Stub Areas

An area is referred to as a stub area when there is commonly a single exit point from that area, or if external routing to outside of the area does not have to take an optimal path. A stub is just what it sounds like: a dead end within the network. Packets can enter and leave only through the ABR. Why would you ever need such an area? The reason is network size. By building stub areas, you can reduce the overall size of the routing tables within the routers that are inside an OSPF stub area. Stub areas have the following functional and design characteristics:

• External networks, such as those redistributed from other protocols into OSPF, are not allowed to be flooded into a stub area. Specifically, the ABR stops LSA Types 4 and 5. Therefore, no router inside a stub area has any external routes.
• Configuring a stub area reduces the link-state database size inside an area and reduces the memory requirements of routers inside that area.
• Routing from these areas to the outside world is based on a default route. Stub areas do contain inter-area and intra-area routes because the ABR injects a default route (0.0.0.0) into the stub area.
• Stub areas typically have one ABR; this is the best design. However, you can have additional ABRs, but this might cause suboptimal routing.

All OSPF routers inside a stub area must be configured as stub routers because whenever an area is configured as stub, all interfaces that belong to that area start exchanging OSPF Hello packets with a flag that indicates that the interface is part of an OSPF stub area. This is actually just a bit in the Hello packet (E bit) that gets set to 0. All routers that have a common area must to agree on that flag. For example, all the routers in a stub area must be configured to recognize that the area is a stub. If the routers don't agree, they cannot become neighbors and routing does not take effect.

Stub areas have certain restrictions applied to their operation. This is because they have been designed not to carry external routes, and any of the situations in the following list can cause external links to be injected into the stub area. These restrictions are as follows:

• Stub areas cannot be used as a transit area for virtual links.
• An ASBR cannot be internal to a stub area.
• OSPF allows certain areas to be configured as stub areas, but the backbone area cannot be a stub area.
• LSA Types 4 and 5 are not allowed in a stub area.

TIP An extension to a stub area is called a totally stubby area. Cisco Systems indicates this type of stub area by adding a no-summary keyword to the stub area configuration within the router. A totally stubby area is one that blocks external routes and summary routes (interarea routes) from going into the area. This way, only intra-area routes exist, and a default route of 0.0.0.0 is injected into the area.