Chapter 2 Advanced Routing for FortiOS 5.0 : Routing Information Protocol (RIP) : RIP background and concepts : How RIP works
  
How RIP works
As one of the original modern dynamic routing protocols, RIP is straightforward. Its routing algorithm is not complex, there are some options to allow fine tuning, and it’s relatively simple to configure RIP on FortiGate units.
From RFC 1058:
Distance vector algorithms are based on the exchange of only a small amount of information. Each entity (gateway or host) that participates in the routing protocol is assumed to keep information about all of the destinations within the system. Generally, information about all entities connected to one network is summarized by a single entry, which describes the route to all destinations on that network.
This section includes:
RIP versus static routing
RIP metric — hop count
The Bellman–Ford routing algorithm
Passive versus active RIP interfaces
RIP packet structure