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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

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RIP General Operation, Messaging and Timers
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4
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RIP Special Features For Resolving RIP Algorithm Problems
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RIP Protocol Limitations and Problems
(Page 4 of 4)

Problems With RIP's Metric

In addition to these concerns with the algorithm itself, RIP is also often criticized because of its choice of metric. There are two related issues here:

  • Hop Count As A Distance Metric: Simply put, hop count is a poor metric of the cost of sending a datagram between two networks. I believe the use of hop count as the metric in RIP is partially due to the desire for simplicity (it's easy to make the protocol work when hop count is all the routers need to consider) and partially an artifact of RIP being a 20-plus-year-old protocol. Decades ago computers were slow, so each time a datagram passed through a router there was probably a significant delay. Hop count was not a perfect metric even then, but I think it had more correspondence with how long it took to move a datagram across an internetwork than it does today.

    Modern routers are lightning fast, making hop count a flawed way of measuring network distance. The number of hops taken often has no correlation with the actual amount of time it takes to move data across a route. To take an extreme case, consider two networks that are connected by a direct dial-up telephone networking link using 56K modems, and also connected by a sequence of three routers using high-speed DS-3 lines. RIP would consider the 56K link a better route because it has fewer hops, even though clearly it is much slower.


  • Lack Of Support For Dynamic (Real-Time) Metrics: Even if RIP were to use a more meaningful metric than hop count, the algorithm requires that the metric be fixed for each link. There is no way to have RIP calculate the best route based on real-time data about various links the way protocols like OSPF do.

These problems are built into RIP and cannot be resolved easily. Interestingly, some RIP implementations apparently do let administrators “fudge” certain routes to compensate for the limitations of the hop count metric. For example, the routers on either end of the 56K link mentioned above could be configured so they considered the 56K link to have a hop count of 10 instead of 1. This would cause any routes using the link to be more “expensive” than the DS-3 path. This is clever, but hardly an elegant or general solution.

Note: In addition to the rather long list of problems above, there were also some specific issues with the first version of RIP. Some of the more important of these include lack of support for CIDR, lack of authentication, and the performance reduction caused by the use of broadcasts for messaging. These were mostly addressed through extensions in RIP-2.



Previous Topic/Section
RIP General Operation, Messaging and Timers
Previous Page
Pages in Current Topic/Section
123
4
Next Page
RIP Special Features For Resolving RIP Algorithm Problems
Next Topic/Section

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