What Is OSPF Special Area Types in Networking?
Also known as: OSPF special area types, OSPF stub area, OSPF totally stubby area, OSPF NSSA, CCNP ENARSI OSPF
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Quick Definition
OSPF special area types are ways to design parts of a network so they don't have to carry every single route from the whole internet or company network. Think of them like different sections of a library where some sections only have a small card catalog instead of the full directory. These areas help routers work faster and use less memory by limiting the routing information they need to store.
Must Know for Exams
OSPF special area types are a core topic in the Cisco CCNP Enterprise certification, specifically in the Implementing Cisco Enterprise Advanced Routing and Services (ENARSI) exam (300-410). This exam tests your ability to design, configure, and troubleshoot OSPF in complex enterprise networks. The exam objectives explicitly cover OSPF stub areas, totally stubby areas, NSSA, and totally NSSA. You are expected to know the difference between each type, which LSA types are allowed or blocked, and how the default route is generated.
In the ENARSI exam, questions often present a scenario with a network diagram showing an OSPF topology. You might be asked to choose the correct area type for a remote branch that only needs connectivity to the corporate headquarters and should not receive any external or inter-area routes. Another common question type gives you a show ip ospf database output and asks you to identify the area type based on the LSA types present. For example, if you see only Type 1, Type 2, and a single Type 3 default route, you should recognize a totally stubby area. If you see Type 7 LSAs instead of Type 5, you know it is an NSSA.
The exam also tests your understanding of the configuration commands. You need to know that the area X stub command creates a stub area, and area X stub no-summary creates a totally stubby area. The area X nssa command enables NSSA, and area X nssa no-summary creates a totally NSSA. You must also understand the role of the ABR in generating the default route and translating Type 7 LSAs. Troubleshooting questions might involve a scenario where a router in a stub area cannot reach an external destination because the ABR is not generating a default route, or where an NSSA ABR is not translating Type 7 LSAs correctly.
Beyond ENARSI, these topics also appear in the CCIE Written and Lab exams. In the lab, you may be required to configure multiple areas with different types and verify that the correct LSAs are being exchanged. The exam expects you to know the constraints, such as virtual links cannot traverse stub areas or NSSAs, and that all routers in a stub area must agree on the stub configuration. Mastering these details is critical because area type misconfiguration is a common source of routing failures in both real networks and exams.
Simple Meaning
Imagine you work in a large office building with many departments. Every department has a map of the entire building showing every room, hallway, and exit. That is a lot of information to carry around.
Now imagine that some departments only need a map of their own floor plus a simple sign that says "Exit to other floors here." They do not need the detailed layout of every other floor. OSPF special area types work exactly like that.
In networking, OSPF is a protocol that helps routers figure out the best path to send data. A network can be divided into areas to make management easier. A standard OSPF area knows all the routes available in the entire network.
But that can overwhelm smaller routers with limited memory. Special area types let you create areas that know less. For example, a stub area blocks detailed routes from outside the area and instead uses a single default route for all external traffic.
A totally stubby area goes even further by also blocking routes from other areas inside the same network. A not-so-stubby-area, or NSSA, is like a stub area that allows one specific type of external route to enter under special conditions. These special area types are like giving different departments different levels of access to the building map.
The mailroom needs a full map because they deliver everywhere. A small satellite office might only need to know how to get to the main building and then trust the main building to know the rest. This reduces clutter and makes the network more efficient, stable, and easier to troubleshoot.
Full Technical Definition
OSPF (Open Shortest Path First) is a link-state routing protocol that maintains a complete view of the network topology using the Shortest Path First (SPF) algorithm. In large networks, OSPF can be partitioned into areas to limit the scope of link-state advertisements (LSAs) and reduce SPF calculation overhead. An OSPF area is a logical grouping of routers that share the same link-state database. The backbone area, area 0, must connect all other areas. Special area types are configurations applied to non-backbone areas that restrict the types of LSAs injected into that area. The standard LSA types relevant here are Type 1 (Router LSA), Type 2 (Network LSA), Type 3 (Summary LSA), Type 4 (ASBR Summary LSA), Type 5 (External LSA), and Type 7 (NSSA External LSA).
