33 RIP C ONFIGURATIONRIP Overview Routing information protocol (RIP) is a simple interior gateway protocol (IGP)suitable for small-sized networks.Basic Concepts RIPRIP is a distance-vector (D-V) algorithm-based protocol. It exchanges routinginformation via UDP packets.RIP uses hop count (also called routing cost) to measure the distance to adestination address. In RIP, the hop count from a router to its directly connectednetwork is 0, and that to a network which can be reached through another routeris 1, and so on. To restrict the time to converge, RIP prescribes that the cost is aninteger ranging from 0 and 15. The hop count equal to or exceeding 16 is definedas infinite; that is, the destination network or host is unreachable.To improve performance and avoid routing loop, RIP supports split horizon.Besides, RIP can import routes from other routing protocols.RIP routing databaseEach router running RIP manages a routing database, which contains routingentries to all the reachable destinations in the internetwork. Each routing entrycontains the following information:■ Destination address: IP address of a host or network.■ Next hop address: IP address of an interface on the adjacent router that IPpackets should pass through to reach the destination.■ Interface: Interface on this router, through which IP packets should beforwarded to reach the destination.■ Cost: Cost for the router to reach the destination.■ Routing time: Time elapsed after the routing entry is updated last time. Thistime is reset to 0 whenever the routing entry is updated.RIP timersAs defined in RFC 1058, RIP is controlled by three timers: Period update, Timeout,and Garbage-collection.■ Period update timer: This timer is used to periodically trigger routinginformation update so that the router to send all RIP routes to all theneighbors.■ Timeout timer: If a RIP route is not updated (that is, the switch does not receiveany routing update packet from the neighbor) within the timeout time of thistimer, the route is considered unreachable.
