Archive for hop count

Preventing the Epic Fail…Routing by Rumor Part II

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on July 9, 2012 by jjrinehart

Come, on, SERIOUSLY???

In our last discussion we described Distance Vector protocols as simple, and relying on rumor, namely that a router will just take the information it receives as accurate and reliable.  For any of you that have ever believed a rumor, you probably have discovered that it was anything but accurate, or left out key details? If that it not plainly obvious, watch presidential advertising during elections in the US!  Needless to say, these protocols are not the bright bulbs so to speak, and thus need “extra help” to avoid the creation of routing loops, which are the kiss of death in networking terms.

Enter Loop Prevention Mechanisms for DV routing protocols!  These help avoid routing loops, but also introduce a great deal of delay into the convergence process (convergence is the amount of time it takes the routing process to recover from changes).  Here is a list and brief description of these mechanisms:

1. Triggered Updates: DV protocols, such as RIP, send out their entire routing table at regular intervals.  What if a route fails before the next time interval?  This feature is the essential answer, namely that in the event of a change, the advertising router sends out an update immediately even if the time for a full update has not arrived yet.

2. Poison Reverse: No, this is not an execution by a would-be assassin, but a form of route poisoning, in which the route is declared invalid and marked with an infinite metric so it will be unusable.

3. Hold-Down: While this may sound like a wrestling maneuver, it actually refers to the time during which any changes to the route are essentially ignored.  If, for example, a faulty interface keeps going up and down, the hold down process will prevent the routing table from suffering a nervous breakdown.

4. Counting to Infinity: If all else fails, Distance Vector routing protocols have a numerical value that it considers infinite, and beyond which a route is considered unreachable.  In RIP for example, the maximum hop-count is 15, beyond which a route is unusable.  It’s essentially the “last resort” loop prevention mechanism.

All of these mechanisms together keep loops from forming, and while a little on the paranoid side, they keep the network stable.  The tradeoff is a much longer convergence time, which can cause outages and delays in the networks.  At one time this was the only protocol type available, but now newer, more sophisticated ones can be used in place of that.

Next time we will talk about Link State protocols…

– Joe

Gossip Girl…Routing by RUMOR, Part I

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on June 26, 2012 by jjrinehart

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