| 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
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