David Sudjiman

Command ip ospf area can be used to simplify the configuration. The idea is that you don’t need to enable network area command yet you only need to explicitly enable ip ospf area per-interface basis and by this command your point-to-point network will be automatically advertised.

Let’s look at this example below.

rtr1 directly connected to rtr2 and each router has its own loopback address. OSPF area 1 is being used accross these routers and ospf network type is point-to-point.

Below is the result before the command ip ospf area being implemented.

rtr1#sh ip ospf nei

Neighbor ID     Pri   State           Dead Time   Address         Interface
20.20.20.20       0   FULL/  -        00:00:36    192.168.1.2     FastEthernet0/0

rtr1(config-if)#do sh ip route
Gateway of last resort is not set

     20.0.0.0/32 is subnetted, 1 subnets
O       20.20.20.20 [110/2] via 192.168.1.2, 00:00:44, FastEthernet0/0
     10.0.0.0/32 is subnetted, 1 subnets
C       10.10.10.10 is directly connected, Loopback0
     192.168.1.0/30 is subnetted, 1 subnets
C       192.168.1.0 is directly connected, FastEthernet0/0

rtr2(config-if)#do sh ip ospf nei

Neighbor ID     Pri   State           Dead Time   Address         Interface
10.10.10.10       0   FULL/  -        00:00:37    192.168.1.1     FastEthernet0/0

rtr2#sh ip route
Gateway of last resort is not set

     20.0.0.0/32 is subnetted, 1 subnets
C       20.20.20.20 is directly connected, Loopback0
     10.0.0.0/32 is subnetted, 1 subnets
O       10.10.10.10 [110/2] via 192.168.1.1, 00:01:40, FastEthernet0/0
     192.168.1.0/30 is subnetted, 1 subnets
C       192.168.1.0 is directly connected, FastEthernet0/0

And these are the results after command ip ospf area being implemented.

rtr1#sh run int f0/0
Building configuration...

Current configuration : 148 bytes
!
interface FastEthernet0/0
 ip address 192.168.1.1 255.255.255.252
 ip ospf network point-to-point
 ip ospf 1 area 0
 duplex auto
 speed auto
end

rtr1#sh run | s router ospf
router ospf 1
 log-adjacency-changes
 network 10.10.10.10 0.0.0.0 area 0

rtr1#sh ip route
Gateway of last resort is not set

     20.0.0.0/32 is subnetted, 1 subnets
O       20.20.20.20 [110/2] via 192.168.1.2, 00:25:13, FastEthernet0/0
     10.0.0.0/32 is subnetted, 1 subnets
C       10.10.10.10 is directly connected, Loopback0
     192.168.1.0/30 is subnetted, 1 subnets
C       192.168.1.0 is directly connected, FastEthernet0/0

rtr2#sh run int f0/0
Building configuration...

Current configuration : 148 bytes
!
interface FastEthernet0/0
 ip address 192.168.1.2 255.255.255.252
 ip ospf network point-to-point
 ip ospf 1 area 0
 duplex auto
 speed auto
end

rtr2#sh run | s router ospf
router ospf 1
 log-adjacency-changes
 network 20.20.20.20 0.0.0.0 area 0

rtr2#sh ip route
Gateway of last resort is not set

     20.0.0.0/32 is subnetted, 1 subnets
C       20.20.20.20 is directly connected, Loopback0
     10.0.0.0/32 is subnetted, 1 subnets
O       10.10.10.10 [110/2] via 192.168.1.1, 00:26:13, FastEthernet0/0
     192.168.1.0/30 is subnetted, 1 subnets
C       192.168.1.0 is directly connected, FastEthernet0/0

Source http://www.cisco.com/en/US/docs/ios/12_0s/feature/guide/ospfarea.html