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- Introduction to Routing and
Packet Forwarding
- Introduction to Routing
and Packet Forwarding
Routing Protocols and Concepts – Chapter 1
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Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
- Objectives
Identify a router as a computer with an OS and
de as co OS
hardware designed for the routing process.
Demonstrate the ability to configure devices and
Demonstrate the ability to configure devices and
apply addresses.
Describe the structure of routing table
Describe the structure of a routing table.
Describe how a router determines a path and
switches packets
switches packets
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Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
- Router as a Computer
Describe the basic purpose of a route
bas pu er
-Computers that specialize in sending packets over the data
network.
They are responsible for interconnecting networks by selecting
the best path for a packet to travel and forwarding packets to
their destination
Routers have many of the same hardware and software
components that are found in other computers
including:
–CPU
–RAM
–ROM
–Operating System
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Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
- Router as a Computer
Router components and their functions”
CPU - Executes operating system instructions
such as system initialization, routing functions, and switching functions.
Random access memory (RAM) -RAM stores the instructions and data needed
access memory (RAM) stores the instructions and data needed
to be executed by the CPU. RAM is used to store these components:
–Operating System: The Cisco IOS (Internetwork Operating System) is copied
into RAM during bootup.
–Running Configuration File: This is the configuration file that stores the
configuration commands that the router IOS is currently using.
–IP Routing Table: This file stores information about directly connected and
remote networks. It is used to determine the best path to forward the packet.
–ARP Cache: This cache contains the IPv4 address to MAC address
mappings, similar to the ARP cache on a PC. The ARP cache is used on
routers that have LAN interfaces such as Ethernet interfaces.
LAN
–Packet Buffer: Packets are temporarily stored in a buffer when received on an
interface or before they exit an interface.
RAM is volatile memory and loses its content when the router is powered down or
restarted.
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- Router as a Computer
Router components and their functions”
co
Read-only memory (ROM) - Holds diagnostic software used
when router is powered up. Stores the router’s bootstrap
program.
–ROM is a form of permanent storage.
Cisco devices use ROM to store:
–The bootstrap instructions
–Basic diagnostic software
–Scaled-down version of IOS
version of IOS
ROM uses firmware, which is software that is embedded inside the
integrated circuit.
– Firmware includes the software that does not normally need to
be modified or upgraded, such as the bootup instructions.
– ROM does not lose its contents when the router loses power
or is restarted.
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- Router as a Computer
Router components and their functions”
Non-volatile RAM (NVRAM) - Stores startup configuration. This may include IP
addresses (Routing protocol Hostname of router)
addresses (Routing protocol, Hostname of router)
NVRAM (Nonvolatile RAM) does not lose its information when power is turned off. This is in
contrast to the most common forms of RAM, such as DRAM, that requires continual power to
maintain its information.
NVRAM is used by the Cisco IOS as permanent storage for the startup configuration file.
is used by the Cisco IOS as permanent storage for the startup configuration file
All configuration changes are stored in the running-config file in RAM, and with few
exceptions, are implemented immediately by the IOS.
To save those changes in case the router is restarted or loses power, the running-config
must be copied to NVRAM where it is stored as the startup
must be copied to NVRAM, where it is stored as the startup-config file. NVRAM retains its
file NVRAM retains its
contents even when the router reloads or is powered off.
Flash memory - Contains the operating system (Cisco IOS)
In most models of Cisco routers, the IOS is permanently stored in flash
memory and copied into RAM during the bootup process, where it is then
executed by the CPU.
Flash consists of SIMMs or PCMCIA cards, which can be upgraded to
increase the amount of flash memory
increase the amount of flash memory.
Interfaces - There exist multiple physical interfaces that are used to connect network.
Examples of interface types:
-Ethernet / fast Ethernet interfaces
-Serial interfaces
-Management interfaces
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- Router as a Computer
Router components
co
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Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
- Internetwork Operating System
The operating system software used in Cisco routers is known as Cisco
Internetwork Operating System (IOS)
Internetwork Operating System (IOS).
– Cisco IOS is a multitasking operating system that is integrated with routing,
switching, internetworking, and telecommunications functions.
Although the Cisco IOS may appear to be the same on many routers,
Although the Cisco IOS may appear to be the same on many routers,
there are many different IOS images.
– An IOS image is a file that contains the entire IOS for that router. Cisco
creates many different types of IOS images, depending upon the model of
the router and the features within the IOS.
the router and the features within the IOS.
– Typically the more features in the IOS, the larger the IOS image, and
therefore, the more flash and RAM that is required to store and load the IOS.
Although some routers provide a graphical user interface (GUI), the
command line interface (CLI) is a much more common method of
li (CLI)
configuring Cisco routers.
– The CLI is used throughout this curriculum.
Upon bootup, the startup-config file in NVRAM is copied into RAM and
bootup the startup file in NVRAM is copied into RAM and
stored as the running-config file.
– IOS executes the configuration commands in the running-config. Any
changes entered by the network administrator are stored in the running-
config and are immediately implemented by the IOS.
and are immediately implemented by the IOS
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Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
- Overview - Managing Cisco IOS Software (cont)
Overview Managing Cisco IOS Software
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Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
- Router as a Computer
Major phases to the
ajo
router boot-up process
Test router hardware
Power-On Self Test
(POST)
Execute bootstrap loader
Locate & load Cisco IOS
software
-Locate IOS
IOS
-Load IOS
Locate & load startup
configuration file or enter
setup mode
-Bootstrap program looks
for configuration file
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Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
- Stages of the router power-on boot sequence
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Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
- Router as a Computer
Major phases to the router boot-up process
Step 1 and 2: Test router hardware
•Power-On Self Test (POST)
(POST)
–During this self-test, the router executes
diagnostics from ROM on several hardware
components including the CPU, RAM, and
NVRAM
•Execute bootstrap loader
–The main task of the bootstrap program is
to locate the Cisco IOS and load it into
RAM.
