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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.
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