Each protocol uses a slightly different algorithm for choosing the best route between two addresses on the network. The algorithm is the “intelligent” part of a dynamic protocol because the algorithm is responsible for deciding which route is best and should be added to the local routing table. RIP and BGP use distance vector algorithms, where OSPF and IS-IS use link-state or a shortest path first algorithm.

Vector algorithms are essentially based on the number of hops between the originator and the destination in a route, possibly weighting hops based on how reliable, fast, and error-free they are.

The link-state algorithm used by OSPF and IS-IS is called the Dijkstra algorithm. Link-state treats each interface as a link, and records information about the state of the interface. The Dijkstra algorithm creates trees to find the shortest paths to the routes it needs based on the total cost of the parts of the routes in the tree.

For more information on the routing algorithm used, see “Distance vector versus link-state protocols”.