OSPF Part II: Wouldn’t You Like to be My Neighbor?

In my opinion, one of the most misunderstood words in modern society is the word neighbor, particularly in the United States.  The term is extraordinarily broad, and can refer to a fellow resident of a housing development or apartment, someone in the same city or county, or even country.  When you consider how loosely the word is used today, you can easily get confused when you start talking about neighbor relationships between routers, because the definition is stricter.  Incidentally, when I grew up in the 1970’s in Pennsylvania, the word neighbor meant the people living on either side of you, or across the street.  Since I don’t expect everyone that reads my writing to have that same set of experiences (either in terms of chronology, geography, or culture), let me invoke a more familiar example: the characters of Tim Taylor and Wilson on the television show Home Improvement.  Wilson and Tim lived right next door to one another, and shared both a fence and property line, meaning that their yards were literally connected.  In just about every episode, these two individuals carried on conversations, usualy with Tim asking Wilson for insight and advice, and usually heeding what was said.

Let me point out how their relationship worked as neighbors.  First, they were connected by a common property line, so there was nothing between them other than the fence, which allowed them to interact easily.  Second, they had an actual relationship, which involved a level of trust; in other words, they were not strangers.  Finally, they carried on conversations, in essence, exchanging information, which resulted in at least one of them changing something they had been thinking or doing previously.  Now let’s apply that to how OSPF defines neighbor relationships.  Neighbors have to be directly connected, without another network separating them; a link of some kind connects them (just like the property line).  Second, these relationships are not casual, the OSPF neighbor routers have a very formal relationship and know and trust one another.  Finally, they exchange information, specifically, Link State Advertisements, the pieces of data that allow for the Link State Database to be duplicated on the other device.  This is much more structured than RIP which just accepted whatever it hear as in fact being true and accurate.  In fact, OSPF routers track the state and/or availability of the neighbor device, sending messages at regular intervals called Hello Messages (more on that later), which transmit data as well as act as a keepalive mechanism.

Next time we will step through how OSPF routers build these relationships.

– Joe

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