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Introduction to OSPF Area Types

March 11, 2024 , ,

Introduction to OSPF Area Types

OSPF (Open Shortest Path First) is a routing protocol that establishes routing paths within a network. In an OSPF network, regions are used to organize logically related routers. Ideally, all areas should be physically connected to the backbone (Area 0), which serves as a centralized point for routing information.

⭐Related : Top OSPF Protocol Interview Questions and Answers
⭐Related : OSPF protocol : OSPF Packet Types

 What is OSPF Area Types ?

OSPF Area is a concept to divide a large network into smaller logical domains. This helps with scalability and reduces the size of routing tables on routers. Routers in the same area must have identical link-state databases (LSDBs). 

OSPF Area Types
Fig 1.1- OSPF Area Types

The key advantage of constructing regions is that it reduces the number of routes to propagate through route filtering and summarization. There are different types of OSPF Areas as: 

  • Backbone Area
  • Standard Area
  • Stub Area
  • Totally Stubby Area
  • Not-So-Stubby Area (NSSA)
  • NSSA Totally Stub Area

 OSPF Backbone Area

  • The backbone area, also known as Area 0, serves as the base for an OSPF network. It connects every other area.
  • All non-backbone areas must be directly connected to the backbone area (either physically or through virtual links).
  • Backbone areas are not partitionable, keeping network connectivity irrespective of failures.

 OSPF Standard Area

  • A standard area in OSPF is the most basic and unrestricted type of area within an OSPF network. 
  • By allowing all LSA types, standard areas have a complete picture of the routing topology within their area and can calculate the best paths for data packets.

OSPF Standard Area

 OSPF Stub Area

  • A stub area does not allow external routes (routes from outside the OSPF domain) to be advertised within the area.
  • External routes are summarized at the area border router (ABR) before entering the stub area.
  • This simplification reduces the complexity of routing calculations for routers in the stub area and keeps their routing databases smaller.
  • An Area Border Router (ABR) injects a default route (typically 0.0.0.0) into the stub area. Routers in the stub area use this default route to reach any external network.

OSPF Stub Area

 OSPF Totally Stubby Area (TSA)

  • In OSPF, a Totally Stubby Area (TSA) is a specialized area type that takes stub area functionality a step further to achieve even stricter limitations on routing information.
  • Totally Stubby Area allows only LSA Type 1 and LSA type 2
  • Blocks external routes LSA Type 5  and also prevents Type 3 LSAs (summary LSAs) from being advertised into the area.
  • Totally Stubby Area depends on an Area Border Router (ABR) for reaching external destinations. The ABR injects a default route (typically 0.0.0.0) into the TSA. Routers within the TSA use this default route to forward any traffic for unknown networks to the ABR for further routing.

OSPF Totally Stubby Area (TSA)


 OSPF Not-So-Stubby Area (NSSA)

  • In OSPF, a Not-So-Stubby Area (NSSA) provides an intersection between the ease of a stub area and the complete information flow of a standard area.
  • They block Type 5 LSAs (External Link-State Advertisements) from other OSPF areas. This prevents internal routers from being flooded with unnecessary external route information.
  • They allow the advertisement of specific external routes within the NSSA itself. These routes are advertised using a special LSA type: Type 7 (NSSA External LSA).
  • The ABR translates the Type 7 LSAs from the NSSA into standard Type 5 LSAs (External Link-State Advertisements) and vice versa for External communication. 

OSPF Not-So-Stubby Area (NSSA)

 OSPF NSSA Totally Stub Area

  • In OSPF, a Not-So-Stubby Totally Stub Area (NSSA Totally Stub Area) combines features of both NSSA (Not-So-Stubby Area) and Totally Stub Area (TSA), offering a unique area type with specific functionalities
  • Similar to a TSA, routers within an NSSA Totally Stub Area primarily have knowledge of LSA Type 1 and LSA Type 2. 
  • They can additionally learn about specific external routes advertised as Type 7 LSAs within the area.
  • However, they are completely isolated from information about other OSPF areas due to the blocked Type 3 LSAs.
  • The ABR injects a default route (typically 0.0.0.0) into the area.
  • ABR translates Type 7 LSAs from the NSSA into standard Type 5 LSAs and floods them into other appropriate areas, allowing external reachability for the advertised routes.

OSPF NSSA Totally Stub Area

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