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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  TCP/IP Routing Protocols (Gateway Protocols)                 9  TCP/IP Interior Routing Protocols (RIP, OSPF, GGP, HELLO, IGRP, EIGRP)                      9  TCP/IP Routing Information Protocol (RIP, RIP-2 and RIPng)                           9  RIP Fundamentals and General Operation

RIP Protocol Limitations and Problems 1 2 3 4 RIP Version-Specific Message Formats and Features

The simplicity of the Routing Information Protocol is its most attractive quality, but also leads to certain problems with how it operates. Most of these limitations are related to the basic algorithm used for determining routes, and the method of message passing used to implement the algorithm. In order for RIP to be a useful protocol, it was necessary that some of these issues be addressed, in the form of changes to the basic RIP algorithm and operational scheme we explored earlier in this section.

The solution to problems that arise due to RIP being too simple is to add complexity, in the form of features that add more intelligence to the way that RIP operates. Let’s take a look at four of these: split horizon, split horizon with poisoned reverse, triggered updates and hold-down.

The “counting to infinity” problem is one of the most serious issues with the basic RIP algorithm. In the example in the previous topic, the cause of the problem is immediately obvious: after Network 1 fails and Router A notices it go down, Router B “tricks” Router A into thinking it has an alternate path to Network 1 by sending Router A a route advertisement to N1.

If you think about it, it doesn't really make sense—under any circumstances—to have Router B send an advertisement to Router A about a network that Router B can only access through Router A in the first place. In the case where the route fails it causes this problem, which is obviously a good reason not do it. But even when the route is operational, what is the point of Router B telling Router A about it? Router A already has a shorter connection to the network and will therefore never send traffic intended for Network 1 to Router B anyway.

Clearly, the best solution is simply to have Router B not include any mention of the route to Network 1 in any RIP Response messages it sends to Router A. We can generalize this by adding a new rule to RIP operation: when a router sends out an RIP Response on any of the networks to which it is connected, it omits any route information that was originally learned from that network. This feature is called split horizon, since the router effectively splits its view of the internetwork, sending different information on certain links than on others.

With this new rule, let's consider the behavior of Router B. It has an interface on Network 2, which it shares with Router A. It will therefore not include any information on routes it originally obtained from Router A when sending on N2. This will prevent the “counting to infinity” loop we saw in the previous topic. Similarly, since Router D is on Network 3, Router B will not send any information about routes it got from Router D when sending on Network 3.

Note, however, that split horizon may not always solve the “counting to infinity” problem, especially in the case where multiple routers are connected indirectly. The classic example would be three routers configured in a triangle. In this situation, problems may still result due to data that is propagated in two directions between any two routers. In this case, the “hold down” feature may be of assistance (see below).

RIP Protocol Limitations and Problems 1 2 3 4 RIP Version-Specific Message Formats and Features