Configuring Multi-Area OSPF in Cisco

OSPF listens to the neighbors and collects their link-state data to build a topology map containing all the routers within the network and saves this information in its Link-State Database. With this information (stored in the Link-State Database), it calculates the best/optimal path to reach a certain network using the SPF algorithm (Shortest Path First).

OSPF  Tables:

• Neighbor Table:  The information of all the neighbors who are sharing their link-state data.
• Topology Table (Database Table): contains the full knowledge/placements of all the OSPF routers along with the best path available to reach them.
• Routing Table: It contains the best path for forwarding traffic between neighbors.

OSPF Area:

• An OSPF Area is a logical group of routers (router’s interfaces to be more specific). An interface can belong to only one OSPF area.
• All routers that are part of the same OSPF Area maintain the same Link-State Database.
• If a router has its interfaces belonging to a different area, then the router has multiple Link-State Databases (one for each area).

Backbone Area :

Area 0 is a special area called the Backbone area and all other areas must connect to the backbone area. All the OSPF areas are required to send their routing information to the Backbone Area, and the backbone area advertises these routers to other areas. This Backbone area prevents the routing loops.

Area Border Router:

 An OSPF router that is connected to Area 0 and another OSPF Area is known as ABR. They are responsible for advertising the routes from one area to a different OSPF area.

Here we have an OSPF Topology, 

• Area 0: R1’s fa0/0 and R2’s fa0/0 interface
• Area 1:  R1’s fa1/0 and R3’s fa1/0 interface
• Area 2: R2’s fa1/0 and R4’s fa1/0 interface.

Since R1 and R2 are connected to area 0 and another area also, therefore they both become the Area Border Routers (ABR).
 

Configuring Multi-Area OSPF: 

R1 configuration:
R1(config)#router ospf 1
R1(config-router)#network 192.168.1.0 0.0.0.255 area 0
R1(config-router)#network 192.168.10.0 0.0.0.255 area 1

R2 configuration: 
R2(config)#router ospf 1
R2(config-router)#network 192.168.1.0 0.0.0.255 area 0
R2(config-router)#network 192.168.20.0 0.0.0.255 area 2

R3 configuration: 
R3(config)#router ospf 1
R3(config-router)#network 192.168.10.0 0.0.0.255 area 1
R3(config-router)#network 3.3.3.3 0.0.0.0 area 1

R4 configuration: 
R4(config)#router ospf 1
R4(config-router)#network 192.168.20.0 0.0.0.255 area 2
R4(config-router)#network 4.4.4.4 0.0.0.0 area 2

Verification of Routes Learned Through OSPF Advertisements:

R1#show ip route ospf

Verifying Reachability:

R3#ping 4.4.4.4 source 3.3.3.3