Configuring Network Area Addresses
OSPF protocol broadcast messages (i.e., Link State Advertisements or LSAs) are
restricted by area to limit their impact on network performance. A large network
should be split up into separate OSPF areas to increase network stability, and to
reduce protocol traffic by summarizing routing information into more compact
messages. Each router in an area shares the same view of the network topolo gy,
including area links, route summaries for directly connected areas, and external
links to other areas.
Command Usage
• Use the Network Area Address Configuration pag e to specif y an Area ID and t he
corresponding network address range. Each area identifie s a logical gro up of
OSPF routers that actively exchange LSAs to ensure that they share an identical
view of the network topology.
• Each area must be connected to a backbone area . This area passes r outing
information between other areas in the autonomous system. The fixed value
0.0.0.0 is used as the Area ID for the backbone. All routers mu st be c onnecte d to
the backbone, either directly, or through a virtual link if a direct physical connection
is not possible.
• An area initially configured via the Network Area Addres s Configu ration pag e
(other than the backbone) is set as a normal area (o r transit are a) by defaul t. A
normal area can send and receive external Link State Adv er tise men ts ( LS As). If
necessary, you can use the Area Configur ation pa ge to conf igure an are a as a
stubby area that cannot send or receive external L SAs, or a not -so-stubby area
(NSSA) that can import external route information into its area (page 20-19).
• An area must be assigned a range of subnetwork addresses. This area an d the
corresponding address range forms a routing interface, and can be configur ed to
aggregate LSAs from all of its subnetwork addresse s and exc hange this
information with other routers in the network (page 20-23).
• If an address rang e i s o ve rlap ped w i th othe r network areas, the router wi ll u se the
network area with the address range that most closely matches the inte rface
address. Also, note that if a more specific address range is removed from a n area,
the interface belonging to that range may still remain act ive if a less specific
address range covering that area has been specified.
Command Attributes
• IP Address – Address of the interfaces to add to the area.
• Netmask – Network mask of the address ra nge to add to th e area.
•
Area ID
– Area to which the specified address or range is assigned. An OSPF area
identifies a group of routers that share common routing inform ation. (The a rea ID
must be in the form of an IP v4 addres s.)
Set the area ID to the same value for all routers on a ne twork se gment us ing the
network mask to add one or more interfaces to an a rea.
Note:
This router supports up to 16 total areas (either normal transit areas, stubs, or
NSSAs).

20-31

Configuring the Open Shortest Path First Protocol

20