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OSPF Overview Configuring OSPF
page 1-8 OmniSwitch 6800/6850/9000 Advanced Routing Configuration Guide December 2007
OSPF Areas
OSPF allows collections of contiguous networks and hosts to be grouped together as an area. Each area
runs a separate copy of the basic link-state routing algorithm (usually called SPF). This means that each
area has its own topological database, as explained in the previous section.
OSPF Intra-Area and Inter-Area Routing
An area’s topology is visible only to the members of the area. Conversely, routers internal to a given area
know nothing of the detailed topology external to the area. This isolation of knowledge enables the proto-
col to reduce routing traffic by concentrating on small areas of an AS, as compared to treating the entire
AS as a single link-state domain.
Areas cause routers to maintain a separate topological database for each area to which they are connected.
(Routers connected to multiple areas are called area border routers). Two routers belonging to the same
area have identical area topological databases.
Different areas communicate with each other through a backbone. The backbone consists of routers with
contacts between multiple areas. A backbone must be contiguous (i.e., it must be linked to all areas).
The backbone is responsible for distributing routing information between areas. The backbone itself has all
of the properties of an area. The topology of the backbone is invisible to each of the areas, while the back-
bone itself knows nothing of the topology of the areas.
All routers in an area must agree on that area’s parameters. Since a separate copy of the link-state algo-
rithm is run in each area, most configuration parameters are defined on a per-router basis. All routers
belonging to an area must agree on that area’s configuration. Misconfiguration will keep neighbors from
forming adjacencies between themselves, and OSPF will not function.
Inter-Area Routing
Backbone
Intra-Area
Routing
Intra-Area
Routing
Router 1
Router 2
Router 3
Router 4
Link State
Messages
Link State
Messages
Area 1
Area 2