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topology
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... receiving
the updates. In addition, much work has been done to produce a
protocol that responds quickly to topology changes, yet involves
small amounts of routing protocol traffic ...
... database describing the Autonomous System's topology. Each
participating router has an identical database ...
... OSPF allows sets of networks to be grouped together. Such a
grouping is called an area. The topology of an area is hidden
from the rest of the Autonomous System. This information hiding
...
... routing within the area is determined only by the area's own
topology, lending the area protection from bad routing data. An
area is a generalization of an IP ...
... section.
The topology of an area is invisible from the outside of the area.
Conversely, routers internal to a given area know nothing of the
...
... Conversely, routers internal to a given area know nothing of the
detailed topology external to the area. This isolation of knowledge
enables the protocol to effect a marked reduction in routing traffic ...
... 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 backbone ...
... backbone as hub and each of the areas as spokes.
The topology of the backbone dictates the backbone paths used
...
... backbone dictates the backbone paths used
between areas. The topology of the backbone can be enhanced by
adding virtual links ...
...
Routers RT3 and RT4 must also summarize Area 1's topology for
distribution to the backbone. Their backbone ...
... group of edges, all having the same color, and interconnected
by vertices, represents an area. If the topology of the
Autonomous System is intact, the graph will have several regions
...
...
When the AS topology changes, one of the areas may become
partitioned. The graph of the AS will then have multiple
...
... routing information into the area. This additional
information is a distillation of the rest of the Autonomous
System's topology.
This distillation is accomplished as follows: Each area border
router ...
... area border
router is by definition connected to the backbone. Each area
border router summarizes the topology of its attached areas for
transmission on the backbone, and hence to all other area border
routers ...