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Understanding OSPF Area Types: Stub, NSSA, Totally Stubby, and Totally NSSA

When designing an OSPF network, understanding the various area types plays a crucial role in optimizing routing efficiency and controlling the size of the routing table. OSPF areas such as Stub, NSSA (Not-So-Stubby Area), and their Cisco proprietary counterparts, Totally Stubby and Totally NSSA, each serve specific purposes in different network scenarios. These area types help reduce the amount of routing information shared within an area while controlling the advertisement of external and inter-area routes. In this post, we will explore the characteristics, use cases, and default route advertisement behavior of these OSPF areas, providing insight into how they can improve network performance and scalability.

OSPF Area Type

Allowed LSAs

Disallowed LSAs

Use Cases

Key Characteristics

Default Route Injection

Stub Area

Type 1 (Router),
Type 2 (Network),
Type 3 (Summary)

Type 4 (ASBR Summary),

Type 5 (External)

Small areas where external routes aren’t needed.

Provides a default route to reach external destinations.
Reduces routing table size.
No external routes allowed (Type 5 LSAs).

Automatically injected by the ABR.

Totally Stubby Area

Type 1 (Router),
Type 2 (Network)

Type 3 (Summary except default),
Type 4,
Type 5

Ideal for branch offices or remote sites.

More restricted than Stub Areas.
Only a default route is propagated from the ABR.
Prevents external and inter-area routes.

Automatically injected by the ABR.

NSSA (Not-So-Stubby Area)

Type 1 (Router),
Type 2 (Network),
Type 3 (Summary),
Type 7 (NSSA External)

Type 4,
Type 5

Areas that need to inject external routes from an ASBR (Type 7 LSAs).

Allows redistribution via Type 7 LSAs.
Type 7 LSAs are converted to Type 5 at the ABR.
Blocks Type 5 LSAs.

Requires manual injection using default-information originate.

Totally NSSA

Type 1 (Router),
Type 2 (Network),
Type 7 (NSSA External)

Type 3 (Summary except default),
Type 4,
Type 5

Used where external routes are needed but fewer routes are required.

ike Totally Stub, but allows Type 7 LSAs.
Only default route and Type 7 external routes allowed.
Blocks Type 3 summary LSAs (except default).

Requires manual injection using default-information originate.

Key Differences:

  • Cisco Proprietary Extensions:
    • Totally Stubby Area and Totally NSSA are Cisco proprietary and further reduce the routing information exchanged within the area compared to standard OSPF Stub and NSSA areas.
  • Default Route Advertisement:
    • Stub, Totally Stubby Area, and Totally NSSA automatically receive a default route (0.0.0.0) from the ABR.
    • NSSA requires manual configuration of a default route using default-information-originate.

Summary:

  • Stub Area: Blocks external routes but allows inter-area routes. Default route injected automatically.
  • Totally Stubby Area: Blocks both external and inter-area routes, allows only a default route.
  • NSSA: Blocks external routes but allows redistribution of local external routes as Type 7 LSAs.
  • Totally NSSA: Blocks both external and inter-area routes but allows local external redistribution as Type 7 LSAs and receives a default route from ABR.

These OSPF areas help optimize routing and reduce the size of the OSPF database depending on the network design.

 


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