引入缺省路由

配置命令：

AR1:
sys
sysname AR1
interface GigabitEthernet0/0/0
ip address 202.1.1.1 255.255.255.0
q
interface GigabitEthernet0/0/1
ip address 202.2.2.1 255.255.255.0
q
ip route-static 172.16.0.0 255.255.0.0 202.1.1.2

AR2：
sys
interface GigabitEthernet0/0/0
ip address 202.1.1.2 255.255.255.0
interface GigabitEthernet0/0/1
ip address 172.16.1.1 255.255.255.0
q
ospf 1 router-id 2.2.2.2
default-route-advertise
area 0.0.0.0
network 172.16.1.0 0.0.0.255
q
ip route-static 0.0.0.0 0.0.0.0 202.1.1.1
AR3：
sys
interface GigabitEthernet0/0/0
ip address 172.16.2.1 255.255.255.0
interface GigabitEthernet0/0/1
ip address 172.16.1.2 255.255.255.0
q
ospf 1 router-id 3.3.3.3
area 0.0.0.0
network 172.16.1.0 0.0.0.255
network 172.16.2.0 0.0.0.255
AR4：
sys
interface GigabitEthernet0/0/0
ip address 172.16.2.2 255.255.255.0
q
ospf 1 router-id 4.4.4.4
area 0.0.0.0
network 172.16.2.0 0.0.0.255