OSPF defines several area types for special cases, including stub areas, totally stubby areas, and not-so-stubby areas (NSSAs). The purpose behind all three types of stub areas is to inject default routes into an area so that external and summary LSAs are not flooded in. Stub areas are designed to reduce the amount of flooding, the size of the LSDB, and the size of the routing table in routers within the area. Network designers should always consider using stub area techniques when building networks, because they improve performance in OSPF networks and allow the network to scale to significantly large sizes. The characteristics assigned to an area control the type of route information that it receives. The possible area types are as follows:

Standard area: Accepts link updates, route summaries, and external routes (the default).
Backbone area (transit area): The central entity to which all other areas connect to exchange and route information. The backbone area is labeled area 0. The OSPF backbone includes all the properties of a standard OSPF area.
Stub area: Does not accept information about routes external to the autonomous system, such as routes from non-OSPF sources. This means that no type 5 LSAs are known inside the area, and consequently type 4 LSAs are unnecessary. Type 4 and 5 LSAs are blocked.

ABRs at the edge of the stub area use type 3 LSAs to advertise a single default route (0.0.0.0) into the area. If routers need to route to networks outside the autonomous area, they use the default route.Stub areas cannot contain ASBRs (except that the ABRs may also be ASBRs).

Totally stubby area: Does not accept external autonomous system routes or summary routes from other areas internal to the autonomous system. The ABR of the totally stubby area blocks type 4 and 5 LSAs as well as all summary LSAs (type 3), with an exception of a single type 3 LSA to advertise the default route.

The default route advertised by the ABR is used to reach destinations external to the autonomous system and all destinations external to the area. Therefore, if the router needs to send a packet to a network external to the area, it sends the packet using a default route. Totally stubby areas cannot contain ASBRs (except that the ABRs may also be ASBRs).

Not-so-stubby area: An NSSA is an addendum to the OSPF RFC. It offers benefits that are similar to those of a stub or totally stubby area, but also allows external routes to be advertised into the OSPF autonomous system. Therefore, NSSAs allow ASBRs, which is against the rule in a stub area.

The ASBR originates type 7 LSAs to advertise the external destinations. The type 7 LSAs are flooded throughout the NSSA but are blocked by the ABR. The ABR converts the type 7 LSA into a type 5 LSA, which is then propagated through the remainder of the autonomous system. Figure illustrates how LSUs are propagated throughout a network, including a stub area. Interactive Media Activity Drag and Drop: ID the Area Upon completion of this activity, the student will be able to identify the difference between stub and totally stubby areas in OSPF.

Content 3.7 Stub, Totally Stubby, and Not-So-Stubby Areas 3.7.2 Using Stub and Totally Stubby Areas Stub and totally stubby areas do not carry any external routes, known as type 5 LSAs. Stub and totally stubby areas have the following characteristics:

There is a single exit point from that area. However, if suboptimal routing paths are acceptable, there can be multiple exits where one or more ABRs inject a default into the stub area. Routing to other areas or autonomous systems could take a suboptimal path to reach the destination by exiting the area at a point that is farther from the destination than other exit points.
All OSPF routers inside the stub area, including ABRs and internal routers, must be configured as stub routers before they can become neighbors and exchange routing information.
There is no ASBR inside the stub area.
The area is not the backbone area (area 0).
The area is not needed as a transit area for virtual links. A virtual link is a link that allows an area to connect to the backbone via a transit area. Virtual links are generally used for temporary connections or backup after a failure and should not be considered part of a primary OSPF design. (Virtual links are covered in the next lesson.)

Content 3.7 Stub, Totally Stubby, and Not-So-Stubby Areas 3.7.3 Configuring Stub Area Configuring a stub area reduces the size of the LSDB, resulting in reduced memory requirements for routers in that area. External network LSAs (type 5), such as those redistributed from other routing protocols into OSPF, are not permitted to flood into a stub area. Routing from a stub area to the outside is based on a default route (0.0.0.0). If a packet is addressed to a network that is not in the routing table of an internal router, the router automatically forwards the packet to the ABR that sends a 0.0.0.0 LSA. Forwarding the packet to the ABR allows routers within the stub to reduce the size of their routing tables, because a single default route replaces many external routes. A stub area is typically created when a hub-and-spoke topology is used, with the spoke being the stub area, such as a branch office. In this case, the branch office does not need to know about every network at the headquarters site because it can use a default route to reach the networks. To configure an area as a stub, use the following steps: Step 1 Configure OSPF. Step 2 Define the area as a stub by issuing the area area-id stub command to all routers within the area. Figure lists the parameters of this command. By default, the ABR advertises a default route with a cost of 1. The cost of the default route can be changed by using the area default-cost command. Figure lists the parameters of this command.

Content 3.7 Stub, Totally Stubby, and Not-So-Stubby Areas 3.7.4 OSPF Stub Area Configuration Example Area 2 in Figure is defined as the stub area. No routes from the external autonomous system are forwarded into the stub area. The last line in each configuration (area 2 stub) defines the stub area. Router 3 (ABR) automatically advertises 0.0.0.0 (the default route) with a default cost metric of 1 into the stub area. Each router in the stub area must be configured with the area stub command. The routes that appear in the routing table of router 4 are as follows:

Intra-area routes, which are designated with an O
Default route and interarea routes, which are both designated with an IA
Default route is also denoted with an asterisk (O *IA)

Note
The hello packet exchanged between OSPF routers contains a stub area flag that must match with the neighboring routers. The area area-id stub command must be enabled on all routers in the stub so that they all have the stub flag set. They can then become neighbors and exchange routing information.

Content 3.7 Stub, Totally Stubby, and Not-So-Stubby Areas 3.7.5 Configuring Totally Stub Areas The totally stubby area is a Cisco proprietary enhancement that further reduces the number of routes in the routing table. A totally stubby area blocks external type 5 LSAs and summary type 3 and 4 LSAs (interarea routes) from entering the area. By blocking these routes, the totally stubby area recognizes only intra-area routes and the default route of 0.0.0.0. ABRs inject the default summary link 0.0.0.0 into the totally stubby area. Each router picks the closest ABR as a gateway to everything outside the area. Totally stubby areas minimize routing information more than stub areas, and increase stability and scalability of OSPF internetworks. Using totally stubby areas is typically a better solution than using stub areas as long as