Original subtitles

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There are different types of paradigms or algorithms used by routing protocols.

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The first type is distance vector also known as routing by rumor.

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Distance vector routing protocols will determine

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the direction and the distance to a destination.

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The direction is also known as a vector

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and the distance such as hop count is determined to any destination in the network.

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RIP as an example, uses hop count to determine the cost or distance of a remote network.

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The advantage of distance vector routing protocols

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is that they are very simple to configure.

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The major disadvantage of distance vector routing protocols

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is that they have limited visibility

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they only know what neighbors tell them

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and that’s where the concept or analogy of routing by rumor comes from.

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It’s possible that routers will learn incorrect information

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or make bad choices based on the information that they received from their neighbors.

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Distance vector routing protocols

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used the Bellman–Ford algorithm to calculate paths.

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This requires that a router inform its neighbors of topology changes

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periodically and in some cases when changes are detected in the network.

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RIP as an example, will advertise its entire routing table every 30 seconds

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and will send triggered updates when there’s a change in the topology.

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Distance vector means that routers advertise routes

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as a vector of the distance and direction.

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Direction is represented by the next hop address and local exit interface and distance

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uses a metric such as hop count in RIP to determine the cost of a route.

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It’s important to understand that distance vector routing protocols

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do not have knowledge of the entire path to a destination

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hence once again the term routing by rumors

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used because the routers are relying on the information that they've received

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from other routers and they themselves cannot determine

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if the information is actually valid or true.

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This can introduce instability and routing loops

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and thus multiple features have been introduced to try and combat loops.

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As an example, mechanisms or features

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used to stop routing loops include split horizon

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poisoned reverse and trigger updates.

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As an analogy distance vector routing protocols have information available to them

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in a similar fashion to the information shown on a road sign.

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If you’re on a road going to a destination

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and you don’t have a map you must rely on the road signs

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to tell you the direction of the destination

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and the distance to get to it, in the same way in distance vector routing protocols

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a router does not know the entire path to every network segment.

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The router only knows the direction or vector in which to send the packet.

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The distance vector routing approach

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to determine the direction called a vector

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and distance such as hop count to any destination network.

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So to summarize, distance vector means

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that a destination such as 10.1.1.0 is a distance of 5 hops away

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in the direction of the next hop router router 2

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that statement sums up essentially

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how distance vector routing protocol operate.

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The destination 10.1.1.0 is a distance of 5 hops away in the direction or router 2

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this is similar to our analogy of using a road sign to get to a destination.

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Links state routing protocols on the other hand

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have a complete map of the area

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Every router built its own internal map of the entire network topology

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in its link state also called topology database

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each router then runs its own shortest path first algorithm

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to calculate the shortest path to all known destinations.

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An example, of a links state routing protocol is OSPF

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which uses cost as its routing metric which is based on bandwidth.

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Link state routing protocols cannot be fooled as easily

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into making bad routing decisions

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because they have a full view of the network topology.

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They can make better choices or more informed decisions

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because they have a full view of the path

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to get to a destination network.

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A road sign use by distance vector routing protocols

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we just say go left and the network is 5 hops away.

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However, a link state routing protocol has a road map

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which allows the router to see the path to the destination

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and therefore the router can make a better choice

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and how to get to that destination network.

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The name link state means that each router

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originates information about itself

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it’s directly connected interfaces or links and the state of those links

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in other words other links up or other links down.

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This information is then passed between all routers

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and each router makes its own copy of that information

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but doesn't change the information

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That means that all routers end up sharing

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the same information within a network or area

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and that’s what called a topological database.

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In OSPF as an example, you could use a command such as display ospf lsdb

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to see the link state database that’s identical on all routers in the area.

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So each router has identical information about the network

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and each router can make independent decisions

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on how to calculate its best path.

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link state routing protocols use an algorithm

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called SPF or Shortest Path First which was develop by EW Dijkstra

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and is a very powerful algorithm use by protocols

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such as OSPF and ISIS, once again links state routing protocols

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have a better visibility of the network than distance vector routing protocols do.

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So in summary, each router has a copy of the topological database

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which is all routes in the entire network or area

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and the routers use the Shortest Path algorithm or SPF algorithm

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to determine the best part to every destination.

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The routers have better visibility than distance vector routing protocols.

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however, the disadvantage of link state routing protocols

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is that they are more difficult to configure

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they require a hierarchical network topology

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where you would have a backbone area, area 0 and multiple other areas.

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Link state routing protocols also require more memory

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because they maintain multiple tables such as a neighbor table

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a link state database table and routing table

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the SPF or Shortest Past First algorithm can also be process intensive

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and thus link state routing protocols require more powerful CPUs

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and more memory in routers than distance vector routing protocols do.