–Note: At this point, if you have a console
connection to the router, you will begin to
see output on the screen.
Step 3 and 4: Locate & load Cisco IOS software
-Locate IOS and Load IOS
IOS and Load IOS
–The IOS is typically stored in flash
memory, but can also be stored in other
places such as a TFTP server.
–If a full IOS image can not be located, a
scaled-down version of the IOS is copied
from ROM into RAM. This version of IOS is
used to help diagnose any problems and
can be used to load a complete version of
the IOS into RAM.
–Note: A TFTP server is usually used as a
backup server for IOS but it can also be
used as a central point for storing and
loading the IOS.
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- Router as a Computer
Step 5 and 6: Locate & load startup configuration file or enter setup
mode
-After the IOS is loaded, the bootstrap program searches for
the startup configuration file, known as startup-config, in
NVRAM. This parameters including:
•interface addresses
addresses
•routing information
•passwords
•any other configurations
–If the startup-config, is located in NVRAM, it is copied into
RAM as the running-config.
•The IOS loads the commands in the file, one line at a
time
time.
–If the startup configuration file does not exist in NVRAM, the
router may search for a TFTP server.
• If the router detects that it has an active link to another
configured router, it sends a broadcast searching for a
configuration file across the active link. You will eventually
see message like the following one:
•%Error opening tftp://255.255.255.255/network-confg
opening tftp://255.255.255.255/network
(Timed out)
•%Error opening tftp://255.255.255.255/cisconet.cfg
(Timed out)
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- Router as Computer
Router as a Computer
Locate & load startup configuration file or enter setup
mode
–Enter Setup Mode (Optional). If the startup
configuration file can not be located the router
configuration file can not be located, the router
prompts the user to enter setup mode.
•Setup mode is a series of questions prompting
the user for basic configuration information.
Setup mode is not intended to be used to enter
complex router configurations and it is not
complex router configurations, and it is not
commonly used by network administrators.
–When booting a router that does not contain a
startup configuration file, you will see the following
question after the IOS has been loaded:
•Would you like to enter the initial configuration
dialog? [yes/no]: no
–Setup mode will not be used in this course
to configure the router. When prompted to
enter setup mode, always answer no. If you
setup you
answer yes and enter setup mode, you can
press Ctrl-C at any time to terminate the
setup process.
–When setup mode is not used, the IOS creates a
default running
default running-config.
•The default running-config is a basic
configuration file that includes the router
interfaces, management interfaces, and certain
default information.
•The default running-config does not contain any
Th
interface addresses, routing information,
passwords, or other specific configuration
information.
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Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
- show version
Router as a Computer
Verify the router boot-up process:
-The show version command is used
to view information about the router
during the bootup process.
Information includes:
Information includes:
Image name & IOS version
IOS (tm) C2600 Software
(C2600
(C2600-I-M), Version 12.2(28),
12
RELEASE SOFTWARE (fc5).
Bootstrap version stored in ROM
ROM: System Bootstrap,
Version 12.1(3r)T2, RELEASE
SOFTWARE (fc1)
Image file name & where it was
file name where it was
loaded from
System image file is
"flash:c2600-i-mz.122-28.bin"
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Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
- show version
Router as a Computer
Verify the router boot-up process:
Platform model number
CPU
Amount of RAM
of RAM
Some series of routers, like the
2600, use a fraction of DRAM as
packet memory. Packet memory is
used for buffering packets.
To determine the total amount of
DRAM on the router, add both
numbers. In this example, the Cisco
numbers. In this example, the Cisco
2621 router has 60,416 KB
(kilobytes) of free DRAM used for
temporarily storing the Cisco IOS
and other system processes. The
other 5,120 KB is dedicated for
packet memory. The sum of these
numbers is 65,536K, or 64
megabytes (MB) of total DRAM.
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- show version
Router as a Computer
Verify the router boot-up process:
Number & type of interfaces
2 FastEthernet/IEEE 802.3
interface(s)
2 Low-speed serial(sync/async)
network interface(s)
Amount of NVRAM
32K bytes of non-volatile
configuration memory.
NVRAM is used to store the
startup
startup-config file.
file
Amount of flash
16384K bytes of processor board
System flash (Read/Write)
System flash (Read/Write)
This is the amount of flash memory
on the router. Flash is used to
permanently store the Cisco IOS.
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- show version
Router as a Computer
Configuration register
Configuration register is 0x2102
–The last line of the show version
command displays the current
configured value of the software
fi th
configuration register in
hexadecimal. If there is a second
value displayed in parentheses, it
denotes the configuration register
denotes the configuration register
value that will be used during the
next reload.
–The configuration register has
several uses, including password
recovery. The factory default setting
for the configuration register is
0x2102. This value indicates that
th
the router will attempt to load a
Cisco IOS software image from
flash memory and load the startup
configuration file from NVRAM.
–Note: The configuration register is
discussed in more detail in a later
course.
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- Configuration register
The order in which the router looks for system
bootstrap depends on the boot field setting in the
configuration register.
The default configuration register setting can be
changed with the global configuration mode
command config-register.
Use a hexadecimal number as the argument for this
command.
command.
The configuration register is a 16-bit register in
NVRAM.
The lowest four bits of the configuration register form
the boot field.
To ensure that the upper 12 bits are not changed,
first retrieve the current values of the configuration
register using the show version command.
Then use the config-register command, changing
only the value of the last hexadecimal digit.
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