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DES-3326S Layer 3 Fast Ethernet Switch User’s Guide
Switch Management and Operating Concepts 127
“state” is a description of that interface and its relationship to
neighboring routers. The state contains information such as
the IP address, subnet mask, type of network the interface is
attached to, other routers attached to the network, etc. The
collection of link-states are then collected in a link-state
database that is maintained by routers running OSPF.
OSPF specifies how routers will communicate to maintain their
link-state database and defines several concepts about the
topology of networks that use OSPF.
To limit the extent of link-state update traffic between routers,
OSPF defines the concept of Area. All routers within an area
share the exact same link-state database, and a change to this
database on one router triggers an update to the link-state
database of all other routers in that area. Routers that have
interfaces connected to more than one area are called Border
Routers and take the responsibility of distributing routing
information between areas.
One area is defined as Area 0 or the Backbone. This area is
central to the rest of the network in that all other areas have a
connection (through a router) to the backbone. Only routers
have connections to the backbone and OSPF is structured such
that routing information changes in other areas will be
introduced into the backbone, and then propagated to the rest
of the network.
When constructing a network to use OSPF, it is generally
advisable to begin with the backbone (area 0) and work
outward.
The Link-State Algorithm
An OSPF router uses a link-state algorithm to build a shortest
path tree to all destinations known to the router. The following
is a simplified description of the algorithm’s steps: