OSPF-Discontiguous Area

OSPF-Discontiguous Area

0

Multi-area OSPF
OSPF routing has been configured as shown in the network digoal. 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

Virtual link established between 2651-1 and 2651-3, router ospf 1 process peer to each other's router-id within area 1.
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

GRE tunnel is created on top of the actual ip subnet and the actual physical interface. this tunnel acts like a virtual link, and is advertised as area 0, to 2651-3 it sees itself to have 3 interfaces: tunnel 0 interface in Area 0, fa0/1 in Area 1, fa0/0 in Area 2, hence this patch up the discontiguous Area 2.
2651-1 # sh run int tu0
Building configuration...

Current configuration: 143 bytes
!
Interface Tunnel0
Ip address 10.0.0.1 255.255.255.255.248
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 10.0.0.1 255.255.255.255.248
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

Author "446007"