Note: The algorithm description in this topic is the basic one used by RIP. This is modified in certain ways to address some of the problems that can occur in special circumstances due to how the algorithm works. Topics later in this chapter describe these problems and the special features RIP includes to address them.

Key Concept: Routing information is propagated between routers in RIP using a simple algorithm. On a regular basis, each router sends out RIP messages that specify what networks it can reach, and how many hops to reach them. Other routers directly connected to that one know that they can then reach those networks through that router at a cost of one additional hop. So if router A sends a message saying it can reach network X for a cost of N hops, each other router that connects directly to A can reach network X for a cost of N+1 hops. It will put that information into its routing table, unless it knows of an alternate route through another router that has a lower cost.