This document describes Open Shortest Path First (OSPF) areas and
There are no specific requirements for this document.
This document is not restricted to specific software and hardware
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An OSPF network can be divided into sub-domains called areas. An area
is a logical collection of OSPF networks, routers, and links that have the same
area identification.. A router within an area must maintain a topological
database for the area to which it belongs. The router doesn't have detailed
information about network topology outside of its area, thereby reducing the
size of its database.
Areas limit the scope of route information distribution. It is not
possible to do route update filtering within an area. The link-state database
(LSDB) of routers within the same area must be synchronized and be exactly the
same; however, route summarization and filtering is possible between different
areas. The main benefit of creating areas is a reduction in the number of
routes to propagate—by the filtering and the summarization of routes.
Each OSPF network that is divided into different areas must follow
A backbone area—which combines a set of independent areas into a
single domain—must exist.
Each non-backbone area must be directly connected to the backbone
area (though this connection might be a simple logical connection through a
virtual link, ).
The backbone area must not be partitioned—divided into smaller
pieces—under any failure conditions, such as link or router down events.
Caution: Some of the routers in your network can have partial routing
information, which negatively compromises your network, if you do not follow
Areas are identified by an area ID. Cisco
IOS® software supports area IDs expressed in IP
address format or decimal format, for example, area 0.0.0.0 is equal to area 0.
If there are multiple areas in your network, you need to name the backbone area
"area 0". Since this backbone connects the areas in your network, it must be a
contiguous area. If the backbone is partitioned, parts of the autonomous system
will be unreachable, and you'll need to configure virtual links to repair the
A router with interfaces in two (or more) different areas is an area
border router. An area border router is in the OSPF boundary between two areas.
Both sides of any link always belong to the same OSPF area.
An autonomous system boundary router (ASBR) advertises external
destinations throughout the OSPF autonomous system. External routes are the
routes redistributed into OSPF from any other protocol. In many cases, external
link states make up a large percentage of the link states in the databases of
every router. A stub area is an area in which you don't allow advertisements of
external routes, thus reducing the size of the database even more. Instead, a
default summary route (0.0.0.0) is inserted into the stub area in order to
reach these external routes. If you have no external routes in your network,
then you have no need to define stub areas.
Stub areas are shielded from external routes but receive information
about networks that belong to other areas of the same OSPF domain. You can
define totally stubby areas. Routers in totally stubby areas keep their
LSDB-only information about routing within their area, plus the default
Not-so-stubby areas (NSSAs) are an extension of OSPF stub areas. Like
stub areas, they prevent the flooding of AS-external link-state advertisements
(LSAs) into NSSAs, relying instead on default routing to external destinations.
As a result, NSSAs (like stub areas) must be placed at the edge of an OSPF
routing domain. NSSAs are more flexible than stub areas in that an NSSA can
import external routes into the OSPF routing domain, thereby providing transit
service to small routing domains that are not part of the OSPF routing
Refer to the OSPF
Database Explanation Guide to learn more about the OSPF database.
Use the area xx stub command in every router
in the area to define an area as a stub area. In the topology shown below,
routers in area 7 do not need to know about all the external destinations. The
routers in area 7 must send a packet to the ABR to reach the ASBR, no matter
what the external destination. Area 7 can be defined as a stub area. To define
area 7 as a stub area, configure the area 7 stub
command in all routers in that area.
Use the OSPF router configuration area
xx stub no-summary command to define a
totally stub area. In the previous network diagram, routers in area 7 do not
need to know about all external destinations or any summary LSA from the
backbone for other areas. The routers in area 7 must send packets to the ABR to
reach any destination outside the area 7. Area 7 can be defined as a totally
stub area. To define area 7 as a totally stub area, configure the
area 7 stub no summary command on the ABR.
Use the OSPF router configuration area
xx nssa command to define an NSSA. For
example, in the topology below, we configured Area 2 as an NSSA by entering the
area 2 nssa command on all routers in the area. This
protects Area 2's internal routers from all the AS-external LSAs imported by
the OSPF ASBR, but still allows for the attachment of the non-OSPF router.
External routing information is imported into an NSSA in Type-7 LSAs. Type-7
LSAs are similar to Type-5 AS-external LSAs, except that they can only be
flooded into the NSSA. In order to further propagate the NSSA external
information, the Type-7 LSA must be translated to a Type-5 AS-external-LSA by
the NSSA ABR. NSSA is supported in Cisco IOS 11.2 and
Use the OSPF router configuration command area xx nssa
no-summary command to define an NSSA totally stub area. In the
previous network diagram, we configured area 2 as NSSA totally stub by entering
the area 2 nssa no-summary command on the NSSA ABR.
This keeps any Type 5 AS-external or Type 3 summary routes from leaking in area
This table shows the differences between the types of areas defined in
No Type 5 AS-external LSA allowed
No Type 3, 4 or 5 LSAs allowed except the default summary
No Type 5 AS-external LSAs allowed, but Type 7 LSAs that
convert to Type 5 at the NSSA ABR can traverse
NSSA Totally Stub
No Type 3, 4 or 5 LSAs except the default summary route, but
Type 7 LSAs that convert to Type 5 at the NSSA ABR are allowed
Refer to the Types of OSPF Areas
section of How Does OSPF Generate
Default Routes? to learn more about different types of areas.
All areas in an OSPF autonomous system must be physically connected to
the backbone area (area 0). In some cases where this physical connection is not
possible, you can use a virtual link to connect to the backbone through a
non-backbone area. As mentioned above, you can also use virtual links to
connect two parts of a partitioned backbone through a non-backbone area. The
area through which you configure the virtual link, known as a transit area,
must have full routing information. The transit area cannot be a stub
Use the area area-id virtual-link
command to configure a virtual
link, where the area-id is the area ID assigned to the
transit area (this can be either a valid IP address or a decimal value), and
where router-id is the router ID associated with the
virtual link neighbor. In this example, the virtual link connects area 7 to the
backbone through area 5:
In this example, the virtual link is created between the routers with
router ID 18.104.22.168 and router ID 22.214.171.124. To create the virtual link, configure
the area 5 virtual-link 126.96.36.199 subcommand on router
188.8.131.52 and the area 5 virtual-link 184.108.40.206
subcommand on router 220.127.116.11. Refer to Configuring OSPF Authentication on a Virtual
Link for more information.