Long Distance Relationships?

The Concept of DISTANCE

As a sign of the times, may relationships today begin on the Internet, not so much in old-style chat rooms (though I am sure that may still happen), but through online dating and such; this is how my wife and I met in 2005.  In our case, we lived less than 30 miles from one another, but I have heard stories of relationships that span a continent or even the globe!  The good news is that the ability to communicate across the planet can make the world seem much smaller, but that excessive mileage can seem discouraging when far from a loved one.  In this example, distance can create very real barriers that can impact our lives.

In networking, this idea of distance plays out in other ways, first in determining metrics for routing protocols.  In programming, you typically create some sort of value hierarchy to determine the best possible choice, usually using a specific algorithm.  Dynamic routing protocols use a number of these methods and base the choice of the best possible route on the results of the processes used by those algorithms.  The problem is that one protocol may use one set of values, and another a completely different set, with no way to arbitrate between them when multiple protocols are involved.  In multiprotocol environments (more than one set of routing sources involved), there needs to be a way of ranking the best sources to prevent chaos and confusion.  Enter the concept of administrative distance.

In Cisco terms, administrative distance refers to the level of reliability (or believability) of a particular routing source.  Naturally, some are better than others, so a value between 0 and 255 is assigned to a protocol/source, with the lower the number the better the source.  Here is a breakdown:

List of Default Distances

As you can see, the farther up the numbers go, the less desirable the routing source is (iBGP is the highest at 200).  This set of values is local to the device and can be changed with the distance command under the target protocol.  In the CCNA labs I created, I used the distance command to create a failover process (check them out).  More to come…

– Joe

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