Cisco has developed a superb software known as Cisco Packet tracer and is essential for anyone preparing for CCNA or higher exams. Since it is very difficult to realize complex network topologies with different devices and interconnections with actual hardware, Cisco developed this product to simulate such topologies and added features which makes debugging easier to understand and implement in real world situations. Therefore one can simulate real life network topologies on their PCs without needing to physically set up devices.
Packet tracer has 2 modes – realtime and simulation.
In realtime mode devices run as they are expected to in normal operation. Simulation mode allows user to filter certain packet, change packet transfer speed to isolate certain packets for better analysis and understanding of the network topology and the path taken by a particular packet.
On the bottom left hand pane there is a list of devices such as routers, switches and WAN cloud which on clicking will display a list of models Cisco has manufactured. The most common router model used in CCNA is 2811 and for switches 2960 is used.
Dragging and dropping a device in the main window will boot it up and make it ready for configuration. Double clicking it will open a new window which will list the hardware specifications which can be modified according to your needs (like adding new modules such as ethernet ports). There is a config mode and a CLI mode.
Config mode is useful for quick configuration of the device wheres CLI mode simulates a console session. Cisco devices can be configured using a console cable and such a session is called a console session. Obviously CLI mode allows user to fully explore a device's features, unlike the config mode.
The simulation mode is a wonderful feature and behaves like waveshark in some ways, for example it allows you to watch the details of a particular packet. Additionally, you can adjust the speed of the packet transfer for debugging and better understanding.
For example, suppose I want to know what type of messages and path is taken when some host pings a router. All you need to do is start the simulation mode and ping the destination. Clicking on the autocapture button will show a graphical “journey” of the packet. You can also click on a packet any time to view the headers and other details.