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

What a RIP(V1/V2) Off!

The algorithms that Routing Information Protocol is based on date back to 1957, but the protocol itself was defined in RFC 1058 in 1988, with several revisions since that time.  The first version of RIP was classful, meaning that it only recognized addressing according to the Class A, B and C groups defined for the original Internet.  Subnetting was not practical since subnet masks could not be transmitted in routing updates.  If you ever want to see what this behavior is like, issue the no ip classless command on a Cisco router and specify version 1 of RIP (which is the default unless you explicitly enable version 1).  One of the more interesting things about RIP is the maximum hop-count of 15.  When I first started in networking, that seemed silly and arbitrary, but as I gained a greater grasp of binary, it made perfect sense!  You see, RIP uses a 4-Bit metric, and in binary, that maps out as follows, using the Powers of Two we talked about earlier:

8     4     2     1                Powers of Two

1       1     1     1                Binary Digits

Binary 1111 (8+4+2+1) equals a decimal value of 15!  Simply put, RIP just can’t count any higher!  In addition, because RIP is a Distance Vector Routing Protocol, it uses the loop-prevention mechanisms we talked about previously, and sends out its entire routing table every 30 seconds.  Even the newest engineer can see the glaring limitations of RIP, and due to those shortcomings, Cisco gained huge popularity by introducing its proprietary Interior Gateway Routing Protocol (IGRP).  IGRP could measure more than just hop count and thus represented a quantum leap forward in routing technology.  Unfortunately, both these protocols still used broadcasts, and new protocols such as OSPF created far more attractive update mechanisms by utilizing multicast instead.

Classless routing changed the landscape of routing in general, and in 1993 with RFC 1388, RIP was updated to support classless routing, and with the transmission of subnet masks in routing updates, subnetting was now possible.  In addition, updates using multicast became supported, and some other improvements, although the hop-limit remained at 15.  One critical thing to keep in mind is that a feature called automatic summarization is enabled on RIPV2 by default, which summarizes networks to the nearest classful boundary—in effect, making the classless protocol act classful.  Two command to almost always use when configuring RIP are: 1. Version 2 (V1 is enabled by default) and no auto-summary (disables automatic summarization).

RIP is useful in smaller networks, but in reality is as undesirable as a cheerleader being asked to the prom by the president of the chess club (see graphic above).  As we will discuss next time, there are MUCH better choices available now.

– Joe