Ca Ex S2 M01 Introduction To Routing And Packet Forwarding

CCNA – Semester 2

Chapter 1 - Introduction to Routing and

Packet Forwarding

CCNA Exploration 4.0

Objectives

• Identify a router as a computer with an OS and hardware

designed for the routing process.

• Demonstrate the ability to configure devices and apply

addresses.

• Describe the structure of a routing table.

• Describe how a router determines a path and switches

packets

Inside the Router

Routers are Computers

• Routers have many of the same hardware and software components

that are found in other computers including:

– CPU

– RAM

– ROM

– Operating System

Routers are at the network center• A router connects multiple networks. This means that it has multiple interfaces

that each belong to a different IP network.

• Each network that a router connects to typically requires a separate interface.

These interfaces are used to connect a combination of both Local Area

Networks (LANs) and Wide Area Networks (WANs).

Routers determine the best path

• The primary responsibility of a router is to direct packets destined for

local and remote networks by:

– Determining the best path to send packets

– Forwarding packets toward their destination

Router as a Computer

• Describe the basic purpose of a router

– 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 are the network center

– Routers generally have 2 connections:

• WAN connection (Connection to ISP)

• LAN connection

Not common:

• PSTN connection

• …

Evolution of Routers

• Multiprotocol devices providing pure data delivery with

minimal access control and QoS.

• Intelligent Network Services, this allowed us to provide

security capabilities like integrated firewall and encryption.

• Integrated voice and data capabilities, allowing routers to

provide voice gateway features for IP telephony.

• Today the router is an integrated platform for providing

advanced services in security, content delivery, and voice.

Cisco Router Series

Router Components

• Router components and their functions”

CPU - Executes operating system instructions

Random access memory (RAM) - Contains the running copy of configuration file. Stores routing table. RAM contents lost when power is off

Read-only memory (ROM) - Holds diagnostic software used when router is powered up. Stores the router’s bootstrap program.

Non-volatile RAM (NVRAM) - Stores startup configuration. This may include IP addresses (Routing protocol, Hostname of router)

Flash memory - Contains the operating system (Cisco IOS)

Interfaces - There exist multiple physical interfaces that are used to connect network. Examples of interface types:

• Ethernet / FastEthernet interfaces

• Serial interfaces

• Management interfaces

Router Components

Cisco router 1841’s Architecture - Front Panel

LED Color Status

SYS PWR Green Router has successfully booted up and the software is functional. Slow,

steady blinking when system is booting or in the ROM monitor.

SYS ACT Green Blinking when packets are transmitted or received on any WAN or LAN

interface, or when monitoring system activity.

CF Blinking

Flash memory is busy. Do not remove the CompactFlash memory card

when this light is on.

http://www.cisco.com/cdc_content_elements/flash/nextgen/webversion/1800/kaon/1841/index.html?NO_NAV

Router Components

• Router components

Internetwork Operating System

• Cisco IOS manages the hardware and software resources

of the router, including memory allocation, processes,

security, and file systems. Cisco IOS is a multitasking

operating system that is integrated with routing, switching,

internetworking, and telecommunications functions.

• Cisco creates many different types of IOS images,

depending upon the model of the router and the features

within the IOS.

• The command line interface (CLI) is a much more common

method of configuring Cisco routers.

Router Boot-up Process

Bootup Process• There are four major phases to the bootup process:

1. Performing the POST

2. Loading the bootstrap program

3. Locating and loading the Cisco IOS software

4. Locating and loading the startup configuration file or entering setup mode

• Verify the router boot-up process:

– The show version command is used to view

information about the router during the bootup

process. Information includes:

Platform model number

Image name & IOS version

Bootstrap version stored in ROM

Image file name & where it was loaded from

Number & type of interfaces

Amount of NVRAM

Amount of flash

Configuration register

Verifying Router Bootup Process

• The show version command can be used to help verify and

troubleshoot some of the basic hardware and software components of

the router.

