As a link-state protocol, OSPF operates
differently than the distance-vector routing protocols. Link-state
routers identify and communicate with their neighbors so that they
can gather firsthand information from other routers in the network.
The OSPF terminology is depicted in Figure ;
a brief description of each term is given.
The information gathered from OSPF
neighbors is not a complete routing table. Instead, OSPF routers
tell each other about the status of their connections, or
"links,"
to the internetwork. In other words, OSPF routers advertise their
link states.
The routers process this information and build a link-state database
,
which is essentially a picture of who is connected to what. All
routers in a given area
should have identical link-state databases. Independently, each
router then runs the Shortest Path First (SPF) algorithm, also known
as the Dijkstra algorithm, on the link-state database to determine
the best routes to a destination. The SPF algorithm adds up the cost
(which is a value usually based on bandwidth)
of each link between the router and the destination. The router then
chooses the lowest-cost path to add to its routing table, also known
as a forwarding database.
OSPF routers keep track of their neighbors in their adjacencies
database.
To simplify the exchange of routing information among several
neighbors on the same network, OSPF routers may elect a Designated
Router (DR) and a Backup Designated Router (BDR)
to serve as focal points for routing updates.
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