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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. Figure
shows how
the routers process this information and build link-state databases.
Figure , is
essentially a picture of who is connected to what. All routers in a
given area, as seen in Figure
, 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. This is shown in Figure
. The
router then chooses the lowest cost path to add to its routing table,
also known as a forwarding database
.
Figure
shows how
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) as shown
in Figure ,
to serve as focal points for routing updates.
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