05 Understanding Switch

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1999, Cisco Systems, Inc.www.cisco.com

Module 5:

UnderstandingLAN Switching


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5-2CSE: Networking FundamentalsSwitching 1999, Cisco Systems, Inc.www.cisco.com

Agenda

Shared LAN Technology

LAN Switching Basics

Key Switching

Technologies


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Shared LANTechnology

1999, Cisco Systems, Inc. www.cisco.com


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Early Local Area Networks

Thick Ethernet Limited to 500 meters before signal degradation

Required repeaters every 500 meters

Limitations on number and placement of stations

Expensive, large, and difficult to pull through buildings

Relatively simple to add new users

Provided 10-Mbps shared bandwidth

Thin Ethernet

Less expensive and required less space than thickEthernet

Still difficult to pull through buildings

Adding users required network interruptions


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Hubs Addressed Many ofThese Problems

Ethernet 10

One device sending at a time

Hub

All nodes share 10 Mbps

Ethernet concentrator

Self-contained EthernetLAN in a box

Passive

Works at physical layer 1


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Collisions: Telltale Signs

Sluggish network response

Increasing user complaints

CRASH

Hub

I could have walked to

Finance by now.

I knew I should have

stayed home.

File transfers take

forever.

Im waiting all the time.


7/385-7CSE: Networking FundamentalsSwitching 1999, Cisco Systems, Inc.www.cisco.com

Other Bandwidth Consumers

Unicast

Broadcast

Multicast



Apple

TCP/IP

Broadcasts Consume Bandwidth

Resolve addresses

Distribute route information

Find network services



Broadcasts Consume ProcessorPerformance

Broadcasts and multicasts interruptall computers on the network

Broadcasts/Second

CPU %

1 0 0 1 0 0 0 3 0 0 0

0

1 0

2 0

3 04 0

5 0

6 0

7 0

8 0

1 0 0 1 0 0 0 3 0 0 0

SPA R C 2

SPA R C 5

Pe nt i um 12 0 MH z



Hub-Based LANs

Shared resources

Desktop connectionswired to centralizedclosets

Poor security withinshared segments

Routers provide scalability

Adds, moves, and changes

are easier than withouthubs, but still a hassle

Groups of usersdetermined by

physical location

10BaseTHub

10BaseTHub



Bridges

More intelligent than a hub

Eavesdrop on conversations tolearn and maintain address tables

Collect and pass packets betweentwo network segments

Control traffic to the network

Bridge

Segment 1 Segment 2

123

124

125

126

127

128

Corporate Intranet

Hub Hub



SwitchesLayer 2

EthernetSwitch

Each Node has10 Mbps

BackboneSwitched Ethernet 10

Multiple devices sending at the same time



Switches versus Hubs

Ethernet 10One devicesending at

a time

Hub

All nodes share 10 Mbps

EthernetSwitch

Each node has 10 Mbps

Backbone Switched Ethernet 10

Multiple

devicessending at thesame time



SLOW

The Need for Speed:Early Warning Signs

File transfers take forever

You can take a coffee breakbetween screen refreshes

Network print jobs: same-day service?

Multimedia applications

SLOW

Mbps



Typical Causes of NetworkCongestion

Too many users on a10-Mbps segment

Most users accessingone or two servers

High-performancePCs such as EISA,PCI, and S-Bus

Network-intensive

applications suchas color publishing,CAD/CAM, imaging,and relationaldatabases File Transfer Client/Server Image

Processing

Backup/Mng Transaction

Processing

E-Mail Desktop Video

Traditional DataRequirements Drive LAN

Bandwidth

BandwidthRequirements



Network Traffic Impact fromCentralization of Servers

Servers are gradually moving into a central area(data center) versus being located throughout the company to:

