Cisco cert lab prep: Sim vs Em vs Real

Alas, there are so few mind readers in the world these days... ;-) So, let me add some context and some why's/wherefore's to this series on build a CCNA and CCNP lab using real gear. (Way back last year, when I did the original series on CCNA/CCNP prep labs, I took the time to give this same context, but it bears some repeating.)

First, the broad view: you can use Simulators, and/or Emulators, and/or real gear, in order to build hands-on skills for the exams. If you buy a Sim, the Sim should have documentation, so I don't bother blogging about it. If you get the one Emulator (Dynamips/GNS3 family), there's already tons of easily-accessible information online, so I haven't spent time on it here in the blog. For buying real used gear, there seems to be not as much online info and recommendations, so that's where I spent some time on this topic last year, and will again this year. But, it's worth a day to ramble about the options...

Simulators, by my definition at least, are software products that outwardly act like real gear, but inwardly can do whatever they want. Being software products, the features beyond that base description vary wildly. For example, I have seen some products over the years that follow a pre-defined set of steps, and the user cannot deviate from those steps. If you follow the steps, the output looks like real gear. Others allow greater freedom, so that you can type any (supported) command on the simulated devices, with behavior that mimics the real gear. Some Simulators support the creation of your own topology of different models of routers, switches, and maybe other devices as well, while others use a static set of pre-defined topologies. (I'm sure there are other comparison points; I'm just listing a few as examples.)

Emulators, defined at least in the context of the public Cisco space, means software that both outwardly and inwardly acts like the networking devices. Specifically, that means freeware Dynamips/Dynagen ( and it's graphical front-end GNS3 ( (If you start at the Dynagen web site, you'll see several good links to learn more.) Short version - get the free code, and then it runs multiple instances of router IOS on your PC. You can connect the instances to each other via LAN or serial, and to the outside world. It's real IOS, so it's very realistic. It's free. It didn't support Cisco switch IOS last I looked, but it supports basic emulated LAN switches. And it's both cool and useful.

And, there's real gear. More to come on that.

If you plan on being in the Cisco space for a while, you may end up with all three. Between the three, there is no cut/dried answer to which option you should use. I do think there are several points to consider, which I'll list several here in the remainder of this blog post. So... considerations?

1) How important is exam prep for you versus general learning?  Sims generally have some advantages for exam prep in that they generally supply lab exercises that may be focused on the exam topics. Sims have the negative that it's practically impossible to create a Sim that's 100% exact in how it's command output mimics real gear, so you may miss some learning point that matters for the exam. Real gear and Dynamips may be better for general study/use, but neither comes with lab exercises that prep you for exams. Some Sims may be good for general learning and use, but probably do not support every command option in IOS, whereas real gear would of course support all command in the IOS feature set on that box.

Since this blog is focused on certifications, I'll stick with that as the primary motivation here.

2) Overhead time versus learning time: Sims require probably the least time, from the point you decide to use one until the time you are productively learning for an exam. I'd say real gear and Dynamips tie in this regard if you are starting from the point of having little knowledge either. Buying gear takes time to research before purchase; you can download Dynamips relatively quickly. However, then it will take some time to learn  Dynamips before you can productively learn for the exam, whereas hooking up the real gear may be a little quicker. Frankly, I'm sure the time required is a bit subjective. (Both are worth it in the long run, though.)

3) Realism of commands/output - Sims comes in last, and again a tie for real vs Dynamips. I've not surveyed Sim products, but from the ones I've sampled over the years, the quality and breath of command support varies significantly even within Sims. However, real gear and Dynamips/GNS3 should always be realistic, as they both run real IOS.

4) Flexibility of different topologies - Some Sims (I think) allow even hundreds of devices in one topology that you create, so those Sims may be best in terms of flexibility. Dynamips probably comes in second, in part because it can work within 1 PC, and with multiple PCs, and even communicate with real gear. So, you can expand your test topology quite a bit. Expanding your topology with real gear is just limited by $$.

5) Legality - I am no lawyer (disclaimer disclaimer), but I believe it's technically against Cisco's licensing to run IOS on anything except Cisco authorized hardware. IE, if you buy used gear on Ebay, running that IOS is against the license rules; running IOS in Dynamips is for sure against the rules. Sims purchased from their respective vendors should not have this problem.

6) Accessibility of IOS - Some people don't have easy access to IOS images. For example, you're learning CCNA, but you don't have a job that gets you a Cisco login that has the rights to download software.

7) Price - Dynamips, hands-down. It's free, and it works. The only potential monetary cost is that it does consume memory and compute resources, so you may want to bump memory, or even purchase a different PC on which to run it. I've heard varying success stories for a well-tuned Dynamips PC supporting anywhere from 2-3 router images simultaneously to 16 at a time. The other options do cost $$, but for exam prep, Sims tend to run a little less $$ than the comparable gear you'd probably want for the same exams.

So, what to do? For Sims, Google "Cisco Simulator". For Emulators, go to, and look for the links. For real gear, stay tuned!

Also, I need to make full disclosure. I have tried to present the information on Simulators as generically as possible, but I do have direct and indirect business relationships regarding a couple of Simulators. Pearson Certification ( now sells a CCENT Sim, with a CCNA version scheduled for June; I have a financial stake in those products. Boson Software ( sells Simulator products. I have no direct relationship with them, but Boson and Cisco Press do have several cooperative agreements, particularly for CD content in Cisco Press books, and many of my books do contain other (not simulator) Boson products.  As for competitors, a Google search shows many links for various other products billed a Cisco Simulators. Although I have attempted to treat Simulators as a group, as generically as possible, but I'm sure there are differences between the products.

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