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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 topology,
including area links, route summaries for directly connected areas, and external
links to other areas.
Command Usage
• Use the Network Area Address Configuration page to specify an Area ID and the
corresponding network address range. Each area identifies a logical group 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 routing
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 must be connected 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 Address Configuration page
(other than the backbone) is set as a normal area (or transit area) by default. A
normal area can send and receive external Link State Advertisements (LSAs). If
necessary, you can use the Area Configuration page to configure an area as a
stubby area that cannot send or receive external LSAs, 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 and the
corresponding address range forms a routing interface, and can be configured to
aggregate LSAs from all of its subnetwork addresses and exchange this
information with other routers in the network (page 20-23).
• If an address range is overlapped with other network areas, the router will use the
network area with the address range that most closely matches the interface
address. Also, note that if a more specific address range is removed from an area,
the interface belonging to that range may still remain active 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 range to add to the 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 information. (The area ID
must be in the form of an IPv4 address.)
Set the area ID to the same value for all routers on a network segment using the
network mask to add one or more interfaces to an area.
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
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