OSPF routing has been configured as shown in the network diagram. A link at 172.16.10.4/30 is broken. There’s no backup link for area 0 between 2651-1 and 2651-2, hence area 2 becomes discontiguous.
Before link breakage
1841-1#sh ip route ospf
2.0.0.0/32 is subnetted, 1 subnets
O 2.2.2.2 [110/2] via 172.16.10.2, 00:39:23, FastEthernet0/0
3.0.0.0/32 is subnetted, 1 subnets
O 3.3.3.3 [110/2] via 172.16.10.6, 00:40:47, FastEthernet0/1
4.0.0.0/32 is subnetted, 1 subnets
O IA 4.4.4.4 [110/3] via 172.16.10.2, 00:12:33, FastEthernet0/0
192.168.1.0/30 is subnetted, 2 subnets
O IA 192.168.1.0 [110/2] via 172.16.10.2, 00:13:22, FastEthernet0/0
O IA 192.168.1.4 [110/2] via 172.16.10.6, 00:13:43, FastEthernet0/1
2651-2#sh ip route ospf
1.0.0.0/32 is subnetted, 1 subnets
O 1.1.1.1 [110/2] via 172.16.10.5, 00:14:22, FastEthernet0/1
2.0.0.0/32 is subnetted, 1 subnets
O 2.2.2.2 [110/3] via 172.16.10.5, 00:14:22, FastEthernet0/1
4.0.0.0/32 is subnetted, 1 subnets
O IA 4.4.4.4 [110/4] via 172.16.10.5, 00:13:27, FastEthernet0/1
172.16.0.0/30 is subnetted, 2 subnets
O 172.16.10.0 [110/2] via 172.16.10.5, 00:14:22, FastEthernet0/1
192.168.1.0/30 is subnetted, 2 subnets
O IA 192.168.1.0 [110/3] via 172.16.10.5, 00:13:27, FastEthernet0/1
After link breakage
2651-2#sh ip route
3.0.0.0/32 is subnetted, 1 subnets
C 3.3.3.3 is directly connected, Loopback3
192.168.1.0/30 is subnetted, 1 subnets
C 192.168.1.4 is directly connected, FastEthernet0/0
1841-1#sh ip route ospf
2.0.0.0/32 is subnetted, 1 subnets
O 2.2.2.2 [110/2] via 172.16.10.2, 00:02:14, FastEthernet0/0
4.0.0.0/32 is subnetted, 1 subnets
O IA 4.4.4.4 [110/3] via 172.16.10.2, 00:02:14, FastEthernet0/0
192.168.1.0/30 is subnetted, 1 subnets
O IA 192.168.1.0 [110/2] via 172.16.10.2, 00:02:14, FastEthernet0/0
Virtual link
2651-3(config-router)#area 1 virtual-link 22.22.22.22
2651-1(config-router)#area 1 virtual-link 44.44.44.44
After virtual link was configured
2651-2#sh ip route ospf
1.0.0.0/32 is subnetted, 1 subnets
O IA 1.1.1.1 [110/4] via 192.168.1.6, 00:04:05, FastEthernet0/0
2.0.0.0/32 is subnetted, 1 subnets
O IA 2.2.2.2 [110/3] via 192.168.1.6, 00:04:05, FastEthernet0/0
4.0.0.0/32 is subnetted, 1 subnets
O IA 4.4.4.4 [110/2] via 192.168.1.6, 00:04:38, FastEthernet0/0
172.16.0.0/30 is subnetted, 1 subnets
O IA 172.16.10.0 [110/3] via 192.168.1.6, 00:04:05, FastEthernet0/0
192.168.1.0/30 is subnetted, 2 subnets
O IA 192.168.1.0 [110/2] via 192.168.1.6, 00:04:38, FastEthernet0/0
1841-1#sh ip route ospf
2.0.0.0/32 is subnetted, 1 subnets
O 2.2.2.2 [110/2] via 172.16.10.2, 00:24:13, FastEthernet0/0
4.0.0.0/32 is subnetted, 1 subnets
O IA 4.4.4.4 [110/3] via 172.16.10.2, 00:08:30, FastEthernet0/0
192.168.1.0/30 is subnetted, 2 subnets
O IA 192.168.1.0 [110/2] via 172.16.10.2, 00:24:13, FastEthernet0/0
O IA 192.168.1.4 [110/3] via 172.16.10.2, 00:08:30, FastEthernet0/0
GRE tunnel
2651-1#sh run int tu0
Building configuration…
Current configuration : 143 bytes
!