Ensure company data integrity

Maintain the network and ensure operability

Maintain security

Perform configuration and administrative functions

More centralized servers increase the bandwidth

demands on campus and workgroup backbones

Servers inWiring Closets

RandomlyDistributed

Servers

CentralizedServers inData Center

MainframeData Center



Todays LANs

Mostly switchedresources; few

shared Routers provide

scalability

Groups of usersdetermined byphysical location

10/100Switch

10-Mbps

Hub

10/100Switch


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Enables dedicatedaccess

Eliminatescollisions andincreases capacity

Supports multipleconversations atthe same time


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A C

B

2

4

1

10 Mbps

10 Mbps

LAN Switch Operation

Forwards packetsbased on a forwardingtable Forwards based on the MAC

(Layer 2) address

Operates at OSI Layer 2

Learns a stationslocation by examiningsource address

Sends out all ports whendestination address isbroadcast, multicast, orunknown address

Forwards when destinationis located on different

interface

Interface

Stations

1 2 3 4

3Data from A to B


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A C

B

2

4

1

10 Mbps

10 Mbps



(Layer 2) address





interface

Interface

Stations

1 2 3 4

A X

3


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A C

B

2

4

1

10 Mbps

10 Mbps



(Layer 2) address





interface

Interface

Stations

1 2 3 4

A X

3Data from A to B

DatafromAtoB

DatafromA

toB


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A C

B

2

4

1

10 Mbps

10 Mbps



(Layer 2) address





interface

Interface

Stations

1 2 3 4

A X

3

B X

DatafromB

toA


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A C

B

2

4

1

10 Mbps

10 Mbps



(Layer 2) address





interface

Interface

Stations

1 2 3 4

A X

B X

3

Data from B to A


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Switching Technology: FullDuplex

Doubles bandwidth between nodes

e.g. switch and server

Collision-free transmission

Two 10- or 100-Mbps data paths

Full Duplex Switch

10 or 100 Mbps

10 or 100 Mbps

10 or 100 Mbps

10 or 100 Mbps


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Switching Technology: TwoMethods

Frame Frame Frame

Frame

Cut-through

Switch checks DA andimmediately begins

forwarding frame

Store-and-forward

Complete frame isreceived before

forwarding


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Key SwitchingTechnologies



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Key Switching Technologies

802.1d Spanning-Tree Protocol

Multicasting


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Problems with large switched networks

Local multicast, broadcast, and unknown single-destination event storms become global events

The Need for Spanning Tree

Station A

Station B

Segment A

Segment B

Switch 1 Switch 2

1/1

1/2

2/1

2/2

802 1d S i T


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802.1d Spanning-TreeProtocol (STP)

Allows redundancy by using parallel links

Shuts down redundant links to eliminateloops

Switches communicate with each otherusing BPDUs (Bridge Protocol Data Units)

Takes 3060 seconds to converge

Cisco refinements:PortFast

UplinkFast


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Multicasting

Emerging applicationsrequire increasedbandwidth

E-mail, file transfers, andfile sharing are easy tohandle

Emerging applications are

bandwidth hungry Multipoint communications

are required Simultaneous communication

between groups of computers

Video

LAN TV

Desktopconferencing

Corporatebroadcasts

Collaborativecomputing


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WAN

VideoServers

Channel 1

Channel 2

Channel 3

1

2

3

Multicast

Internet Group Management

Protocol (IGMP)

Multicast routing protocols

IGMPMulticast RoutingProtocols

Switches typically flood allports with multicast traffic

Problem: How toprevent multicastingflooding?


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WAN

VideoServers

Channel 1

Channel 2

Channel 3

1

2

3

End-to-End Multicast


Protocol (IGMP)


IGMP

Cisco Group Management

Protocol (CGMP)

CGMP

Wire-SpeedMulticast

Multicast RoutingProtocols


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VideoServers

Channel 1

Channel 2

Channel 3

1

2

3

End-to-End Multicast

IGMP


Protocol (IGMP)


Cisco Group Management

Protocol (CGMP)

WAN

CGMP

Wire-SpeedMulticast

Multicast RoutingProtocols


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Why Use Multicast?

1 20 40 60 80 100

Number of Clients

0.8

0.6

0.4

0.2

0

TrafficMbps With multicast

Without multicast

100-kbps Video Stream

Highly scalable information delivery


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Summary

Switches provide dedicated access

Switches eliminate collisions andincrease capacity

Switches support multiple

conversations at the same timeSwitches provide intelligence formulticasting


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05 Understanding Switch

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