In this lab I'll show you how to configure a simple multi area OSPF network using four routers.

As per the network diagram we will have 3 OSPF area's, our backbone area 0, and area's 10 and 20. Routers R1 and R2 are both known as an Area Border Router's (ABR) because they are both on the border between two area's.

The first thing we need to do is configure the interfaces with IP addresses. (Assumed, hostname and base config is already complete):

R0(config)#interface fastethernet 0/0
R0(config-if)#ip address 10.0.0.1 255.255.255.252
R0(config-if)#interface loopback 0
R0(config-if)#ip address 192.168.1.1 255.255.255.0
R1(config)#interface fastethernet 0/0
R1(config-if)#ip address 10.0.0.2 255.255.255.252
R1(config-if)#interface fastethernet 0/1
R1(config-if)#ip address 10.1.1.1 255.255.255.252
R2(config)#interface fastethernet 0/0
R2(config-if)#ip address 10.1.1.2 255.255.255.252
R2(config-if)#interface fastethernet 0/1
R2(config-if)#ip address 10.2.2.1 255.255.255.252
R3(config)#interface fastethernet 0/0
R3(config-if)#ip address 10.2.2.2 255.255.255.252
R3(config-if)#interface loopback 0
R3(config-if)#ip address 172.16.1.1 255.255.0.0

Make sure you bring your interfaces up with no shutdown.

Next we'll configure our backbone OSPF area 0:

R1(config)#router ospf 1
R1(config-router)#router-id 1.1.1.1
R1(config-router)#network 10.1.1.0 0.0.0.3 area 0
R1(config-router)#network 10.0.0.0 0.0.0.3 area 10
R2(config)#router ospf 1
R2(config-router)#router-id 2.2.2.2
R2(config-router)#network 10.1.1.0 0.0.0.3 area 0
R2(config-router)#network 10.2.2.0 0.0.0.3 area 20

We'll then complete the OSPF config for Area 10:

R0(config)#router ospf 1
R0(config-router)#router-id 10.10.10.10
R0(config-router)#network 10.0.0.0 0.0.0.3 area 10
R0(config-router)#network 192.168.1.0 0.0.0.255 area 10
R0(config-router)#interface loopback 0
R0(config-if)#ip ospf network point-to-point

We'll the complete the OSPF config for Area 20:

R3(config)#router ospf 1
R3(config-router)#router-id 3.3.3.3
R3(config-router)#network 10.2.2.0 0.0.0.3 area 20
R3(config-router)#network 172.16.0.0 0.0.255.255 area 20
R3(config-router)#interface loopback 0
R3(config-if)#ip ospf network point-to-point

We'll now test our connectivity and verify our routing tables:

R0#ping 172.16.1.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 16/21/24 ms

The routing table on R0, shows the Interarea (IA) routes to the 10.2.2.0 and 10.1.1.0 subnets. The reason they are IA routes is because R0 itself is not a member of the same area that these subnets are in:

Gateway of last resort is not set

O IA 172.16.0.0/16 [110/4] via 10.0.0.2, 00:44:58, FastEthernet0/0
     10.0.0.0/30 is subnetted, 3 subnets
O IA    10.2.2.0 [110/3] via 10.0.0.2, 00:44:58, FastEthernet0/0
O IA    10.1.1.0 [110/2] via 10.0.0.2, 01:21:09, FastEthernet0/0
C       10.0.0.0 is directly connected, FastEthernet0/0
C    192.168.1.0/24 is directly connected, Loopback0

The routing table on R2:

Gateway of last resort is not set

O    172.16.0.0/16 [110/2] via 10.2.2.2, 00:45:18, FastEthernet0/1
     10.0.0.0/30 is subnetted, 3 subnets
C       10.2.2.0 is directly connected, FastEthernet0/1
C       10.1.1.0 is directly connected, FastEthernet0/0
O IA    10.0.0.0 [110/2] via 10.1.1.1, 00:45:18, FastEthernet0/0
O IA 192.168.1.0/24 [110/3] via 10.1.1.1, 00:45:18, FastEthernet0/0

The OSPF neighbors on R2:

R2#show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
1.1.1.1           1   FULL/BDR        00:00:37    10.1.1.1        FastEthernet0/0
3.3.3.3           1   FULL/DR         00:00:32    10.2.2.2        FastEthernet0/1