5.4 Using Stub and Totally Stubby Areas
5.4.1 What are stub and totally stubby areas
Recall that a stub area is an area that does not accept information about routes external to the autonomous system (that is, the OSPF internetwork), such as routes from non-OSPF sources. A totally stubby area is an area that does not accept external autonomous system (AS) routes and summary routes from other areas internal to the autonomous system. OSPF allows areas to be configured as stub and totally stubby areas, as shown in the main figure.

Their differences are as follows:

  • Configuring a stub area reduces the size of the link-state database inside an area and, as a result, reduces the memory requirements of routers inside that area. External networks (Type-5 LSAs), such as those redistributed from other protocols into OSPF, are not allowed to be flooded into a stub area.
  • Routing from these areas to the outside world is based on a default route (0.0.0.0). A default route means that if a packet is addressed to a network that is not in an internal router's route table, the router automatically forwards the packet to the ABR that sent a 0.0.0.0 LSA. This configuration allows routers within the stub to reduce the size of their routing tables because a single default route replaces the many external routes.
  • A stub area is typically created when you have a hub and spoke topology, 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. It can instead use a default route to get there.
  • To further reduce the number of routes in a table, you can 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 (inter-area routes) from going into the area. This way, intra-area routes and the default of 0.0.0.0 are the only routes known to the stub area. 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 all destinations outside the area.

Totally stubby areas further minimize routing information (as compared to stub areas) 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