|  | 11.5 | Routing Protocols
| 11.5.2 | Definition of routing protocol

Routers use routing protocols to exchange routing tables and to share routing information. Within a network, the most common protocol used to transfer routing information between routers, located on the same network, is Routing Information Protocol (RIP). This Interior Gateway Protocol (IGP) calculates distances to a destination host in terms of how many hops (i.e. how many routers) a packet must pass through. RIP enables routers to update their routing tables at programmable intervals, usually every 30 seconds. One disadvantage of routers that use RIP is that they are constantly connecting to neighboring routers to update their routing tables, thus creating large amounts of network traffic.

RIP allows routers to determine which path to use to send data. It does so by using a concept known as distance-vector. Whenever data goes through a router, and thus, through a new network number, this is considered to be equal to one hop. A path which has a hop count of four indicates that data traveling along that path would have to pass through four routers before reaching the final destination on the network. If there are multiple paths to a destination, the path with the least number of hops would be the path chosen by the router.

Because hop count is the only routing metric used by RIP, it doesn’t necessarily select the fastest path to a destination. A metric is a measurement for making decisions. You will soon learn that other routing protocols use many other metrics besides hop count to find the best path for data to travel. Nevertheless, RIP remains very popular, and is still widely implemented. This may be due primarily to the fact that it was one of the earliest routing protocols to be developed.
One other problem posed by the use of RIP is that sometimes a destination may be located too far away to be reachable. When using RIP, the maximum number of hops that data can be forwarded through is fifteen. The destination network is considered unreachable if it is more than fifteen router hops away.