There are four primary special area types. The stub area blocks Type 5 LSAs (external routes) from entering the area. Instead, the Area Border Router (ABR) generates a default route (0.0.0.0/0) as a Type 3 LSA, directing all external traffic out of the area. This reduces the LSA database size in the stub area. A totally stubby area, a Cisco proprietary extension, goes further by blocking both Type 5 LSAs and Type 3 LSAs (inter-area routes). The ABR injects only a single default route into the area. This creates the smallest possible link-state database for that area, as routers only know about their own area and the default path out.
The not-so-stubby-area (NSSA) is defined in RFC 3101. It allows an area to be configured as a stub area while still permitting the injection of external routes from an autonomous system border router (ASBR) located inside the NSSA. These external routes are carried as Type 7 LSAs within the NSSA. At the ABR, Type 7 LSAs are translated into Type 5 LSAs for propagation into other areas. A totally NSSA, another Cisco extension, blocks Type 3, Type 4, and Type 5 LSAs but permits Type 7 LSAs from an internal ASBR. The ABR injects a default route into the area.
Implementation in real IT environments requires careful planning. The area type must match the capabilities of all routers in that area. For example, all routers in a stub area must support the stub configuration. Virtual links cannot traverse a stub area or NSSA. Additionally, stub areas and NSSAs cannot contain an ASBR because external routes (Type 5) are blocked, except for NSSAs which allow Type 7 LSAs. These configurations are configured under the OSPF process on the router using commands like area 1 stub or area 2 nssa. Network engineers use these area types to optimize routing tables, improve convergence time, and reduce CPU and memory usage on lower-end routers deployed at remote sites.
Real-Life Example
Think of a large university campus with multiple buildings. The main administration building has a detailed directory of every room, office, and facility across the entire campus. Each individual building, like the library or the science hall, has its own internal directory showing rooms inside that building. Now, the administration building shares its full campus directory with all other buildings. That would be like a standard OSPF area where every router knows every route. But the library does not need to know the exact location of every janitor closet in the science hall. It only needs to know where the science hall building is and how to get there. So the campus decides to give the library a simplified directory. The library gets a map showing only the library itself and one main exit sign that says "All other buildings: go through the central plaza." That is a stub area. The library does not receive detailed routes for every other building; it only gets a default route pointing to the main plaza.
Now imagine a small satellite office off campus. This office is inside a separate business park. The satellite office also uses the university network, but it only needs to know about its own building and the central campus. The network team decides to give the satellite office a totally stubby area. That means the satellite office does not get any detailed routes for any other building on campus, not even the science hall or library. It only knows about its own space and a single door that leads to the entire university network. This is the most restricted view possible.
Finally, consider a research lab that is part of the university network but also connects to an external research consortium. The lab needs to send data to both the university and that specific external partner. A standard stub area would block all external routes, which would break the connection to the consortium. So the network team configures this area as an NSSA. The lab can now accept one specific external route from the consortium, but it still does not get all other external routes from the internet. The ABR translates this special external route into a regular format for the rest of the university network.
Why This Term Matters
In real IT work, networks are rarely small. A large enterprise might have hundreds of routers spread across multiple geographic locations. Running OSPF without special area types would force every router to maintain a complete map of the entire network. This is a serious problem for several reasons. First, memory is finite. Lower-end routers at branch offices often have limited RAM. Loading a full OSPF database with thousands of routes can exhaust memory, causing the router to crash or flap routes. Second, the SPF algorithm recalculates every time the network topology changes. In a flat OSPF design, a single link flapping in a remote data center triggers SPF recalculations on every router in the network, including those in faraway branches. This wastes CPU cycles and delays convergence. Special area types solve both problems by containing the impact of topology changes and reducing the routing table size to only what is necessary for each area.
