Information Technology Reference
In-Depth Information
Internetwork Operating System (IOS)
Like any operating system on any other computer, Cisco IOS is responsible for managing the hard-
ware and software resources of the router. Although the Cisco IOS might appear to be the same on
many routers, there are many different IOS
images:
a file that contains the entire IOS for that router.
Although some routers provide a GUI (
graphical user interface
), the CLI (
command-line interface
) is
a much more common method of configuring Cisco routers.
Upon bootup, the startup-config file in
NVRAM
is copied into
RAM
and stored as the
running
-
config
file. Any changes entered by the network administrator are stored in the
running
-
config
file and imme-
diately implemented by the
IOS
.
Router Bootup Process
Like all computers, a router uses a systematic process to boot up. The four phases are
POST
: Testing the router hardware
1.
Loading the
bootstrap
program
2.
Locating and loading the
IOS
3.
Locating and loading the
startup
configuration file or entering
setup mode
4.
Power-on self test
(POST) is a common process that occurs on most every computer during bootup.
The POST process is used to
test
the router
hardware
.
After the POST, the
bootstrap
program is copied from ROM into RAM. Its job is to locate the Cisco
IOS and load it into RAM.
After the IOS is loaded, it searches for the
startup
-
config
file. If this file is located, it is copied into
RAM as the
running
-
config
file. The IOS executes the commands in the file one line at a time.
If the startup configuration file cannot be located, the router will prompt the user to enter
setup mode
,
a series of questions prompting the user for basic configuration information. Setup mode will not be
used in this course.
After the normal loading process is completed and the prompt is displayed, the router is now running
the IOS with the current running configuration file. The network administrator can now begin using
IOS commands on this router.
The
show version
command can be used to help verify and troubleshoot some of the basic hardware
and software components of the router.
Router Ports and Interfaces
Routers have
management
ports, which are physical connectors used by the administrator to configure
the router and are not used for packet forwarding. The most common of the management ports is the
console
port. It must be used during initial configuration of the router. Another management port is
the
auxiliary
port, which can also be used to attach a modem.
The term
interface
on Cisco routers refers to a physical connector on the router whose main purpose
is to receive and forward
packets
. Routers have multiple interfaces used to connect to multiple net-
works.
Every interface on the router is a member, a host on a different IP
network
. A router's
Ethernet
inter-
face usually uses an RJ-45 jack that supports unshielded twisted-pair (UTP) cabling. When a router is
connected to a switch, a
straight
-
through
cable is used. When a PC's network interface card (NIC) is