Router Interfaces

• Router Interface is a physical connector that enables a

router to send or receive packets

• Each interface connects to a separate network

• Consist of socket or jack found on the outside of a router

• Types of router interfaces:

– Fixed

– Modular

Router Interfaces

• Two major groups of Router Interfaces

– LAN Interfaces:

Are used to connect router to

LAN network

Has a layer 2 MAC address

Can be assigned a Layer 3 IP

address

Usually consist of an RJ-45 jack

– WAN Interfaces

Are used to connect routers to external networks that

interconnect LANs.

Depending on the WAN technology, a layer 2 address may

be used.

Uses a layer 3 IP address

Router Interfaces

Routers and the Network Layer

• The main purpose of a router is to connect multiple networks and forward

packets destined either for its own networks or other networks.

• A router is considered a Layer 3 device because its primary forwarding

decision is based on the information in the Layer 3 IP packet, specifically the

destination IP address. This process is known as routing.

Routers and the Network Layer

• Routers Operate at Layers 1, 2, and 3

CLI Configuration and Addressing

Implementing Basic Addressing Schemes

• When designing a new network or mapping an existing

network, document the network. At a minimum, the

documentation should include a topology diagram that

indicates the physical connectivity and an addressing table

that lists all of the following information:

– Device names

– Interfaces used in the design

– IP addresses and subnet masks

– Default gateway addresses for end devices, such as

Implementing Basic Addressing Schemes

Basic Router Configuration

• Router name

• Passwords

• Banner

• Router Interface Configuration

• Verifying Basic Router Configuration

– Issue the show running-config command

– Save the basic router configuration by Issuing the

copy running-config startup-config command

• Additional commands that will enable you to further verify router

configuration are:

show running-config - Displays configuration currently in RAM

show startup-config - Displays configuration file NVRAM

show ip route - Displays routing table

show interfaces - Displays all interface configurations

show ip interface brief - Displays abbreviated interface

configuration information

Building the Routing Table

Introducing the Routing Table

• The primary function of a router is to forward a packet

toward its destination network, which is the destination IP

address of the packet.

• Routing Table is stored in RAM and contains information

about:

Directly connected networks - this occurs when a device

is connected to another router interface

Remotely connected networks - this is a network that is

not directly connected to a particular router

Detailed information about the networks include source

of routing information, Network address & subnet mask,

and ip address of next-hop router, exit interface

• show ip route command is used to view a routing table

Introducing the Routing Table

• Directly Connected Routes

• Static Routes

• Dynamic Routes

Directly-Connected Networks

• Once the interface is "up," the network of that interface is

added to the routing table as a directly connected network.

Static Routing

• Static routes in the routing table

– Includes: network address and subnet mask and IP

address of next hop router or exit interface

– Denoted with the code S in the routing table

– Routing tables must contain directly connected

networks used to connect remote networks before static

or dynamic routing can be used

• When to use static routes

– When network only consists of a few routers

– Network is connected to internet only through one ISP

– Hub-and-spoke topology is used on a large network

Static Routing

Dynamic Routing

• Dynamic routing protocols are used by routers to share information

about the reachability and status of remote networks. Dynamic routing

protocols perform several activities, including:

– Network discovery

– Updating and maintaining

routing tables

Dynamic Routing

• Dynamic routing features:

– Automatic Network Discovery: allows the routers to automatically

learn about these networks from other routers.

– Maintaining Routing Tables: automatically share routing information

with other routers and compensate for any topology changes

without involving the network administrator.

• IP Routing Protocols:

– RIP (Routing Information Protocol)

– IGRP (Interior Gateway Routing Protocol)

– EIGRP (Enhanced Interior Gateway Routing Protocol)

– OSPF (Open Shortest Path First)

– IS-IS (Intermediate System-to-Intermediate System)

– BGP (Border Gateway Protocol)

Routing Table Principles

• 3 principles regarding routing tables:

Every router makes its decisions alone, based on the information it

has in its routing table.

