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Overview of OSPF
Areas
9-6
OSPF allows parts of a networks to be grouped together into areas.
The topology within an area is hidden from the rest of the
autonomous system. Hiding this information enables a significant
reduction in LSA traffic, and reduces the computations needed to
maintain the LSDB. Routing within the area is determined only by the
topology of the area.
The three types of routers defined by OSPF are as follows:
Internal Router (IR) – An internal router has all of its interfaces
within the same area
Area Border Router (ABR) – An ABR has interfaces in multiple
areas. It is responsible for exchanging summary
advertisements with other ABRs
Autonomous System Border Router (ASBR) – An ASBR acts as a
gateway between OSPF and other routing protocols, or other
autonomous systems
Area 0. Any OSPF network that contains more than one area is
required to have an area configured as area 0, also called the
backbone. All areas in an autonomous system must be connected to
the backbone. When designing networks, you should start with area
0, and then expand into other areas.
The backbone allows summary information to be exchanged
between ABRs. Every ABR hears the area summaries from all other
ABRs. The ABR then forms a picture of the distance to all networks
outside of its area by examining the collected advertisements, and
adding in the backbone distance to each advertising router.
When a VLAN is configured to run OSPF, by default it is automatically
joined to the backbone area (0.0.0.0). If you want to configure the
VLAN to be part of a different OSPF area, use the following
command:
config ospf vlan <name> area <areaid>
If this is the first instance of the OSPF area being used, you must
create the area first using the following command:
create ospf area <areaid>
Stub Areas. OSPF allows certain areas to be configured as stub areas.
A stub area is connected to only one other area. The area that
connects to a stub area can be the backbone area. External route
information is not distributed into stub areas. Stub areas are used to
reduce memory and computation requirements on OSPF routers.
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