A Primer on the CLI, Why You Should Care, and How to Get the Skills
If you just started your Cisco certification journey, and followed the specifics listed in the first two posts in this series, you’d have made a couple of big choices. You’d have chosen whether to start with CCENT, CCNA, or CCDA. If you chose CCNA, you’d have at least started to think about going for the 1-exam or 2-exam path. And you would also have chosen your primary study tool – be it a particular book, video set, or course to take – and you may have a secondary study resource in mind as well. Today we’ll begin to look at the next big decision for how to go getting your first Cisco cert: the choice of how to build skills using Cisco routers and switches, specifically how to learn how to use the Cisco Command Line Interface (CLI).
As with the other posts in this series, I’m directing my comments mainly at those who are indeed at the point of choosing to pursue Cisco certs, but not sure how to go about it. For those of you that have already passed that point, and are further along the Cisco cert path, I’ll ask that you pepper some comments to add to what I have hear, disagree, or give examples of how it’s worked for you. But today’s post will mostly be background info that’s necessary before getting into the whole discussion of which kind of tool to use to get that CLI experience, which I’ll get to in the next post.
Command Line Interface (CLI) 101
Cisco routers and switches run an operating system called Internetwork Operating System (IOS). However, unlike PC system units, routers and switches have no way to connect a keyboard, display, or mouse. We humans of course need some way to interact with the routers and switches for administrative purposes: to tell (configure) the device as to what to do, to check on the status of its operation, and so on. So, there must be some user interface that connects to the router and switch.
The short version, as shown in this figure, is that you use the keyboard and display on some other computer (PC, Mac, whatever), and then connect to the router or switch. The physical connection comes via a cable that connects some serial port on the computer (DB-9 older style serial connector, or USB) to a physical port on the router/switch called the console port. The computer then runs software called a terminal emulator. When you type in the emulator window, what you type is sent over the console cable to the router/switch. The router/switch interprets what you type as a command that asks the router/switch to either tell you status info, or to change how it is currently operating. The router/switch parses the command, and decides how to react, and sends some text back to the terminal emulator screen.
This use interface, where you and I type commands and the router/switch sends back some response, is called the Cisco CLI.
Why You Need CLI Practice
The Cisco CCNA exams (ICND1, ICND2, and CCNA) all include a couple of types of questions in which the exam interface simulates the CLI of various routers and switches. One type, called a Sim or Simulator question, shows a topology of connected routers and switches. Your job is to figure out why the network isn’t working right, and fix it. To do so, you have to remember commands, choose the right commands to use, interpret the output from those commands, and then remember the correct configuration commands that change what the device is doing, and by changing the configuration, fix the problem.
The exam also includes Simlet questions. These questions also require you to use the CLI, know commands, use them, interpret the output, etc. However, these questions ask you several multi-choice questions about what you see at the CLI.
If you take any of the Cisco CCNA-level exams, and got everything right except the Sim and Simlet questions… you may or may not pass. Seems a bit non-specific, huh? Well, as an aside, Cisco does not publish any detail about how they score exams. Your score report won’t say how many you missed, instead listing a score between 300 and 1000. So, there is no way to know, with surety, how many questions you missed, or which ones. As a result, there is no way to know whether you would pass if you missed all the points on Sim and Simlet questions, but got all else correct.
However… you’re kidding yourself if you think you can pass without doing well on the Sim and Simlets. That’s just my opinion. However, it is a widely held opinion from those who’ve commented about Cisco exams over the years. In fact, some people have commented over the years that if you miss one entire Sim question, you’ll already lose enough points to fail. (Again, we can’t know for sure from where we sit outside Cisco.) Regardless, in my opinion, if you want to increase your chances of passing, you need to master the CLI-related topics.
If you’ve not yet checked out Cisco’s info about the types of questions on the exam, like that Sim and Simlet questions, it’s worth a look early in the study process. After all, you’re prepping for your first Cisco exam. Note that many study resources include some test software, but the specifics of the user interfaces differ compared to the actual exams, so look at this page to get familiar with the test.
The Beginnings of Real Vs. Simulator Vs. Emulator
Assuming you buy in to what I’ve written so far, you need CLI skills. So, how do you get these skills? Well, there are three classes of tools you can use to learn the skills:
- Real Cisco routers and switches
- Simulators
- Emulators
Some people choose to use real gear, some choose simulators, some choose emulators, and some use more than one. Today’s post is already getting a bit long for a blog, so my plan for today is to hit the highlights on real gear and close for the day, and hit the Simulator and Emulator topic next time. But if you anxious in the mean time for more, Ive got a couple of links for you:
My certskill.com web site lists some background info about considering the decision of what to use.
I’ve blogged about comparisons of real vs. simulators vs. emulators in the past, but without the emphasis on the basics.
The whole topic of how to choose and buy real gear is a big topic unto itself. I’ve blogged about it before, but it’s been a while since I’ve done so while focusing on a CCNA audience. I plan to do that soon, probably starting by the end of February or early March. In the mean time, you can look at my two most recent lab series if you want more now:
CCNA/CCNP:
CCNP:
A Brief Primer/Perspective of Real Routers
I’ll close out today’s post with a bit more of a primer on how routers work, with the goal of making it easier to understand what simulators and emulators really do.
A router consists of hardware, including CPU, RAM, permanent memory in flash, and physical interfaces like Fast Ethernet. The router stores its operating system (IOS) in flash, and loads IOS in into RAM when the router is powered on. IOS executes on the router CPU, and does the primary work of forwarding packets in/out of the physical interfaces on the router.
The IOS creates the CLI in that it receives the commands you type, and it sends the output back to you. The router hardware, from one perspective, is there simply to provide a path between the CLI and your terminal emulator.
Finally, IOS comes in several shapes and sizes, but the IOS CLI is the same for all. For example, the actual IOS for a model 1841 differs from the IOS for a model 2610XM. Some IOS’s have more features (and cost more $), and some have fewer features. Those more functional IOS’s support a larger number of commands than the less function IOS’s. But the mechanics of the CLI remains the same across all IOS images.
The good news is that you can buy used routers and switches and use them to learn the CLI. Lots of companies buy their used Cisco gear inventory from companies that are upgrading their routers and switches to newer models. Then these companies resell the used Cisco gear. These used devices typically come with whatever IOS was on the device when it was removed from that company’s network. You can literally buy the router/switch, connect your PC to the console port, turn on the device, and start using the CLI. But there’s lots more to consider beyond that, and that’s why I plan to do a more comprehensive discussion on this topic down the road a tad.
So there are the basics on the background on CLI for CCNA prep, which introduces enough info so we can talk about the tradeoffs between the main choices: emulator, simulator, or real gear.
Related Posts:Getting Started Part 2: Primary CCNA Study Source
So You Want to Get STarted in Cisco Certs – Now What?
What to Consider After PAssing CCNA
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