The collection of all of these link states forms a link-state
database.
A router sends link-state advertisement (LSA) packets to advertise its state periodically (every 30
minutes) and immediately when the router state changes. Information about attached interfaces,
metrics used, and other variables is included in OSPF LSAs. As OSPF routers accumulate linkstate
information, they use the shortest path ?¬?rst (SPF) algorithm to calculate the shortest path to
each node.
A topological (link-state) database is, essentially, an overall picture of networks in relation to
routers. The topological database contains the collection of LSAs received from all routers in the
same area. Because routers within the same area share the same information, they have identical
topological databases.
OSPF can operate within a hierarchy. The largest entity within the hierarchy is the autonomous
system, which is a collection of networks under a common administration that share a common
routing strategy. An autonomous system can be divided into a number of areas, which are groups
of contiguous networks and attached hosts. Figure 4-1 shows an example of an OSPF hierarchy.
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