Original subtitles

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Link state routing protocols have greater visibility of the network

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and are generally more powerful than distance vector routing protocols.

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As an example, OSPF is a more powerful routing protocol than RIP.

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Links state routing protocols, to use our analogy once again

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have a road map or a map of the entire network.

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They can thus make better routing decisions

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than distance vector routing protocols which only have a road sign.

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So rather than just looking at the road sign that says go left

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and your city is a 100 miles away

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and then drive a 100 miles to get to your city.

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A router running link state routing protocols has a road map

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and thus has better visibility of the network or topology.

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Link state routing protocols will flood the network

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with what are called LSAs or Links State Advertisements

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this information is flooded through the entire network

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if routers are configured in a single area

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or are flooded within the area when special areas are created.

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Does information about the entire network

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or area is received by all routers in that network or area.

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the LSAs are propagated between all routers without being changed

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all routers will individually create or populate

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what’s called a topological database which is a database

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that’s the same on all routers within an area

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or once again if running only a single area, all routers within the network.

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The topological database contains information about

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all routers within that area or network.

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All the links to those routers as well as the states of those links

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hence the term link state routing protocol is the link up or is the link down?

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So router 1 as an example will tell all routers

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within the area about the links connected to it and the state of those links.

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Are they up or are they down?

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the routers run what’s called

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the shortest path first algorithm or SPF algorithm

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OSPF, as an example, is Open Shortest Path First

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or in other words, it’s an open standard that runs the SPF algorithm.

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Open Shortest Path First or OSPF, when the routers run the SPF algorithm

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they put themselves at the root of an SPF tree

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and then calculate or workout

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the best route to destination networks from their perspective.

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a routers perspective will vary depending on where it is in the network topology

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and thus the best route for 1 router

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maybe different to the best route for another router.

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The router essentially determine the best route for themselves

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by running the SPF algorithm against the topological database

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and then they put the best routes into their local routing tables.

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examples once again of link state routing protocols include OSPF

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or Open Shortest Path First and ISIS or Intermediate System-Intermediate System.