Types Of Areas - Nokia IPSO 4.0 Reference Manual

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IPSO supports OSPFv2, which supports IPv4 addressing, and OSPFv3, which supports IPv6
addressing.

Types of Areas

Routers using OSPF send packets called Link State Advertisements (LSA) to all routers in an
area. Areas are smaller groups within the AS that you can design to limit the flooding of an LSA
to all routers. LSAs do not leave the area from which they originated, thus increasing efficiency
and saving network bandwidth.
You must specify at least one area in your OSPF network—the backbone area, which has the
responsibility to propagate information between areas. The backbone area has the identifier
0.0.0.0.
You can designate other areas, depending on your network design, of the following types:
Normal Area—Allows all LSAs to pass through. The backbone is always a normal area.
Stub Area—Stub areas do not allow Type 5 LSAs to be propagated into or throughout the
area and instead depends on default routing to external destinations. You can configure an
area as a stub to reduce the number of entries in the routing table (routes external to the
OSPF domain are not added to the routing table).
NSSA (Not So Stubby Area)—Allows the import of external routes in a limited fashion
using Type-7 LSAs. NSSA border routers translate selected Type 7 LSAs into Type 5 LSAs
which can then be flooded to all Type-5 capable areas. Configure an area as an NSSA if you
want to reduce the size of the routing table, but still want to allow routes that are
redistributed to OSPF.
It is generally recommended that you limit OSPF areas to about 50 routers based on the
limitations of OSPF (traffic overhead, table size, convergence, and so on).
All OSPF areas must be connected to the backbone area. If you have an area that is not
connected to the backbone area, you can connect it by configuring a virtual link, enabling the
backbone area to appear contiguous despite the physical reality.
Note
If you need to connect two networks that both already have backbone areas and you do not
want to reconfigure one to something other than 0.0.0.0, you can connect the two backbone
areas using a virtual link.
Each router records information about its interfaces when it initializes and builds an LSA packet.
The LSA contains a list of all recently seen routers and their costs. The LSA is forwarded only
within the area it originated in and is flooded to all other routers in the area. The information is
stored in the link-state database, which is identical on all routers in the AS.
354 Nokia Network Voyager for IPSO 4.0 Reference Guide