From a security perspective, special area types can also limit the visibility of network routes. A branch office that only needs access to corporate applications does not need to know the detailed routing structure of the data center. Using a totally stubby area prevents that branch from seeing internal inter-area routes, adding a small layer of abstraction. This is not a security protocol by itself, but it reduces the information available if a router is compromised.
In terms of configuration complexity, special area types require careful planning. A common real-world scenario is merging two companies with overlapping OSPF domains. An NSSA can allow one organization to inject external routes without redesigning the entire area structure. Network engineers also use these area types when deploying OSPF in MPLS VPN environments where the provider edge routers need to advertise only a default route to customer edge routers. Understanding special area types is essential for anyone managing medium to large OSPF deployments, as you will encounter them in design reviews, troubleshooting, and upgrades. They also appear frequently in CCNP and CCIE lab exams, where you must configure, verify, and troubleshoot these area types under time pressure.
How It Appears in Exam Questions
Exam questions about OSPF special area types come in several distinct formats. The first is the scenario-based design question. You are given a network topology with multiple locations, router models, and traffic requirements. You must select the most appropriate area type for a given area. For example, a question might describe a remote sales office with low-end routers that only needs access to the central data center and the internet. The correct answer would likely be a totally stubby area because it minimizes routing information. The distractor options might include a standard area or an NSSA, which would incorrectly allow external routes or inter-area routes.
The second format is the identification question using router output. The exam might present a partial output from show ip ospf database or show ip route. You need to deduce the area type from the LSA types present. For instance, if the database shows Type 1, Type 2, and a single Type 3 LSA with a destination of 0.0.0.0, the area must be a totally stubby area. If Type 5 LSAs are absent but Type 7 LSAs are present, it is an NSSA. These questions test your ability to interpret real router behavior, which is a practical skill for both exams and real-world troubleshooting.
The third format is the configuration verification question. The exam might show a running configuration snippet from an ABR, such as router ospf 1 area 1 stub. You must determine the effect on routers in area 1. Sometimes the question asks which command would need to be added to make the area totally stubby. The correct answer is area 1 stub no-summary. Another variant asks about the effect on the ABR itself, such as whether the ABR will generate a default route into the area automatically.
The fourth format is the troubleshooting scenario. A question describes a symptom, such as hosts in area 2 cannot reach a web server on the internet, even though other areas can. You must identify the root cause, which could be that area 2 is a stub area but the ABR is not configured to inject a default route, or the area is an NSSA but the external route is not being translated correctly. Sometimes the issue is that a router in a stub area has been misconfigured as a standard area, and it receives a Type 5 LSA that it cannot process, causing the OSPF adjacency to fail.
The fifth format is the constraints question. You are asked which OSPF features are incompatible with a given special area type. For example, a question might ask, "Which of the following cannot be placed in a stub area?" The correct answer is an ASBR, because external routes are blocked. Another question might ask if virtual links can traverse an NSSA. The answer is no. These questions test your knowledge of the limitations and rules governing each area type.
Study enarsi
Test your understanding with exam-style practice questions.
Example Scenario
A company called GreenLeaf has a headquarters in New York City and a small branch office in Albany. The Albany office has only 20 employees and uses a single router with limited memory. The network team wants to connect Albany to the headquarters using OSPF. They decide to put Albany in its own area, area 5. The headquarter's router is the ABR for area 5. The team wants to ensure that the Albany router does not receive every single route from the entire corporate network, as that would overwhelm its memory. They also want to prevent Albany from seeing routes that come from the internet, because those are many and constantly changing.
They configure area 5 as a totally stubby area. On the headquarter's ABR, they use the command area 5 stub no-summary. This tells the ABR to block all Type 3 summary routes from other areas and all Type 5 external routes. Instead, the ABR injects a single default route into area 5. Now the Albany router only knows about routes within area 5 itself, plus one default route pointing to the headquarters for everything else. When an employee in Albany wants to access a server in the Chicago data center, the router checks its routing table. It does not have a specific route to Chicago, so it uses the default route and sends the traffic to the headquarters ABR, which knows how to forward it. This works perfectly and keeps the Albany router's routing table small and stable. If the Chicago link goes down, only the routers in Chicago and the ABR need to recalculate. The Albany router is unaffected because it only sees a default route. This is a textbook use of a totally stubby area.