One router’s information in the routing table may not be the same

as another router’s routing table.

Routing information about a path to a network does not provide

information on how to return to the original network.

Routing Table Principles

Path Determination

and Switching Functions

Packet Fields and Frame Fields

• Internet Protocol (IP) packet format contains fields that provide information

about the packet and the sending and receiving hosts

• Fields that are importance for CCNA students:

– Destination IP address

– Source IP address

– Version & TTL

– IP header length

– Precedence & type of service

– Packet length

Packet Fields and Frame Fields

• MAC Layer Frame Format

• MAC Frames are also divided into fields. They include:

– Preamble

– Start of frame delimiter

– Destination MAC address

– Source MAC address

– Type/length

– Data and pad

– Frame check sequence

Best Path and Metric

• A Metric is a numerical value used by routing protocols to assess a

given route

• Metrics may be based on a single path variable or multiple variables of

a path

• 2 types of metrics used by routing protocols are:

– Hop count - this is the

number of routers a packet

must travel through to get

to its destination

– Bandwidth - this is the

“speed” of a link also

known as the data capacity

of a link

Router Paths and Packet Switching

• Equal cost metric is a condition where a router has multiple paths to the same destination that all have the same metric

• To solve this dilemma, a router will use Equal Cost Load Balancing. This means the router sends packets over the multiple exit interfaces listed in the routing table.

• Equal Cost Paths and Unequal Cost Paths: a router can send packets over multiple networks even when the metric is not the same if it is using a routing protocol that has this capability.

Path Determination

• The path determination function is the process of how the router

determines which path to use when forwarding a packet by

searching its routing table.

• One of three path determinations results from this search:

– Directly connected network

– Remote network

– No route determined

Switching Function

• Switching Function of Router is the process used by a

router to switch a packet from an incoming interface to an

outgoing interface on the same router.

• A packet received by a router will do the following:

Strips off layer 2 headers.

Examines destination IP address located in Layer 3

header to find best route to destination.

Re-encapsulates layer 3 packet into layer 2 frame.

Forwards frame out exit interface.

Switching Function

• Step 1: PC1 has a packet to be sent to PC2

Switching Function

• Step 2: Router R1 receives the Ethernet frame

Switching Function

• Step 3: Packet arrives at router R2

Switching Function

• Step 4: The packet arrives at R3

Switching Function

• Step 5:The Ethernet Frame with encapsulated IP packet

arrives at PC2

Switching Function

• Step 5: The Ethernet Frame with encapsulated IP packet

arrives at PC2

1. PC2 examines the destination MAC address, which

matches the MAC address of the receiving interface,

its Ethernet NIC. PC2 will therefore copy the rest of the

frame into its buffer.

2. PC2 sees that the Ethernet Type field is 0x800, which

means that the Ethernet frame contains an IP packet

in the data portion of the frame.

3. PC2 decapsulates the Ethernet frame and passes the

IP packet to the IP process of its operating system.

Summary

Configuration Register Setting – 1800 Series

• The order in which the router looks for system bootstrap information

depends on the boot field setting in the configuration register.

• The configuration register is a 16-bit register in NVRAM.

• 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.

Configuration Register

Boot Field Configuration Register Bit Descriptions

Boot Field

(Bits 3, 2,

1, and 0)

Meaning

At the next power cycle or reload, the router boots to the ROM monitor.

(0x01)

Boots the first image in flash memory as a system image.

0010 - 1111

(0x02 - 0xF)

At the next power cycle or reload, the router sequentially processes each

boot system command in global configuration mode that is stored in the

configuration file until the system boots successfully.

If no boot system commands are stored in the configuration file, or if

executing those commands is unsuccessful, then the router attempts to

boot the first image file in flash memory.

Configuration Register

Ca Ex S2 M01 Introduction To Routing And Packet Forwarding

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