interface Tunnel0
ip address 10.0.0.1 255.255.255.248
ip ospf 1 area 0
tunnel source FastEthernet0/1
tunnel destination 192.168.1.2
end
2651-3#sh run int tu0
Building configuration…
Current configuration : 143 bytes
!
interface Tunnel0
ip address 10.0.0.4 255.255.255.248
ip ospf 1 area 0
tunnel source FastEthernet0/1
tunnel destination 192.168.1.1
end
Routing table of 2651-2 after GRE tunnel was configured.
2651-2#sh ip route ospf
1.0.0.0/32 is subnetted, 1 subnets
O IA 1.1.1.1 [110/11114] via 192.168.1.6, 00:22:40, FastEthernet0/0
2.0.0.0/32 is subnetted, 1 subnets
O IA 2.2.2.2 [110/11113] via 192.168.1.6, 00:22:40, FastEthernet0/0
4.0.0.0/32 is subnetted, 1 subnets
O IA 4.4.4.4 [110/2] via 192.168.1.6, 00:23:07, FastEthernet0/0
172.16.0.0/30 is subnetted, 1 subnets
O IA 172.16.10.0 [110/11113] via 192.168.1.6, 00:22:40, FastEthernet0/0
10.0.0.0/29 is subnetted, 1 subnets
O IA 10.0.0.0 [110/11112] via 192.168.1.6, 00:23:07, FastEthernet0/0
192.168.1.0/30 is subnetted, 2 subnets
O IA 192.168.1.0 [110/2] via 192.168.1.6, 00:23:07, FastEthernet0/0
1841-1 knows the route to Area 2 after GRE ws configured
1841-1#sh ip route ospf
2.0.0.0/32 is subnetted, 1 subnets
O 2.2.2.2 [110/2] via 172.16.10.2, 01:24:24, FastEthernet0/0
4.0.0.0/32 is subnetted, 1 subnets
O IA 4.4.4.4 [110/3] via 172.16.10.2, 01:24:24, FastEthernet0/0
10.0.0.0/29 is subnetted, 1 subnets
O 10.0.0.0 [110/11112] via 172.16.10.2, 00:27:09, FastEthernet0/0
192.168.1.0/30 is subnetted, 2 subnets
O IA 192.168.1.0 [110/2] via 172.16.10.2, 01:24:24, FastEthernet0/0
O IA 192.168.1.4 [110/11113] via 172.16.10.2, 00:24:09, FastEthernet0/0
1841-1#ping 192.168.1.5
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.5, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms
Alternatively use GRE tunnel to extend Area 2 until the ABR 2651-1
2651-1#sh run int tu0
Building configuration…
Current configuration : 143 bytes
!
interface Tunnel0
ip address 10.0.0.1 255.255.255.248
ip ospf 1 area 2
tunnel source FastEthernet0/1
tunnel destination 192.168.1.2
end
2651-3#sh run int tu0
Building configuration…
Current configuration : 143 bytes
!
interface Tunnel0
ip address 10.0.0.4 255.255.255.248
ip ospf 1 area 2
tunnel source FastEthernet0/1
tunnel destination 192.168.1.1
end
The result is the same, either you use GRE tunnel to extend Area 0 to ABR 2651-3 or extend Area 2 to ABR 2651-1
cyruslab 