Discovering computer networks: hands on in the Open Networking Lab
Discovering computer networks: hands on in the Open Networking Lab

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Discovering computer networks: hands on in the Open Networking Lab

13.2 IOS levels and Ethernet

You already know from earlier sessions that there are multiple modes within the Cisco IOS and that each mode allows the user to execute different tasks. This section explains these modes in terms of hierarchical levels. It serves as a brief reminder of how to identify the current mode (or level) and then goes on to explore information stored about the router interfaces and how to interpret it.

Watch the video below, which is almost 6 minutes long.

IOS levels and Ethernet

Download this video clip.Video player: 71_iso_levels_and_the_ethernet.mp4
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When a router has booted up, if we go to the command-line interface we see something like this.

Where it says ‘Router’, that’s the name of the router. If we haven’t given it a name, the default name is ‘Router’. So this one hasn’t been named.

Packet Tracer Anywhere, unlike a real router, can’t save its configuration. Every time we begin a Packet Tracer Anywhere session we start from scratch, so everything that can be configured is unconfigured or at its default settings.

When the prompt is an arrow head, like this one, we’re at the top level of IOS. This is called the ‘user executive level’, or ‘user exec level’. Whatever level you are currently logged in as you can issue the question mark as a command to list all the available commands. This is shown here for the euser executive level.

This name is a bit misleading. An executive is supposed to be able to make things happen; but at the user exec level of IOS you can’t do very much. You can send a ping though.

Typing enable at the executive level prompt takes you to the next level, called the ‘privilege exec level’, or sometimes ‘privileged executive level’. Notice that the prompt has changed from an arrow head to a hash sign. The prompt keeps changing as we go down through the levels of IOS, as you will see.

If you want to go back a level from here, you can do so by typing exit. Commands that work at one level usually don’t work at another. But you’ve already seen in another session a command that does work at this level. It’s the show ip interface briefcommand, and it’s very useful.

On the left there is a list of interfaces on this router. More specifically, these are ‘built-in’ interfaces. They are the interfaces [in] this router when you buy it.

Most routers have slots that you can slide additional modules into. This gives you more interfaces if you need them. But what are Gigabit interfaces, and what do these numbers mean 0/0, or 0/1 etc. mean?

Ethernet, in one form or another, is a common form of wired local area internet. It’s widely used in offices for networking computers and peripherals.

Ethernet was in use before the wide-scale adoption of the Internet Protocol. In its first commercial form it had a speed of 10 megabits a second.

Since then it has gone through various generations, and the term ‘Ethernet’ is now used as a general name covering all varieties of Ethernet. The first major upgrade of Ethernet was FastEthernet, with a top speed of 100 megabits a second. After FastEthernet came Gigabit [Ethernet] with a top speed of 1 gigabit a second. Routers are likely to have FastEthernet ports or GigabitEthernet ports, or a mixture of FastEthernet and GigabitEthernet. Later versions of Ethernet are backwards-compatible with earlier, so GigabitEthenet is compatible with earlier versions of FastEthernet, and FastEthernet is compatible with Ethernet.

All versions of Ethernet have the same type of connector. Here’s an Ethernet plug, with a coin to show you the scale.

All versions of Ethernet use MAC addresses to identify the source and destination addresses. In fact, some authors refer to MAC addresses as Ethernet addresses.

Switches are often used to direct Ethernet traffic between one local network, and every device on the local network that has its own connection to the switch. This is why switches typically have 24 or 48  interfaces, unlike routers which usually have far fewer interfaces. In the early days of Ethernet, though, switches weren’t used and other methods were used to direct traffic between devices on a local networks. After the widespread adoption of Internet Protocol, switches have had to translate between IP addresses of devices on their local networks and MAC addresses, and routers are added to the network to provide a gateway into and out of the local network.

The basic data unit of Ethernet is a frame. The Ethernet data stream is split into frames, and frames travel [separately] across the network. So when internet packets are carried across Ethernet networks, the packets are encapsulated inside Ethernet frames.

As we have seen, routers typically have Ethernet interfaces, usually for connecting to a switch, but sometimes for connecting to another router. Clearly each interface on a router needs a unique identifier, so that can be configured independently of other interfaces. Numbers are used to identify the interfaces, but the numbering system varies from one manufacturer to manufacturer, and from one model to another.

Here’s one example of a numbering system for interfaces. To understand this numbering system, remember that routers and switches have built-in interfaces and empty slots which additional network cards can be slid into if more interfaces are needed. The built-in interfaces have 0 in their number, like this. Interfaces on additional cards would have the 0 replaced by a number, depending on the slot occupied by the card. So you get 1/1 or 1/2 for a 1 card, and if there was a second card added it might get a 2/1 or a 2/2 on it.

Ethernet ports are usually configured to serve the local network, like this. Whether we use FastEthernet or GigabitEthernet is determined mainly by what’s available on the devices. Of course, Wi-Fi is also widely used for serving the local network, but with enterprise equipment, as opposed to domestic equipment, it’s unusual for wireless devices to be incorporated into a router or switch. Usually there would be a separate Wi-Fi access point, connected to a switch by Ethernet.

On the internet-facing side of the router, the network connected to is called the wide area network, or WAN. A suitable interface needs to be chosen for this connection and a ‘serial’ port is often used for this; and you will see at least one serial port among those listed when you do the show ip interface briefcommand. Increasingly, though, Ethernet is used for the WAN as well as the LAN connections.

End transcript
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Activity 2 Test yourself

5 minutes

1. If a router interface has the identity GigabitEthernet3/2 what does this tell you? Select one or more correct answers from the list below.


This is not a Cisco router.


The router has a total of two interfaces.


This is a wireless interface.


The router probably has at least three network cards added.


Fast Ethernet is not being used on this router.

The correct answer is d.


Correct. The first number of the ID identifies the network card; the second number gives the identity of the interface on that card.

2. What would be the result of the command Router>show ip interface brief? Select one or more correct answers from the list below.


The IP addresses and subnet masks of all the router interfaces will be displayed.


A summary of the status of the router’s interfaces will be displayed.


A ‘% incomplete command’ message will be displayed.


A ‘% invalid input detected at ‘^’ marker’ will be displayed.


The router’s interface IDs and configured IP addresses are shown.


All the network IP addresses are shown.

The correct answers are b and e.


Correct. The status is shown as ‘up’ or ‘administratively down’.


Correct. Not all the network IP addresses are shown.


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