| You can configure an OSPF router interface to either operate in a stub area (does not accept information about routes external to the AS) or as a totally stubby area (does not accept external AS routes and summary routes from other areas internal to the AS).
Both of these area types are shown in the figure.
By configuring an area as stub, you
can greatly reduce the size of the link-state database inside that
area and, as a result, reduce the memory requirements of area
routers. Remember that stub areas do not accept Type 5 (that is,
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 you
configure a stub area, the stub's ABR automatically propagates a
0.0.0.0/0 default route within the area.
Stub areas are typically created when
you have a hub-and-spoke topology, with the spokes (such as branch
offices) configured as stub areas. 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, 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 (that is, 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.
Thus, 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.
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