Preventing the Epic Fail…Routing by Rumor Part II

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

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