Summary

RIP is a distance vector protocol that uses the Bellman-Ford algorithm to compute IP routes dynamically. RIP is suitable to run in small IP networks because of its hop count limit of 15. RIP was designed as a simple IP routing protocol that exchanges a complete routing table at a fixed interval (30 seconds) with other routers running RIP. In larger networks with a large number of IP routes, sending a complete routing table every 30 seconds is not practical. This results in extra work for the sender and receiver, and it consumes unnecessary bandwidth and processing time. Therefore, RIP is used in smaller networks with a hop count of less than 15 and a small number of routes as well.

RIP offers a descent algorithm for loop avoidance by using split horizon and poison reverse. Split horizon takes care of the loops by not advertising any routes back to the interface where it learned the routes. Poison reverse causes routes to be advertised with the infinite RIP metric (16), thus removing RIP routes that might be looped or down.

Because any change in the network takes at least 30 seconds to propagate, the concept of holddown causes the RIP routing table to wait for three times the advertisement interval. This implementation is designed for when a RIP route is not advertised because it might have been down for a little over 30 seconds. The receiving routers should wait for 90 seconds to remove the route from the routing table. If a routes comes back before 90 seconds, it is reinstalled and is advertised throughout the network.

In the early days of IP networking, RIP was the protocol of choice in smaller IP networks. Since then, a lot of new IP protocols have been developed to be more robust and dynamic than RIP; they can scale up to a much larger number of routers than 15. The advent of these new protocols, such as OSPF, IS-IS, and EIGRP, resulted in almost complete phaseout of RIP from larger networks today. These new protocols have improved upon the limitations of RIP in terms of convergence and scalability, and they offer the support for VLSM and discontiguous networks that RIP-1 lacked.

Although RIP-2 improved RIP with new features, such as route tags, queries, subnet masks, next hops, multicasting, and authentication, larger networks still prefer OSPF, IS-IS, and EIGRP as IP routing protocols.

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