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Router

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Router

A routeris an electronic device that

interconnects two or more computer

networks

Works atLayer 3, Network Layer in an

intelligent manner

Can connect different network segments,

whether they are in the same building or

even on the opposite side of the globe

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Router : Network Layer Interface

X MEDIUM X MEDIUM

PHYSICAL

LAYER

PHYSICAL

LAYER

DATA LINK

LAYER

DATA LINK

LAYER

NETWORK

LAYERROUTER

NETWORK

LAYER

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Router

Works in LAN, WAN environments

Allows access to resources by selecting the

e!t "at#

Can interconnect different networks

Changes packet size and format to match

the requirements of the destination network

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Internetworking with a Router

!!! "#$%&'ub(network !!! "#$%)

'ub(network

*C(N+'

'ub(network

outer

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Devices and Layers

PHYSICAL

LAYER

DATA LINK

LAYER

NETWORK

LAYER

'witches

outers

epeaters

'witches

Layer -

Layer $

Layer &

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Difference Between Routers,

Switches and Hubs

.ubs

'imply provides the mechanical and electrical

connections between the nodes

'witches !/amine the data packet for the destination address

0o not alter the data packets

outers !/amine and alter the data packets

*erform protocol conversion

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Delivery, Forwarding & Routing

De$%&ery

efers to the way a packet is handled by the

underlying networks under the control of thenetwork layer%

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'orwar(%)*

efers to the way a packet is delivered to the

ne/t station%

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Rout%)*

efers to the way routing tables are created to

help in forwarding%

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Functions of a Router

Two "r%+ary u)-t%o)!.

1 0etermine the 2e!t "at#2

1S#are (eta%$! o route!with other

routers

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Router has

Rout%)* Ta$e( a database which keeps

track of the routes to networks and the

associated costs

Rout%)* Proto-o$( uses a special algorithm

to route data across a network eg *

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Format of Routing Table

Ma!k Network

A((re

!!

Ne/t0#o"

A((re!!

I)tera-e

'$a*!

Reere)-e Cou)t

U!e

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3ask

4his field defines the mask applied for the

entry%

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Network Address

0efines the network address to which the

packet is finally delivered%

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Ne/t(hop Address

0efines the address of the ne/t(hop router to

which the packet is delivered%

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nterface

'hows the name of the interface%

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+lags

56up78 f this flag is not present, it means that

the router is down%

96gateway78 0estination is in another network%

.6host(specific78 ndicates that the entry in the

Network Address field is a host(specificaddress%

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Flags Contd.

06added by redirection78 ndicates that the

routing information for this destination has

been added to the host routing table by aredirection message from C3*%

36modified by redirection78 ndicates that the

routing information for this destination has

been modified by a redirection message from

C3*%

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eference Count

9ives the number of users of this route at the

moment%

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5se

'hows the number of packets transmitted

through this router for the correspondingdestination%

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Types of Routing Tables

Stat%- Rout%)* Ta$e:

outes are manually configured by a network

administrator

Cannot update automatically when there is a

change in the internet

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Types of Routing Tables

Dy)a+%- Rout%)*4able

Ad;usts automatically to changes in network

topology 5ses one of the dynamic routing protocols such

as *

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RI

'tands for outing nformation *rotocol

* implementation considerations8

We are dealing with routers and

networks6links7

outers have routing tables< networks do not

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RI Implementation

Considerations Contd.

4he destination in a routing table is a network% 4his means the first column defines a network address

3etric used is very simple< distance defined asno% of links to reach destination

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RI Implementation

Considerations Contd.

nfinity is defined as -= Which means no route can have more than -) hops

Ne/t(node column defines the address of therouter to which the packet is to be sent to reach

its destination%

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Router !ardware

'imilarity with a *C8

A C*5

A memory

*orts > interfaces

0issimilarity8

outers are diskless

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Router "otherboard

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Router C#

)# 3.z C*5s are generally used for small

offices > homes%

+or more powerful purposes, processors

from 3otorola, 'ilicon 9raphics, etc% are

used%

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Router "emory

A3?0A3

N@A3

+lash 3emory

3

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R$"%DR$"

'tands for random access memory?dynamic

random access memory

5sed by the routerBs central processor to do

its work

CiscoBs smallest router ships with a

minimum of -=3 of 0A3

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'R$"

'tands for nonvolatile A3

retains information after losing power

stores a copy of the routerBs configuration

file

enables the router to restart in its proper

configuration in case of accidental shutdown

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Flash memory

riginally developed by ntel

also nonvolatile

differs from N@A3 in that it can be

erased and reprogrammed as needed

used to store one or more copies of the '

software

.elps in upgrading ' on all routers

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R("

5sed to hold a so(called bootstrapprogra

which is a file that can be used to boot to a

minimum configuration state after a catastrophe

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Router orts

*ort means a physical connection through

which ? can pass

a serial port, for e/ample

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