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

OSPF Area Types Explained

Open Shortest Path First (OSPF) is a link-state Interior Gateway Protocol (IGP) that maintains a database of network topology and computes optimal paths using Dijkstra’s algorithm. You can read more on OSPF on Wikipedia.

A key feature of OSPF is its area design. Dividing a routing domain into multiple areas improves scalability, reduces routing overhead, and optimizes convergence times. Let’s explore the types of areas and how to configure them.

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1. Stub Area

A Stub Area limits the number of external routes (Type 5 LSAs) in the LSDB. Routers receive a default route pointing toward the ABR.

Router(config)# router ospf 55
Router(config-router)# area 100 stub

All routers in the stub area must be configured with the stub keyword.

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2. Totally Stubby Area

A Totally Stubby Area blocks external and summary LSAs (Type 3), leaving only intra-area routes and a default route from the ABR.

Router(config)# router ospf 55
Router(config-router)# area 100 stub no-summary

On non-ABR routers, only stub is needed.

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3. Not-So-Stubby Area (NSSA)

An NSSA allows redistribution of external routes in a stub area (Type 7 LSAs converted to Type 5 by the ABR).

Router(config)# router ospf 55
Router(config-router)# area 100 nssa default-information-originate

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4. Totally Stubby NSSA

A hybrid area that blocks summary LSAs but allows external routes as Type 7 LSAs.

Router(config)# router ospf 55
Router(config-router)# area 100 nssa no-summary

Routers inside the area are configured with just the nssa keyword.

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Key Differences in Configuration Behavior

• Earlier releases required explicit options on all routers for NSSA default injection.
• Modern releases streamline defaults, reducing manual configuration.
• Keywords like no-summary now apply precisely on ABRs, simplifying deployment.

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Interactive OSPF Topology

Hover over routers below to see the OSPF area type.

Hover over each router to view its OSPF area type and behavior.

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Final Thoughts

Choosing the correct OSPF area type depends on your network’s objectives:

• Use Stub Areas to reduce external route overhead.
• Use Totally Stubby Areas for minimal LSDB entries.
• Use NSSA to inject external routes into stub areas.
• Use Totally Stubby NSSA for maximum control and efficiency.

Proper area design ensures efficient resource utilization, faster convergence, and a stable OSPF environment.