Chris Bryant, CCIE #12933, tells CCNA candidates why they need to know the OSI model, and it's not just for the tests!
Nothing makes me happier than teaching Cisco technologies, especially to people who want to become CCNAs. Everyone is happy to be there, whether they're there in person or online. Everyone is excited and ready to work hard. They can't wait to get their hands on the racks of Cisco routers and switches.
have available...
I then pull out the OSI model chart. The chins drop. People sigh, or at least wish they hadn't gotten decaf that morning.
Okay, it's not that bad. But it does make things less exciting. I always get the feeling that "Why can't we just get on the routers and switches right away? Why do we have to learn such boring things?"
One reason is that both the Intro and ICND exams require you to know the OSI model inside and out. You have to agree that's a pretty good reason, but students still find the information about the OSI model to be very boring.
I know what you mean because I've been there myself. The first time I heard about the OSI model was in a "Networking Technologies" class taught by Novell. Man, that chart was dry. The OSI model has every known protocol (and, I think, some that aren't known yet) stuffed into it. It looked like a big jigsaw puzzle, but the real problem was that I had no idea what most of it was.
So I did what I was supposed to and tried to remember this huge chart. I did well enough on the test to pass, but I wondered why I had worked so hard. It's not like you talk about the OSI model in a server room or wiring closet.
As a CCNA candidate, you don't have to worry about all the protocols I used to remember, but you do have to know what happens at each layer. So, here's the question:
"Why do I need to know about all the other layers if I work with routers and switches? Routers and switches work at layers 2 and 3, right?"
Yes, both switches and routers work at Layer 2. But to really understand networking, you have to know how the other layers work. Why?
Most network engineers and administrators will spend a lot more time fixing problems than installing new things. That's just how things are. And to be a good troubleshooter, you need to know what's going on at all OSI layers, not just layers 2 and 3.
I've hired a lot of people and done a lot of job interviews, so I can tell you that the most important thing I look for is the ability to solve problems. So I tell CCNA and CCNP candidates to get as much hands-on experience as they can. I know how important theory is, but the only way to learn how to solve problems is to work on the real thing. No simulator programme
will teach you how to figure out what's wrong.
Also, the only way to really get good at troubleshooting is to know what's going on across the whole network, not just with the routers and switches. Layer 1 is always where troubleshooting begins. If you don't find a problem at the Physical layer and your routers and switches are working fine, how are you going to continue troubleshooting if you don't know what the next steps are as the data moves closer to the end user?
So, don't just quickly look over the OSI model and move on to more fun things in your CCNA studies. Mastering the OSI model is worth it not only because it will help you pass your exams, but also because it will help you develop your own troubleshooting methods.
