OSPF 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:
• Standart Area
• Backbone Area (Area 0)
• Stub Area
• Totally Stubby Area
• Totally NSSA
We will see all these areas detailly. Before and during this detailed check, you can use the below diagram to remember OSPF Areas and their specific Advertisement acceptance rules.
Let’s check these OSPF Areas one by one.
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 OSPF Area.