Cisco Router Emulator
Clara Vanliere
Whether you've been advised by a coworker, supervisor, or a thread on Reddit, everyone has their favorite simulator or emulator. Someone who primarily uses GNS3 will always claim that it's superior to VIRL, and vice-versa.
These are the most popular software for gaining hands-on Cisco lab experience for the routing and switching track certifications. Many of these tools can also be used to test networking technologies for deployment in the real world. However, we will focus as much as possible on the certification exam use case.
Disclaimer: We won't necessarily pick a winner. It's important to note this here. We will not give a blanket recommendation as to which particular piece of software is "best". Our intent is to focus on the pros and cons of each particular software, leaving you to decide.
A simulator is a piece of software that, as the name implies, simulates a network topology composed of one or more network devices. The network devices being simulated are not real network devices, and are not capable of passing live network traffic the same way that a real network device could. Instead, each network device is a piece of software pretending to be a real network device to the best of its ability.
As a result, network devices within a simulator are limited to the commands and features programmed into the simulation. For this reason, many advanced features (such as DMVPN, Policy Based Routing, and so on) that real network devices are capable of are not present in simulated counterparts.
The key benefit of simulators is that they tend to be extremely lightweight. Simulator software can run on just about any modern computer without worrying about processor, memory, or storage requirements.
Packet Tracer is Cisco's visual simulation tool that simulates network topologies comprised of Cisco routers, switches, firewalls, and more. Packet Tracer was originally designed as an educational aid for Cisco's Networking Academy (better known as NetAcad) but is an excellent simulator for anyone pursuing an entry-level Cisco certification, such as the CCENT or CCNA R&S.
Packet Tracer is Free. There is absolutely no cost associated with using Cisco's Packet Tracer. It is completely free to download and use. The only requirement is that you create and log into Packet Tracer with a Cisco Networking Academy account, which is free to create.
These network components are invaluable when studying for the CCNA. You can even experiment with devices outside exam prep. What does it look like when a laptop accesses a network through a wireless connection? How is the configuration of an ASA firewall different from an ISR router? Packet Tracer makes it easy to add a device to an existing topology.
Connection Variety. Whether it's straight-through or crossover copper cables, fiber-optic cables, DCE or DTE serial cables, or crossover console cables, Packet Tracer provides plenty of options for connecting entry-level network devices.
While the majority of production environments utilize either fiber-optic or straight-through copper cables to connect devices, production environments still use serial connections for WAN connectivity due to a lack of alternative options. As a result, you should be aware of the differences between ethernet and serial physical mediums. Packet Tracer provides the necessary practical experience with configuring and troubleshooting environments with both connection types.
Making a Realistic Lab. Packet Tracer offers different methods to connect and configure devices. Most of the time, you'll probably left-click on each device in the simulator and configure it through the CLI tab. However, you can also simulate how network engineers would provision devices in the real world. You can accomplish this by connecting a computer to each device via console cable and then configuring the network device through the PC's terminal. You can even use this to configure remote management of the network device through Telnet or SSH, then connect the PC to a management network and configure each device remotely.
By default, Packet Tracer works in Realtime Mode. In Realtime Mode, connected network devices generate control plane traffic (such as Spanning Tree Protocol BPDUs, routing protocol Hello messages) and forward data plane traffic (such as ICMP messages and TCP/UDP packets between end hosts) in real-time, just like real network devices would. This network activity is visible in real-time through the flashing connection lights between devices, indicating that data was sent or received on a link.
However, let's say you're troubleshooting a connectivity issue between two end hosts. It can be difficult to visualize how the packet flows through a large network. Simulation Mode allows you to walk through the path of a packet step-by-step. You can observe attributes of the packet change and see the forwarding decision that each intermediary network device makes on the packet. With Simulation Mode you can quickly compare what's happening on the network device's CLI with what visually happens to the packet as it traverses their simulated network.
