13.1 The command line and operating system

In this part of this session I want to say a bit more about the command-line interface. This interface is used for setting up routers and switches on large-scale commercial networks, or enterprise networks. You’ve already seen the command-line interface in an earlier session. Now I want to explore it further using Packet Tracer Anywhere, so that you can get more experience of using it for yourself. Also, I want to explore things you’re likely to see when using the command line. For example, why does the prompt sometimes change depending on the command you’re using? There’s an arrow head here showing you how it changes to a hash sign. And this GigabitEthernet0/0. What’s this about? Depending on the router or the switch you’re using, you might get ‘FastEthernet’ or ‘GigabitEthernet. And sometimes you’ll even see ‘Serial’. I want to look at theose as well. And what’s this ‘VLAN’? I think we can guess what the ‘unassigned’ means when we see the column headed ‘IP addresses’. But ‘administratively down’, or just ‘down’ in the next column, is a bit puzzling.

Before we delve into that level of detail, though, I want to say something about what’s happening when you use the command line of a router or a switch.

A router or a switch has one feature that is similar to what you find on your computer or your mobile phone. It has an operating system, just like a computer might have Windows or a phone might have Android. As with a computer or a phone, the operating does various jobs. One of the most important is providing services to programs running on your device. When you use the command-line interface of a router or a switch, you are in the operating system.

With Cisco devices, the operating system is usually referred to as ‘IOS’ which stands for ‘Internetwork Operating System’, not to be confused with Apple iOS. In the past, when you switched on a computer or mobile phone, the operating system was loaded into the memory from storage on a hard drive. These days when you switch on a computer or phone you’re usually waking it up from standby rather than loading an operating system, but a clean boot reloads the operating systems [which] have to be taken from storage somewhere. That can take a little while and it’s usually from the memory.

In Packet Tracer Anywhere we can see what is displayed when a Cisco router or switch boots up. The operating system is loaded from storage into active memory and various checks have to be done on the hardware. Booting up ends with this question ‘Continue with configuration dialog?’ to which the sensible answer is nearly always ‘no’. Answering ‘yes’ takes you through a series of questions that are supposed to help you set up the router but in practice are not very helpful.

Along with other things, the operating system allows you to create and save a configuration file. A configuration file contains settings such as the name that has been given to the router, the IP addresses of its interfaces, security settings and so on. These details aren’t stored in the IOS itself, but in the configuration file created by the IOS.

After IOS has booted up, it will load the configuration file if there is one. If there’s no configuration file, then things like the anIP addresses of the interfaces, security settings and so on are not set, or will be at default settings.

Thank you for watching.