Mastering OSPF Stub Areas: When and Why to Use Them

March 2015   |   Reading Time: 10 min read

Introduction

Open Shortest Path First (OSPF) is a link-state routing protocol widely used in enterprise and service provider networks. As OSPF networks scale, it becomes critical to manage the size of routing tables and the number of LSAs (Link-State Advertisements) flooded throughout the topology. OSPF stub areas are one method to reduce this overhead without sacrificing network reachability.

What Are OSPF Stub Areas?

Stub areas are special types of OSPF areas where external routes (Type 5 LSAs) are restricted. Instead of learning each external route individually, routers in stub areas receive a default route (0.0.0.0/0) pointing toward the ABR (Area Border Router).

Types of Stub Areas

• Stub Area – Blocks Type 5 LSAs, injects a default route.
• Totally Stubby Area – Blocks Type 3 and Type 5 LSAs, only default route is allowed.
• Not-So-Stubby Area (NSSA) – Allows Type 7 LSAs from redistribution inside the area and converts them to Type 5 at the ABR.
• Totally NSSA – Like NSSA, but blocks inter-area routes (Type 3 LSAs).

When to Use Stub Areas

Stub areas are ideal for edge segments or branches that need connectivity but don't require visibility into external destinations. Using stub areas can reduce router CPU, memory usage, and LSA flooding, which improves convergence time.

Design Considerations

• All routers in the area must agree on stub configuration.
• You cannot have an ASBR inside a stub area (except for NSSA).
• Mixing stub and non-stub routers causes adjacency failure.
• Stub areas work best for leaf segments with a single path to the core.

Sample Configuration

    ! Configure Area 1 as a stub on the ABR
    router ospf 1
     area 1 stub
    !
    ! Configure on internal routers
    router ospf 1
     area 1 stub
    !
    ! To make Area 1 a totally stubby area
    router ospf 1
     area 1 stub no-summary

NSSA Configuration Example

    router ospf 1
     area 2 nssa
    !
    ! NSSA with no-summary (Totally NSSA)
    router ospf 1
     area 2 nssa no-summary
    !
    ! Redistributing into NSSA
    router ospf 1
     redistribute static subnets
     area 2 nssa

Benefits of Stub Areas

• Reduced routing table size
• Lower LSA flooding and faster convergence
• Minimized complexity for edge routers
• More predictable routing behavior

Limitations and Pitfalls

• Cannot have multiple exit points without breaking stub logic
• Default routes may mask suboptimal paths
• Cannot host an ASBR (except in NSSA)
• Must manually configure all routers consistently

Verification and Troubleshooting

• Use show ip ospf and show ip ospf database to inspect LSA types
• Check for neighbor issues using show ip ospf neighbor
• Confirm stub configuration with show ip protocols
• Debug with debug ip ospf adj for adjacency problems

Conclusion

Stub areas are a simple yet powerful way to optimize OSPF networks. By strategically applying stub area configurations, network engineers can ensure scalability, improve stability, and maintain control over routing behavior. Understanding the differences between stub types and applying them correctly is key to successful OSPF design.

Eduardo Wnorowski is a network infrastructure consultant and technologist.
With over 20 years of experience in IT and consulting, he brings deep expertise in networking, security, infrastructure, and transformation.
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