Link state routing protocols have greater visibility of the network
and are generally more powerful than distance vector routing protocols.
As an example, OSPF is a more powerful routing protocol than RIP.
Links state routing protocols, to use our analogy once again
have a road map or a map of the entire network.
They can thus make better routing decisions
than distance vector routing protocols which only have a road sign.
So rather than just looking at the road sign that says go left
and your city is a 100 miles away
and then drive a 100 miles to get to your city.
A router running link state routing protocols has a road map
and thus has better visibility of the network or topology.
Link state routing protocols will flood the network
with what are called LSAs or Links State Advertisements
this information is flooded through the entire network
if routers are configured in a single area
or are flooded within the area when special areas are created.
Does information about the entire network
or area is received by all routers in that network or area.
the LSAs are propagated between all routers without being changed
all routers will individually create or populate
what’s called a topological database which is a database
that’s the same on all routers within an area
or once again if running only a single area, all routers within the network.
The topological database contains information about
all routers within that area or network.
All the links to those routers as well as the states of those links
hence the term link state routing protocol is the link up or is the link down?
So router 1 as an example will tell all routers
within the area about the links connected to it and the state of those links.
Are they up or are they down?
the routers run what’s called
the shortest path first algorithm or SPF algorithm
OSPF, as an example, is Open Shortest Path First
or in other words, it’s an open standard that runs the SPF algorithm.
Open Shortest Path First or OSPF, when the routers run the SPF algorithm
they put themselves at the root of an SPF tree
and then calculate or workout
the best route to destination networks from their perspective.
a routers perspective will vary depending on where it is in the network topology
and thus the best route for 1 router
maybe different to the best route for another router.
The router essentially determine the best route for themselves
by running the SPF algorithm against the topological database
and then they put the best routes into their local routing tables.
examples once again of link state routing protocols include OSPF
or Open Shortest Path First and ISIS or Intermediate System-Intermediate System.