OSPF Part I: The Drill Sergeant of Networking!

I have never been in any branch of the military, but I have known a number of brave men and women that have served.  For those who may not know how the military induction process works, the first step is known as Basic Training, or more affectionately, hell on earth!  This phase involves heavy physical activity, intense discipline,and hours of a drill sergeant (see graphic above) barking orders in a fierce, shrill tone that makes fire freeze!  By now you might be wondering what on earth any of this has to do with networking, which is a fair question.  Enter the Open Shortest Path First protocol, abbreviated OSPF.
Everything in OSPF is about rules, and ensures that routing takes places in an even and consistent manner across the entire autonomous system (network under a common administration).  There are rules about design, rules about information exchange, rules about the routing hierarchy, and about everything else you can think of, so to speak.  Needless to say, OSPF is also highly structured, and that is part of the appeal of this best-known link-state routing protocol.
In keeping with the military analogy, one of the first things to understand about OSPF is the “chain of command” or hierarchy.  Just as there are platoons, squads, and units of soldiers in any army, there are specific groups of devices in this protocol, which are called areas.  With a small network, you can get away with a single area and keeps things relatively simple.  In larger networks, multiple areas are a given even if just for scalability (a term meaning the ability to grow in a measured fashion), but there are often additional, design-related, reasons as well.  Areas are identified using decimal numbering, such as 0, 1, 2, and so on, although you can also use dotted decimal numbering such as 10.1.4.13; in my own experience I have only seen the digit form of numbering.
Getting back to the “rules”, an important thing to understand is that all areas must connect to a special area, called the Backbone Area, or Area 0.  In other words, any traffic leaving one area and destined for another must cross Area 0.  If you have just one area in the entire network, then you can number it just about any way you like, but in any multi-area OSPF network you simply must have an Area 0, through which traffic passes.  There is an exception process using a special connection called a Virtual-Link (which creates a direct connection to Area 0 by one that is separated by another area), but that is beyond the scope of the CCNA.
Next time we will look at formal neighbor relationships!
– Joe
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