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OSPF (Open Shortest Path First) uses an hierarchical architecture and build OSPF network with different Areas. There are many specific Area types that are used with OSPF and these Area types has specific characteristics. So, what are these areas? General OSPF Area types are:
We will see all these OSPF Area types detailly. Before and during this detailed check, you can use the below diagram to remember OSPF Area types and their specific Advertisement acceptance rules. As you can see below, different OSPF LSA Types are used by different areas.
Let’s check these OSPF Areas one by one.
You can also check other CCNP ENCOR lessons
This Area is the normal Area of OSPF. All Standard Areas must connect to the Area 0. A Standard Area has no specific characteristics. It accepts, LSA Type 1, Type 2, Type 3, Type 4 and Type 5 LSAs.
Backbone Area (Area 0) is the central of OSPF Network and all the other Areas are connected to thsese Backbone Area. It is similar to Standard Area but it is build as Area 0.
Stub Area is the first specific Area. They are generally used to reduce LSA flooding, the LSDB and routing table size. So, this type of Area is used for the places that is a little sensitive to high traffic.
To configure an Area as “Stub“, all routers in these Area must be configured as “Stub“.
Stub Area accepts Intra Area LSAs, Type 1,Type 2 LSAs. It also accepts, summary routes(Type 3, Type 4) and external routes (Type 5) advertised only by a Default Route. Stub area does not accept external routes(Type 5), it is replaced by Default Route.
Totally Stubby Area is one step more strict Area than Stub Area. This is a Cisco Proprietary Open Shortest Path First Area.
To configure an Area as “Totally Stubby“, ABR on this area is configure as Totally Subby Area and the other internal routers is configured as Stub Area.
Totally Stubby Area accepts Intra Area LSAs, Type 1,Type 2 LSAs. It does not accept all inter-area (Type 3, Type 4) and External routes (Type 5). All these routes replaced with Default Route.
You can also learn OSPF Configuration on Cisco Routers
Not So Stubby Area (NSSA) is the Area like Stub, but as its name emphasize, not so Stub. This means that, a NSSA accepts External Routes that are generated from ASBR. This LSA type is LSA Type 7.These routes are converted to LSA 5 in ABR to be advertised to the other Areas.
For Open Shortest Path First NSSA configuration, all the routers in the area need to be configured as NSSA.
We can see NSSA routes in the routing table as type N1 or N2.
Totally Not So Stubby Area (Totally NSSA) is a different type of NSSA. As its name implies, it is a little strict than NSSA.
Totally Not So Stubby Area (Totally NSSA) do not allow Type 3 LSAs to be advertised. The Default Route is used to reach other areas.
In this article, we have talked about OSPF Area Types one by one. We have also talked about the advertisements that they accept. You can also find the OSPF Area Types and the LSA types accepted by them in the below table.
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