Common Mistakes
Thinking that a stub area blocks all Type 3 LSAs (inter-area routes).
A standard stub area only blocks Type 5 LSAs (external routes). It still allows Type 3 LSAs from other areas. Blocking Type 3 LSAs is a feature of the totally stubby area, not the standard stub area.
Remember that stub blocks Type 5 only. Totally stubby blocks Type 3 and Type 5. Think of 'stub' as blocking external, and 'totally stubby' as blocking external plus inter-area.
Placing an ASBR inside a stub area or an NSSA without understanding the LSA restrictions.
A stub area blocks Type 5 LSAs, so an ASBR cannot inject external routes into a stub area. An NSSA allows an ASBR to inject Type 7 LSAs, but the ASBR must be in the NSSA itself. Placing an ASBR in a stub area will break OSPF because the Type 5 LSAs are dropped.
Use an NSSA if you need to inject external routes from within the area. Never place an ASBR in a standard stub area. Check whether the area type supports the required LSA types before deploying.
Assuming that all routers in a stub area must be configured with the same 'no-summary' keyword.
The 'no-summary' keyword is configured only on the ABR, not on the internal routers inside the stub area. Internal routers in a stub area simply use the 'area X stub' command. The ABR's 'no-summary' option tells the ABR to block Type 3 LSAs and inject a default route. Internal routers do not need this keyword.
Configure internal routers with 'area X stub'. Configure the ABR with 'area X stub no-summary' to make it totally stubby. Do not add 'no-summary' to internal routers.
Assuming that a virtual link can be configured across a stub area or NSSA.
Virtual links require a full transit area that can carry all LSA types, including Type 5 LSAs. Stub areas and NSSAs block Type 5 LSAs by design, so they cannot serve as transit areas for virtual links. A virtual link across a stub area will fail to establish.
If you need a virtual link, ensure the transit area is a standard non-stub area. Alternatively, redesign the network to avoid virtual links altogether by connecting the area directly to area 0.
Confusing the default route generation in stub areas versus NSSAs.
In a standard stub area or totally stubby area, the ABR automatically generates a default route (Type 3 LSA) into the area. In an NSSA, the ABR does not automatically inject a default route. You must explicitly configure it using the 'area X nssa default-information-originate' command if you want a default route in the NSSA.
Memorize that NSSA does not automatically create a default route. Always check if a default route is needed in the NSSA and add the appropriate command on the ABR.
Exam Trap — Don't Get Fooled
A question shows a router configured in area 2 with the command 'area 2 stub' and asks if this router can receive a default route from the ABR. The trap is that the learner assumes the router will receive a default route automatically. However, if the ABR in area 2 does not have the 'area 2 stub' command configured, the OSPF adjacency between the internal router and the ABR will not form at all because the area type must match on all routers.
Always remember that the OSPF stub area configuration must be consistent across all routers in that area. When you see a configuration question, verify that every router in the area has the same 'area X stub' or 'area X nssa' command. If any router lacks it, the adjacency will not form and the area will not function as intended.
This is a classic exam trap.
Commonly Confused With
A stub area blocks only Type 5 LSAs (external routes) but allows Type 3 LSAs (summary routes from other areas). A totally stubby area blocks both Type 5 and Type 3 LSAs, so it only knows about its own area routes and a single default route. The totally stubby area is more restrictive and is a Cisco proprietary extension.
In a stub area, a router sees routes like 10.1.0.0/16, 10.2.0.0/16 (other areas) and a default route. In a totally stubby area, the router sees only its own area routes like 192.168.1.0/24 and a single default route 0.0.0.0/0.
