
 no ip address
 shutdown
!
interface Ethernet0/2
 no ip address
 shutdown
!
interface Ethernet0/3
 no ip address
 shutdown
!
ip forward-protocol nd
!
!
no ip http server
no ip http secure-server