Gossip Girl…Routing by RUMOR, Part I

Distance Vector Routing

If you have ever been in any social setting in your life, then you understand the concept of gossip, where one person essentially reveals something about another person (and often something negative).  You may think you outgrew this in kindergarten or high school, but even if you have never participated in it, you have known about, or even been a victim of gossip.  Sadly enough, the truthfulness of the information being shared is often dubious at best.  Usually the fastest way to break the cycle is to ask something like, “may I quote you on that?”  While certainly a life lesson, the concept of gossip also applies to network routing as well.

Distance Vector routing, the first and earliest type of routing protocols, function by gossip, sometimes called routing by rumor.  When a distance vector router receives information from a neighboring device, it simply assumes that the information is accurate, and passes it along to any other devices participating in that process.  Never once does it stop to see if the updates came from a reputable source, or if the information itself is viable in the first place.  This is but the first flaw in distance vector protocols.

A second, but related, issue is the measure by which DV protocols make routing decisions, namely, the defined metric.  As you can guess by the name, these protocols choose routes based on how far away that network is, similar to the way we choose a route of travel (based on mileage, for example).  If one road takes 43 miles to reach a specific city, and another only takes 25 miles, we will usually choose the route with the least mileage.  Routing Information Protocol (versions 1 or 2) use this same basic approach using hop count, or how many Layer 3 networks a route crosses to reach a destination network/subnet.  Simple, right?  Yes indeed, and in reality probably a little bit too simple.  Going back to the example a moment ago, what if the shorter mileage was on surface streets with stop signs, traffic lights, and low speed limits?  The mileage might be shorter, but the travel time would most likely be quite a bit longer.  Simply put, distance vector protocols have no way of knowing or judging the quality of the route, only the distance.  To make this network specific, if one route to a network was using a T1 link (1.544 Mbps) and another was using a 56 Kbps link, RIP would not know any difference, only how many hops away it is.

Distance Vector protocols also require additional mechanisms to prevent routing loops, which we will consider next time.

– Joe

One Response to “Gossip Girl…Routing by RUMOR, Part I”

  1. David Daverso Says:

    Nice anology, per usual.

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