A standard area accepts all LSA types, including Type 5 external routes. An NSSA is like a stub area but with the exception that it can accept Type 7 LSAs, which carry external routes from an ASBR inside the NSSA. The NSSA ABR translates Type 7 to Type 5 for other areas. A standard area does not restrict external routes at all.
A standard area in an enterprise might receive hundreds of external routes from the internet. An NSSA at a branch might receive only one specific external route from a partner network, plus a default route, but it blocks all other external routes.
A virtual link is a logical connection used to connect a non-backbone area to area 0 through a transit area when a physical connection is missing. Special area types are configurations that restrict LSA distribution within an area. Virtual links cannot traverse stub areas or NSSAs because those areas block the LSAs needed for virtual link operation.
If area 3 is not directly connected to area 0, you might configure a virtual link through area 1. If area 1 is a stub area, the virtual link will fail. You would need to convert area 1 to a standard area to support the virtual link.
Step-by-Step Breakdown
Identify the need for a special area type
Analyze the network requirements. Ask whether a branch office has routers with limited memory or CPU. Determine if the branch only needs connectivity to the core and does not need detailed knowledge of other areas or external networks. This step sets the foundation for choosing the correct area type.
Choose the appropriate special area type
Based on the analysis, select the area type. Use a stub area if the branch needs to see inter-area routes but not external routes. Use a totally stubby area if the branch needs no inter-area routes and only a default route. Use an NSSA if the branch must inject its own external routes but otherwise behave like a stub area. Use a totally NSSA if the branch needs the NSSA behavior plus no inter-area routes.
Configure the area type on all routers in the area
On every router that belongs to the special area, enter OSPF configuration mode and apply the area command. For a stub area, use 'area <area-id> stub'. For a totally stubby area, configure 'area <area-id> stub no-summary' on the ABR and 'area <area-id> stub' on internal routers. For an NSSA, use 'area <area-id> nssa' on all routers in the area. Ensure consistency across all routers to prevent adjacency failures.
Configure default route injection (if needed)
In stub and totally stubby areas, the ABR automatically generates a default route. In NSSA, the default route is not automatic. Add the command 'area <area-id> nssa default-information-originate' on the ABR if a default route is needed. For totally NSSA, use 'area <area-id> nssa no-summary' which simultaneously blocks Type 3 LSAs and injects a default route.
Verify the area configuration
Use verification commands such as 'show ip ospf', 'show ip ospf database', and 'show ip route ospf' on routers inside the area. Check that the LSA types match expectations. For a totally stubby area, the database should contain only Type 1, Type 2, and a Type 3 default route. For an NSSA, look for Type 7 LSAs. Also verify that OSPF adjacencies are established. If adjacency fails, double-check that all routers share the same area type configuration.
Practical Mini-Lesson
OSPF special area types are a critical tool for network engineers who design scalable routing architectures. In practice, you will encounter them most often in multi-site enterprise networks where branch offices connect to a central data center. The key to mastering these area types is understanding exactly which LSA types are permitted and which are blocked. For the ENARSI exam, you must memorize that a stub area blocks Type 5 LSAs, a totally stubby area blocks Type 3 and Type 5, an NSSA blocks Type 5 but allows Type 7, and a totally NSSA blocks Type 3 and Type 5 but allows Type 7.
When configuring these area types in a lab or production environment, always start by planning the area design on paper. Identify which routers will act as ABRs and which will be internal routers. Then configure the ABR first, followed by the internal routers. A common mistake is to forget that internal routers in a stub area must also have the 'area X stub' command. If an internal router misses this command, it will not form an adjacency with the ABR because the ABR will not send LSAs that the internal router expects. The OSPF neighbor state will stay in the EXSTART or EXCHANGE state, and you will see error messages about mismatched area type.