This may seem like a minor feature, but it's extremely important. When you master keyboard shortcuts within Packet Tracer, you are simultaneously mastering keyboard shortcuts for the real Cisco IOS command line. Furthermore, many of these keyboard shortcuts are applicable to other CLI-based applications, including Linux devices. Because the keyboard shortcuts work in both environments, your transition from Packet Tracer to real networking devices will be seamless.
The only exception to this rule is with respect to highlighting text in the terminal window. With most terminal applications, highlighting text with the mouse automatically copies that text to the system clipboard. Once it's copied, you can paste the contents of the system clipboard into the terminal with a right-click. Unfortunately, Packet Tracer does not support this functionally.
Instead, you must right-click highlighted text and select "Copy" from a drop-down menu to copy it, then right-click and select "Paste" from a drop-down menu to paste into the terminal. Alternatively, a "Copy" and "Paste" button are provided in the lower right corner of the terminal window.
Custom Exercise Creation. Students who have taken NetAcad courses may be familiar with the Packet Tracer lab exercises provided throughout the coursework. These exercises include formal instructions that appear when the lab is executed, as well as a built-in grading system that lets you know whether all of the tasks within the lab have been completed. Packet Tracer also allows you to create these types of labs through the Activity Wizard feature, complete with formal instructions and grading system.
These labs can be as simple or as complex as you desire. Labs may include HTML instructions with images similar to lab instructions provided by NetAcad, variables that slightly randomize the nature of the lab, and custom JavaScript. Furthermore, the Activity Wizard allows for the definition of an "Answer Network" used to grade your work. The Answer Network is comprised of a number of different tests, including required configuration parameters on network devices and successful connectivity between end hosts.
Custom Exercise Distribution. When you create a custom exercise, you must save the activity as a file and distribute this file to all interested parties. The lack of a centralized distribution method results in some challenges. For example, if you (or someone) needs to update the exercise to fix a bug, correct lab instructions, or add additional content, you'll need to redistribute updated version of the exercise file.
Versioning is also a challenge. A Packet Tracer file created with version 7.2.1 is not compatible with prior versions of Packet Tracer (although the latest version of Packet Tracer is fully backwards-compatible).
Why not have a marketplace? Ideally, these challenges would be resolved with an in-application "marketplace" of lab exercises similar to what Boson's NetSim offers. With this solution, authors could publish exercises directly to NetAcad that could then be downloaded by others from within Packet Tracer instead of downloading and opening a separate file. This also opens up the possibility for friendly competition amongst lab exercise authors through the simple implementation of a rating system, allowing high-quality lab exercises to rise to the top.
Bugs. All software has bugs, and Packet Tracer is no exception. Packet Tracer's bugs tend to be more prominent than other simulators or emulators, perhaps due to its popularity and widespread use through Cisco's NetAcad courses. A quick Google search shows users repeatedly report odd bugs where the configuration of a device is correct, but the behavior of the device is unexpected. These bugs are typically fixed by saving the simulation file, then reloading Packet Tracer, after which the device is behaving as expected. This is a problem because you may spend precious time troubleshooting rather than labbing.
Since Packet Tracer is so similar to a production environment, there's another risk here, too. You can inadvertently develop a habit of "saving and reloading" as a valid troubleshooting methodology, which can carry over into production environments and be impactful to business networks.
No Built-In Labs. When you first download and install Packet Tracer as a new user, you're presented with a blank network topology as a sandbox. For some, a sandbox is seen as an invitation to experiment and learn on their own. For others, a blank sandbox can be overwhelming. The lack of built-in labs may be intimidating for learners who need more structure to get started with labbing. Cisco offers a number of excellent lab exercises in Packet Tracer, but they are locked behind Cisco's NetAcad courses, which are offered by educational institutes such as community colleges and IT training schools. These courses are typically offered both online and on-premises to accommodate a variety of learning styles, but the cost of these courses may prohibitive for anyone on a tight budget.
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