February 2012 | 8 min read
Open Shortest Path First (OSPF) is one of the most widely adopted interior gateway protocols (IGPs) in enterprise networks. A key feature of OSPF is its support for hierarchical design using areas. Understanding the different OSPF area types — standard, stub, totally stubby, and not-so-stubby (NSSA) — is essential for optimal scalability, security, and performance.
Why Use OSPF Areas?
Segmenting an OSPF network into multiple areas helps reduce the size of routing tables, limits the propagation of topology changes, and keeps LSA flooding under control. Every OSPF deployment must include an Area 0, also known as the backbone area, which connects to all other areas either directly or virtually.
Standard Areas (Default)
These areas support all OSPF LSAs (Type 1 to Type 5) and allow full route redistribution. They’re flexible but can become inefficient in large topologies due to the volume of routing information exchanged.
Stub Areas
A stub area limits external routing information by blocking Type 5 LSAs (external routes from other protocols like BGP). Instead, a default route is injected to reach external destinations. This reduces the LSA database size and simplifies routing.
Totally Stubby Areas (Cisco Extension)
Going further than stub areas, totally stubby areas also block Type 3 LSAs (inter-area routes), leaving only a default route. These are excellent in hub-and-spoke topologies where branches don’t need visibility into the full enterprise routing table.
NSSA (Not-So-Stubby Areas)
NSSAs provide a hybrid between stub areas and standard areas. They allow limited external route injection using Type 7 LSAs, which are translated to Type 5 LSAs at the ABR. This is useful when you need to redistribute routes into OSPF in an otherwise stub area, such as from a firewall or edge device at a branch site.
Totally NSSA
This combines the properties of a totally stubby area with NSSA behavior — filtering both Type 3 and Type 5 LSAs while still permitting Type 7 LSAs. Not all vendors support this natively, but it's an important concept in some network designs.
Design Considerations
- Always keep Area 0 as the core and backbone of your design.
- Use stub or totally stubby areas to simplify branch routing and reduce overhead.
- Use NSSA where route redistribution at the branch level is required.
- Be mindful of ABR and ASBR placement — misconfiguration can break LSAs or loop prevention.
- Monitor LSA counts and SPF calculation frequency to validate design efficiency.
Practical Example
Consider a network with HQ and 20 branch offices. HQ is Area 0. Each branch is in a separate totally stubby area. Branch firewalls running BGP inject specific routes to external services. For these branches, configure the OSPF area as NSSA to permit external routes without opening the area fully to Type 5/3 LSA flooding.
Conclusion
By tailoring OSPF area types to your topology, you improve scalability, reduce CPU overhead, and maintain clear routing boundaries. Understanding the nuances of stub, totally stubby, and NSSA areas is essential for any network architect deploying OSPF in modern environments.