Another practical consideration is the use of NSSA in situations where you have a firewall or a separate routing domain connected to a branch router. For example, if your branch has a direct internet connection through a separate ISP router, that ISP router might inject a default route into OSPF. If the branch area is a stub area, that default route would be a Type 5 LSA and would be blocked, breaking internet connectivity. The solution is to use an NSSA and configurpartitioning the ISP connection as an ASBR inside the NSSA. The ISP default route becomes a Type 7 LSA and is allowed.
Troubleshooting area type issues requires you to look at the LSA database. Use 'show ip ospf database' on a router inside the area and compare the output with what you expect. If you see Type 5 LSAs in an area that should be stub, the configuration is wrong. If you see no default route in an NSSA, check whether the ABR has the 'default-information-originate' keyword. Also check the routing table on the ABR itself to ensure it has a default route to originate.
Finally, remember that special area types are not a substitute for good network design. They are a refinement that should be applied after the basic area structure is in place. Overusing stub areas can make future network expansions more difficult. Always weigh the memory savings against the administrative overhead of maintaining stub configurations across dozens of routers. Professionals who master these area types can build networks that are both efficient and resilient, which is why this topic carries significant weight in CCNP and CCIE certifications.
Memory Tip
Use the acronym S-T-N for LSA restrictions: Stub blocks Type 5; Totally stubby blocks Type 3 and 5; NSSA blocks Type 5 but allows Type 7. For default routes, remember: Stub and Totally stubby get automatic defaults; NSSA does not, so you must add 'default-information-originate'.
Covered in These Exams
Related Glossary Terms
802.1Q is the networking standard that allows multiple virtual LANs (VLANs) to share a single physical network link by tagging Ethernet frames with VLAN identification information.
802.1X is a network access control standard that authenticates devices before they are allowed to connect to a wired or wireless network.
An A record is a DNS record that maps a domain name to the IPv4 address of the server hosting that domain.
Frequently Asked Questions
What is the main benefit of using an OSPF stub area?
The main benefit is that it reduces the size of the routing table and link-state database in routers within the stub area by blocking external routes (Type 5 LSAs). This saves memory and CPU processing power, especially on lower-end routers.
Can I place an ASBR inside a stub area?
No, you cannot place an ASBR inside a standard stub area because stub areas block Type 5 LSAs, which are the only way an ASBR injects external routes. If you need an ASBR, use an NSSA instead.
What is the difference between a stub area and a totally stubby area?
A stub area blocks only Type 5 LSAs (external routes) but allows Type 3 LSAs (summary routes from other areas). A totally stubby area blocks both Type 3 and Type 5 LSAs, so it only knows about its own area routes and a single default route.
Does an NSSA automatically generate a default route?
No, an NSSA does not automatically generate a default route. You must explicitly configure the command 'area X nssa default-information-originate' on the ABR if you want a default route to be injected into the NSSA.
Can a virtual link traverse a stub area?
No, a virtual link cannot traverse a stub area or an NSSA because those area types block the necessary LSA types (Type 5) needed for the virtual link to function. Virtual links require a standard transit area.
What command creates a totally stubby area on the ABR?
The command is 'area <area-id> stub no-summary' configured under the OSPF process on the ABR. Internal routers in the area use the simpler 'area <area-id> stub' command.
Summary
OSPF special area types are a powerful mechanism for controlling the flow of routing information within an OSPF domain. They allow network engineers to design scalable and efficient networks by limiting the LSAs that routers in certain areas receive. The four main types are stub, totally stubby, NSSA, and totally NSSA.
Each type has a specific LSA filtering behavior that directly impacts the size of the routing table and the stability of the area. For IT certification exams, particularly the CCNP ENARSI, you must know which LSA types are allowed or blocked for each area type, how configuration commands differ on ABRs versus internal routers, and the constraints such as virtual links being incompatible with special areas. Mastering these concepts not only helps you pass exams but also equips you to design and troubleshoot real enterprise networks where resource optimization is critical.
Always verify consistency of area type configuration across all routers in the area, and remember that default route injection in NSSA requires explicit configuration. With this understanding, you can confidently apply special area types to reduce overhead, improve convergence, and build robust OSPF networks.