Routing Information Protocol (RIP) is a distance vector protocol that uses hop count as its primary metric. RIP defines how routers should share information when moving traffic among an interconnected group of local area networks (LANs).
Routing Information Protocol (RIP) was originally designed for Xerox PARC Universal Protocol and was called GWINFO in the Xerox Network Systems (XNS) protocol suite in 1981. RIP, which was defined in RFC 1058 in 1988, is known for being easy to configure and easy to use in small networks.
In the enterprise, Open Shortest Path First (OSPF) routing has largely replaced RIP as the most widely used Interior Gateway Protocol (IGP). RIP has been supplanted mainly due to its simplicity and its inability to scale to very large and complex networks.
How Routing Information Protocol (RIP) works
RIP uses a distance vector algorithm to decide which path to put a packet on to get to its destination. Each RIP router maintains a routing table, which is a list of all the destinations the router knows how to reach. Each router broadcasts its entire routing table to its closest neighbors every 30 seconds (126.96.36.199 for RIPv2). In this context, neighbors are the other routers to which a router is connected directly -- that is, the other routers on the same network segments this router is on. The neighbors, in turn, pass the information on to their nearest neighbors, and so on, until all RIP hosts within the network have the same knowledge of routing paths. This shared knowledge is known as convergence.
If a router receives an update on a route, and the new path is shorter, it will update its table entry with the length and next-hop address of the shorter path. If the new path is longer, it will wait through a "hold-down" period to see if later updates reflect the higher value as well. It will only update the table entry if the new, longer path has been determined to be stable.
If a router crashes or a network connection is severed, the network discovers this because that router stops sending updates to its neighbors, or stops sending and receiving updates along the severed connection. If a given route in the routing table isn't updated across six successive update cycles (that is, for 180 seconds) a RIP router will drop that route and let the rest of the network know about the problem through its own periodic updates.
Features of RPI
RIP uses a modified hop count as a way to determine network distance. Modified reflects the fact that network engineers can assign paths a higher cost. By default, if a router's neighbor owns a destination network and can deliver packets directly to the destination network without using any other routers, that route has one hop. In network management terminology, this is described as a cost of 1.
RIP allows only 15 hops in a path. If a packet can't reach a destination in 15 hops, the destination is considered unreachable. Paths can be assigned a higher cost (as if they involved extra hops) if the enterprise wants to limit or discourage their use. For example, a satellite backup link might be assigned a cost of 10 to force traffic follow other routes when available.
Timers in RPI help regulate performance. They include:
Update timer—Frequency of routing updates. Every 30 seconds IP RIP sends a complete copy of its routing table, subject to split horizon. (IPX RIP does this every 60 seconds.)
Invalid timer—Absence of refreshed content in a routing update. RIP waits 180 seconds to mark a route as invalid and immediately puts it into holddown.
Hold-down timers and triggered updates—Assist with stability of routes in the Cisco environment. Holddowns ensure that regular update messages do not inappropriately cause a routing loop. The router doesn't act on non-superior new information for a certain period of time. RIP's hold-down time is 180 seconds.
Flush timer—RIP waits an additional 240 seconds after holddown before it actually removes the route from the table.
Other stability features to assist with routing loops include poison reverse. A poison reverse is a way in which a gateway node tells its neighbor gateways that one of the gateways is no longer connected. To do this, the notifying gateway sets the number of hops to the unconnected gateway to a number that indicates infinite, which in layman's terms simply means 'You can't get there.' Since RIP allows up to 15 hops to another gateway, setting the hop count to 16 is the equivalent of "infinite."
RIP also stands for: raster image processor