OSPF Areas And LSA Types

Quick Definition: Open the shortest path first (OSPF) areas to help break up your networks and routers and find the shortest and most efficient route from one location to another. Different link state advertisement (LSA) types provide data about the architecture of your network and routers.

OSPF and LSA types are often mentioned together because they are closely related components within the OSPF routing protocol. OSPF uses LSA types to ensure that network traffic is always routed to its intended destination. Large routing tables and network segments efficiently store routing tables, including ones that change often. The technology keeps bandwidth usage low when several routers are on a network.

What are OSPF Areas?

You’ve probably heard of RIP (Routing Information Protocol) if you’ve learned basic networking and router concepts. RIP sends routing tables to neighboring routers every few seconds. Keeping track of routing tables this way eats up a ton of bandwidth and causes congestion that isn’t necessary.

Instead, OSPF divides networks into subdomains called areas. An area is a logical collection of OSPF networks, routers, and links with the same area identification.

A router within an area only has to maintain a topological database for its assigned area, which reduces its database size. Rather than knowing an entire network’s topology, it only needs to know its own, making the area’s routers run much more efficiently.

So, the main benefit of creating areas is reducing the number of routes to propagate — by filtering and summarizing routes. 

What is the Backbone Area?

“Area Zero”—or the “Backbone Area“—is the first area created in an OSPF configuration. Some businesses never grow beyond this one area. As businesses grow, they typically add more routers, which means router tables grow a bit unruly with too many routes and changes.

This all leads to a waste of processor cycles. Therefore, you can break your network into multiple areas and add to Area Zero to keep things efficient.

 

What are Standard Areas?

Standard Areas are put in place to make configurations run more efficiently. You might want to split your routers and do summarization between the areas you’re building. This enables the routers to communicate with each other and the outside your network. 

 

What are Stub Areas?

Stub areas are stubby because they are limited in the information they can receive. Let’s say you have RIP, EIGRP, or even BGP routes stored in a cloud. You decide to redistribute or send those routes into your system, making them external routes.

Stub areas can block external routes and be replaced with a default route. As a result, all the routers in that Stub Area automatically get a default route.

 

What are Totally Stubby Areas?

Cisco took Stub Areas to the next level and said, “We’re gonna create a totally Stubby Area.” A Totally Stubby Area filters the external routes and routes from other systems and replaces them with a default route, making the routing table inside that area more efficient.

 

What are Not-So-Stubby Areas?

We’ve got Stubby and Totally Stubby Areas, so it only makes sense to have Not-So-Stubby Areas, right? What about areas that aren’t so stubby? A Not-so-Stubby Area — or some people call it an NSSA — usually comes about unexpectedly.

Let’s say Area One represents China, and there, they have a partner relationship with another company that is running RIP. You need to break the stub rules to send these routes into your system. But the stub needs to be live so your routing tables stay efficient.

So you’re going to have to bend the rules. That’s what we’re calling a Not-so-Stubby Area. An NSSA camouflages those routes. Once they’re through the stubby area, they appear as external routes.

OSPF LSA Type

What is LSA (Link State Advertisement)?

Understanding all area types makes understanding the Link State Advertisements so much easier. An LSA is a router’s way of communicating information in OSPF. There are different types of LSAs

 

LSA Type 1: Router LSA

Type 1 is the basic LSA generated when connecting a new router to the network. This new router uses a Type 1 LSA to let the network know it’s now a part of the architecture.

Let’s say you have a router plugged into several networks and want to communicate that fact. Because it’s a normal router, it will generate a Type 1 LSA. The router will then tell the networks to add it to their Link State databases. 

 

LSA Type 2: Network LSA (DR Generated)

A Type 2 LSA is for the designated router. When you plug a bunch of routers into one switch, you don’t want them all forming neighbor relationships.

It turns into a big mess of updates any time something happens. So, one router becomes the designated router, and the Type 2 LSA  lets all the networks know who the designated router is. 

 

LSA Type 3: Summary LSA (ABR Summary Route)

Type 3 LSAs are used whenever a router is placed between OSPF areas. It summarizes routes as router information moves across areas. For example, Router One is plugged into the 10.1 network, 10.1.1.0/24. Moving from Area Two to Area Zero, it converts from a Type 1 LSA to a Type 3 LSA.

 

LSA Type 4: Summary LSA (ASBR Location)

Type 4 LSA is the location of an Autonomous System Boundary Router (ASBR). These routers can leave your system so that internal routers can identify the best route for data transfer. In contrast, area Border Routers (ABRs) can summarize between the areas.

