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Table Of Contents  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

Previous Topic/Section
RIP General Operation, Messaging and Timers
Previous Page
Pages in Current Topic/Section
12
3
4
Next Page
RIP Special Features For Resolving RIP Algorithm Problems
Next Topic/Section

RIP Protocol Limitations and Problems
(Page 3 of 4)

“Small Infinity”

The use of a relatively small value of “infinity” limits the slow convergence problem. Even in a situation where we “count to infinity” as we just saw, the total amount of time elapsed is at least manageable; imagine if “infinity” were defined as say, 1000! Unfortunately, the drawback of this is that it limits the size of the internetwork that can be used for RIP.

Many people balk at the limit of a span of 15 routers in RIP, but to be honest I think it is much ado about, well, if not nothing, then at least “nothing much”. The 15 value is not a limit on the total number of routers you can use, just on the number of routers between any two networks. Consider that most internetworks are set up hierarchically; even if you have a rather complex four-level hierarchy you wouldn't be close to the 15-router limit. In fact, you could create a huge autonomous system with thousands of routers without having more than 15 routers between any two devices. So this is only a limitation for the very largest of autonomous systems.

On the other hand, RIP’s need to send out its entire routing table many times each hour makes it a potentially poor choice for a large internetwork regardless of the “infinity=16” issue. In an internetwork with many routers, the amount of traffic RIP generates can become excessive.

Key Concept: One of the most important problems with the operation of RIP is slow convergence, which describes the fact that it can take a long time for information about changes to a network to propagate between routers. One specific instance of this problem is the counting to infinity problem, in which out-of-date information causes many bogus RIP messages to be exchanged between routers about an unreachable network.


To be fair, these problems are mostly general to distance-vector routing algorithms and not RIP in particular. Some of them are corrected through the implementation of specific changes to the algorithm or the rules under which RIP messages are sent, as described in the next topic. According to RFC 2453, there was actually a proposal to increase RIP's “infinity” to a number larger than 16, but this would have caused compatibility problems with older devices (which would view any route with a metric of 16 or higher as unreachable) so it was rejected.


Previous Topic/Section
RIP General Operation, Messaging and Timers
Previous Page
Pages in Current Topic/Section
12
3
4
Next Page
RIP Special Features For Resolving RIP Algorithm Problems
Next Topic/Section

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Version 3.0 - Version Date: September 20, 2005

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