CISCO IOS in a Nutshell

Chapter 9. Interior Routing Protocols

RIP

The Routing Information Protocol (RIP) is the oldest routing protocol that is still widely used. It has a large support base and a simple configuration. However, it also has a major drawback: poor route determination. RIP is a distance-vector protocol that looks only at the number of route hops (i.e., the number of routers crossed in traveling from one network to another) in computing the best route. For example, let’s say that there are two routes to a destination. The first route crosses two separate 56K links, for a metric of 2; the second route crosses three T1 links, for a metric of 3. RIP always selects the first (two hops) route, even though it is obvious that the second route is better under almost all circumstances. It’s possible to use offset lists to force RIP to choose the better route, but that’s merely adapting to the problem, not fixing it.

Another problem with RIP is that it can’t scale to large networks. There are two scaling issues. First, routers using RIP periodically broadcast the entire routing table to the network. These broadcasts can eat precious bandwidth on lines that often can’t afford it. Second, RIP considers any route past 15 hops unreachable. In addition, RIP does not support Variable-Length Subnet Masks (VLSM), an important technique for conserving IP-address space. Nor does it have any mechanism for authenticating other routers; it isn’t difficult to trick RIP into believing bogus routing information. ...