Cisco ios overview

1. Cisco IOS overview

2. Cisco IOS overview
• IOS (Internetwork Operating System) is a
multitasking operating system used on most Cisco
routers and switches.
• IOS has a command-line interface with the
predetermined number of multiple-word
commands.
• This operating system is used to configure routing,
switching, internetworking and other features
supported by a Cisco device.
• Previous versions of Cisco switches ran CatOS, a
discounted version of a CLI-based operating system.

3. Accessing the IOS
-> There are three most common ways to access the IOS:
1. Console access - this type of access is usually used to
configure newly acquired devices. These devices usually
don't have an IP address configured, and therefore can
not be accessed through a network. Most of the Cisco
devices have a physical console port. This port can be
connected to a computer using a rollover cable, a
special type of cable with pins on one end reversed on
the other end of the cable. The rollover cable is a serial
cable, which means that you can't just plug it in an
Ethernet port on your computer. You will need an
adapter that converts an interface on your computer
(usually a 9-pin serial interface) into RJ-45.

4. 2. Telnet access - this type of access used to be a common way
to access network devices. Telnet is an terminal emulation
program that enables you to access IOS through the network
and configure the device remotely. The device that is being
configured needs to have an Telnet server installed and an IP
address configured.
• Telnet uses a well known TCP port 23. One of the biggest
disadvantages of this protocol is that is sends all data as clear-
text, which includes the passwords! This is the reason why
this type of access is usually not used anymore. Instead, SSH
is usually used.
3. SSH access - like Telnet, this access type enables you to
configure devices remotely, but it adds an extra layer of
security by encrypting all communications using public-key
cryptography. SSH uses well known TCP port 22.

5. Types of memory on a Cisco device
-> Cisco devices usually have four types of memory that are
being used for different purposes. These four types are:
• ROM (Read-only memory) - stores a bootstrap program
that is used to initialize a boot process. This is a read-only
type of memory, so it can't be altered.
• RAM (Random Access Memory) - the running
configuration of a device is stored here. This type of memory
loses its content when a device is restarted.
• Flash memory - used to store IOS software images. Can
also be used to store other files, for example backup
configuration files. Retains its content even after a device is
restarted.
• NVRAM (Nonvolatile RAM) – usually used to store a
startup configuration file. This type of memory retains its
content even after a device is powered down or restarted.

6. IOS modes
-> IOS has many different modes. There are three main modes
and many sub modes. We will describe the three main modes.
• user EXEC mode - the default mode for the IOS CLI. This is
the mode that a user is placed in after accessing the IOS. Only
basic commands (like ping or telnet) are available in this
mode.
• privileged EXEC Mode – this mode is accessed by typing
the enable command from the user EXEC mode. This mode
can be password protected. In this mode a user can view and
change a device's configuration.
• global configuration mode - this mode can be accessed by
typing the configure terminal command from the privileged
EXEC mode. It is used to change the device's configuration.