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| 6.7 |  | Stub, Totally Stubby, and Not-So-Stubby Areas |
| 6.7.1 |  | Using stub and totally stubby areas |

An OSPF router interface can be configured to either operate as a stub area or a totally stubby area. A stub area does not accept information about routes external to the AS. A totally stubby area does not accept external AS routes and summary routes from other areas internal to the AS.

By configuring an area as stub, the size of the link-state database inside that area can be greatly reduced. As a result, this will reduce the memory requirements of area routers. Remember that stub areas do not accept Type 5, external, LSAs.

Because OSPF routers internal to a stub area will not learn about external networks, routing to the outside world is based on a 0.0.0.0/0 default route. When configuring a stub area, the ABR on the stub automatically propagates a 0.0.0.0/0 default route within the area.

Stub areas are typically created when using a hub-and-spoke topology, with the spokes configured as stub areas. The spokes could be the branch offices. In the case of a hub-and-spoke topology, the branch office may not need to know about every network at the headquarters site. It can instead use a default route to get there.

To further reduce the number of routes in a table, create a totally stubby area, which is a Cisco specific feature. A totally stubby area is a stub area that blocks external Type 5 LSAs and summary, Type 3 and Type 4, LSAs from entering the area. This way, intra-area routes and the default of 0.0.0.0/0 are the only routes known to the stub area. ABRs inject the default summary link 0.0.0.0/0 into the totally stubby area.

Therefore, totally stubby areas further minimize routing information and increase stability and scalability of OSPF internetworks. This is typically a better solution than creating stub areas, unless the target area uses a mix of Cisco and non-Cisco routers. The following sections describe the criteria for determining whether an area should be configured as stub or totally stubby, and the configuration commands necessary to implement these area types.

| Web Links Configuring OSPF http://www.cisco.com/en/US/products/ sw/iosswrel/ps1828/products_configuration_ guide_chapter09186a00800ca56c.htm