 

LSA Type 5: External LSA (ASBR Summary Route)

The fifth type of LSA is the external LSA. This is for routes that are coming from outside a system. Type Five LSAs are the opposite of Type Four LSAs. Type Four is the location of the ASBR, and Type Five gives us the routes from the ASBR. 

OSPF Area Types and LSA

So, how do these two concepts work together? OSPF is a dynamic routing protocol used in computer networks to determine the best paths for routing IP packets. It operates by exchanging routing information between routers using LSAs. LSAs are the messages OSPF routers use to communicate information about the network’s topology and routing metrics.

Let’s consider a practical example. Stub Areas don’t allow Type 5 LSAs. Stubby Areas completely block Type 3, Type 4, and Type 5 LSAs. By doing this, they shrink their routing tables.

Not-So-Stubby Areas use an LSA type that’s not even on this list: a Type 7 LSA. You can think of a Type 7 LSA as one of those masks that people use in a play. They put masks in front of all these Type 5 LSAs coming in to disguise them so that the Stub Area won’t block them. But once they make it through that system, the masks are removed.

OSPF Area Type

1. Backbone Area (Area 0)

The Backbone Area (Area 0) is the core of an OSPF network. All other areas must connect to Area 0 either directly or through a virtual link. It carries all inter-area routing information and ensures proper communication between different areas.

👉 Example:
If you have Area 1 and Area 2, both must connect to Area 0. If Router R1 connects Area 1 to Area 0, it acts as an ABR and forwards routing information between them.

 

2. Standard Area

A Standard Area is the default OSPF area type. It allows all types of LSAs (Type 1 to Type 5). This means it can carry both internal routes and external routes (like routes redistributed from other protocols).

👉 Example:
If a router redistributes a static route (e.g., 8.8.8.0/24), that route will be advertised as a Type 5 LSA and will be visible in all standard areas.

 

3. Stub Area

A Stub Area blocks external LSAs (Type 5) to reduce routing table size. Instead of receiving external routes, it uses a default route (0.0.0.0/0) to reach outside networks. This makes the area simpler and more efficient.

👉 Example:
If Area 1 is configured as a stub area, it will not receive external routes from an ASBR. Instead, it will use a default route to reach external networks like the internet.

 

4. Totally Stubby Area (Cisco Proprietary)

A Totally Stubby Area is an extension of the stub area (Cisco-specific). It blocks both external LSAs (Type 5) and summary LSAs (Type 3). Only a default route is allowed into the area, making the routing table very small.

👉 Example:
In a branch office with limited resources, configuring Area 2 as totally stubby ensures only one default route is used instead of many routing entries.

 

5. NSSA (Not-So-Stubby Area)

NSSA allows external route redistribution inside the area but still blocks Type 5 LSAs from entering. Instead, it uses Type 7 LSAs, which are later converted to Type 5 by the ABR when sent to other areas.

👉 Example:
If a router inside NSSA redistributes a static route (e.g., 172.16.0.0/16), it generates a Type 7 LSA. The ABR converts it into Type 5 when advertising to Area 0.

 

6. Totally NSSA (Cisco Proprietary)

Totally NSSA is a combination of NSSA and totally stubby area. It blocks Type 3 and Type 5 LSAs but still allows Type 7 LSAs within the NSSA. Only a default route is injected for external communication.

👉 Example:
In a remote office that needs to redistribute routes locally but still keep a small routing table, Totally NSSA is used.

 

Quick Comparison Table

Area TypeType 3 (Summary)Type 5 (External)Type 7Default Route
Backbone (Area 0)✔ Allowed✔ Allowed✖Optional
Standard Area✔ Allowed✔ Allowed✖Optional
Stub Area✔ Allowed✖ Blocked✖✔ Yes
Totally Stubby✖ Blocked✖ Blocked✖✔ Yes
NSSA✔ Allowed✖ Blocked✔ YesOptional
Totally NSSA✖ Blocked✖ Blocked✔ Yes✔ Yes

Key Points (Easy to Remember)

  • Area 0 = Backbone (mandatory core)
  • Standard Area = All LSAs allowed
  • Stub Area = No external routes (Type 5 blocked)
  • Totally Stubby = Only default route
  • NSSA = Allows external routes using Type 7
  • Totally NSSA = Minimal LSAs + Type 7 allowed