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Bridges and Gateways

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In the previous class, you have learnt about

• Definition of a Repeater

• Importance of Repeaters

• Function of a Repeater

• Location of a Repeater

Recap

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Objectives

On completion of this period, you would be able to know about

• Definition of a bridge • Importance of a bridge

• Types of bridges

• Functioning of a bridge

• Definition of a gateway

• Function of gateways

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Definition of a Bridge

• A bridge is a connecting device which has it’s own

processor, memory and two NIC cards, to connect

two portions of a network

• Facilitates host to host communication

• Operates at the physical as well as data link layer

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Bridge at the Last two OSI Layers

Application layerPresentation layer

Session layerTransport layerNetwork layerData link layerPhysical layer

Bridge

A

Physical layerData link layerNetwork layer

Transport layerSession layer

Presentation layerApplication layer

B

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Importance of a Bridge

• Bridge is to divide a big network in to smaller sub-networks

• A bridge connects two LAN’s

5

4

3

2

1

4

3

2

1

5

Fig .2 A bridge connecting two LAN’s

BRIDGE

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Function of a Bridge

• As a physical layer device it generates the signal it receives

• As a data link layer device it checks the source and destination addresses in the frame

• It send data frames only to the concerned destination

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• It minimizes unwanted traffic

• It minimizes network congestion

• Error links can be identified and then isolated

• Security features or access control can be added

• It acts as a filter

• It maintains a table that maps addresses to ports

Function of a Bridge contd…

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Types of Bridges

BRIDGE

MULTIPORTLEARNINGSIMPLE

Fig .3 Classification of bridges

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Simple Bridge

• A simple bridge connects two sub nets

• The table of host addresses verses segment numbers entered manually

• When new host added or existing deleted the table has to be updated

• These are cheapest but also are error prone

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Learning Bridge

• No manual entering of host addresses versus segment numbers

• When a frame comes to it, checks for the addresses in the table. If not adds it to the table along with the segment number

• This is how bridge constructs mapping table to the entire network

• The bridge continue checking and update its mapping table, so the name learning bridge

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Multi-port Bridge

• When a simple or learning bridge connects more than two network segments it is called a multi-port bridge

• It is a special case of either the simple or the learning bridge

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Gateway

• To connect two networks that use different protocols

• It works at all seven layers of OSI model

• Unlike a router a gateway can forward packets across different networks that may also use different protocols

• Ex. If network A is token ring network using TCP / IP and network B is a novell- netware network, a gateway can relay frames between the two

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Application layerPresentation layer

Session layerTransport layerNetwork layerData link layerPhysical layer

GATEWAY

A

Physical layerData link layerNetwork layer

Transport layerSession layer

Presentation layerApplication layer

B

Fig .4 Gateway at all OSI layers

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Functions of Gateways

• It has abilities to translate between different frame

formats and also between different protocols

• It is a very powerful computer system compared to a bridge

• It is used to connect huge and incompatible networks

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Summary

In this class, we have learnt about

• Definition of a bridge

• Different types of bridges

• Importance of a bridge

• Functions of bridge

• Definition of a gateway

• Functions of gateway

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Quiz

1. A bridge is a device used to

a) To connect two LAN’s

b) To connect two segments with in a LAN

c) To connect two networks of different protocols.

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2. A gateway is a device used to

a) Connect two LAN segments

b) Connect two segments of same LAN

c) Connect two networks of different protocols

Quiz

19

Frequently Asked Questions

1. Define bridge?

2. List different types of bridges

3. Explain the function of a bridge

4. Define a gateway

5. Explain the functioning of a gateway