Link state routing protocols,
also called shortest-path-first-protocols,
are a little more advanced than distance-vector routing protocols in the sense
that a link state routing protocol knows about the entire network topology. A
link state protocol is responsible for monitoring the state of the link between
the routers. This link state information is then used to determine the optimal
route to a destination network. Although protocols such as RIP have knowledge
of neighboring routers, link state protocols have knowledge of the entire
network topology and multicast the routing table information to the entire
network.
One of the benefits
of the link state routing protocols is that if a link is down, that information
is stored in the routing table and that pathway will not be used. Because a
distance vector routing protocol does not store link state information, it is
possible that it will not know of a link that is unavailable for some time and
it could still send traffic through that pathway.
In link-state protocols
the routers each create three separate tables. One of these tables keeps track
of directly attached neighbors, one determines the topology of the entire
internetwork, and one is used as the routing table. Link-state routers know
more about the internetwork than any distance-vector routing protocol. OSPF is
an IP routing protocol that is completely link state. Link-state protocols send
updates containing the state of their own links to all other routers on the
network.
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