OSPF Area Types

OSPF Area Types

Certiology’s OSPF Area Types tutorial explains everything about OSPF Area types for CCNA, CCNP level students.

OSPF Areas are created to control the movement of LSAs across the OSPF network. There are different types of Areas created, namely:

a)      Standard / Normal Area

b)     Backbone Area

c)      Stub Area

d)     Totally Stubby Area

e)      No So Stubby Area (NSSA)

Let’s put in some brief information about each of these areas:

a)    Standard Area:  

LSA Type 1 and 2 are sent between routers within an area and thus help prepare the Shortest Path tree for an area. The LSA Type 3 which shares internal routes and LSA Type 5 that shares external routes flood through this area. Just note that the internal routes are by OSPF Router whereas the external routes are by the ASBR.

Standard areas help in optimizing routing as the information about all routes is with all routers. But this at times can be a burden to storing / maintain the full database.

To configure a normal area, we use the command:

R1 (config)# router ospf 10

 R1 (config-router)# network <ip address> < wildcard mask> area <area no.>

b)   Backbone Area:

Backbone area is similar to the standard area. All areas are connected to this area. This is also known as Area 0.

c)    Stub Area :

In a Stub area the External LSAs i.e. LSA Type 5 is stopped. The LSA Type1 and Type can be used in a stub area so that a topology can be built. The main use of Stub Area is that the number of LSAs flowing in an area is under control which in turn will help consumption of CPU and bandwidth if the router.

In a Stub Area, a default route is injected by the ABR along with the Type 3 LSA. And this default route is helpful to reach external networks. All the routers in the area must be configured as stub in order to make an area stub. The command used is as:

(Router-config)# area <area no.> stub

d)   Totally Stubby Area:

As in a Stub area, in the Totally Stubby Area as well the External LSAs are stopped (Type 5 LSA.) In addition the LSA Type 3 i.e. Summary LSA is also not allowed in a Totally Stubby area. Thus this drastically reduces the size of the routing table as both internal and external routes are reduced and only default route is used.

The main use of Totally Stubby Area is also similar to a stub area, that the number of LSAs flowing in an area is under control which in turn will help consumption of CPU and bandwidth if the router.

Here also all the routers in the area must be configured as stub and the ABR of the stub areas will be configured as totally stubby. The command used is as:

On the ABR:

(Router-config)# area <area no.> stub no-summary

On other routers:

(Router-config)# area <area no.> stub

e)   Not so Stubby Area (NSSA):

Here Type 7 LSAs are used which are similar to the type 5 LSA. Here the external links are advertised by the ASBR towards the ABR, which in turn will convert the LSA type 7 to LSA Type 5 and then flood it to the rest of OSPF network.

Similar to other areas, Type 1 and Type 2 LSAs are used to build the topology tables. The type 3 Network LSA are accepted by the NSSA thus can be used to reach other networks of other areas.

All the routers in the area must be configured as nssa. The command is:

(Router-config)# area <area no.> nssa

To learn OSPF in details, please look at our complete step by step OSPF configuration tutorial

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