Packet Tracer is a routing and switching emulator distributed by Cisco free of charge. It is capable of emulating reasonably complex routing and switching scenarios and is perfect for students studying for their CCNA. Although it does not provide all the BGP commands for the CCNP provided by GNS3 and Dynamips, It has one extremely valuable feature over these applications, it supports level 2 and 3 switching simulation, with devices like the 2950 and 3560 as well as generic device that allows the user to create devices with other interfaces such as the NM-1FFE and NM-1FGE. It supports a fairly broad range of commands permitting the user to create very advanced switching scenarios. I have completed several switching lab with EtherChannel, VTP, MLS, and more. While you cannot implement some features like HSRP, Packet Tracer is by far the best switch emulator available outside of buying actual hardware, and unlike GNS3 and Dynamips you do not have to obtain Cisco IOS's to use it.
Packet Tracer can also be used for the CCNA Voice and Wireless Practice labs as it also includes the Cisco 7960 IP phone and wireless routing devices.
The current version of Packet Tracer is 5.3.1, and may be found on a number of websites for the Windows and Linux GNU platforms. It requires the following system requirements:
- Microsoft Windows 2000, XP, Vista, 7 or Ubuntu 7.10, Fedora 7.
- Intel Pentium 300 MHz or better.
- 96 Mb. of ram or better.
- 250 Mb. of storage or better.
- 800X600 monitor or better.
These are minimum system requirements and will run much better on current computer configurations.
In conclusion if you are studying for your CCNA, CCNA Voice, CCNA Wireless, Packet Tracer is all you need and the price is right. If you are studying for the CCNP the Dynamips or GNS3 application will be the better choice although Packet Tracer can provide an eraser interface for most of the switching labs. And finally if you are working on that CCIE emulators can only do so much I would suggest getting your hands on some actual hardware similar to that used on the exam, because it will be cheap in contrast with a retake of that exam.