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CCNA version 3

Module 4

Switching Concepts



CCNA version 3

Summary

LAN congestion and its effect onnetwork performance

Advantages of LAN segmentation in a

network Advantages and disadvantages of

using bridges, switches, and routers

for LAN segmentation Effects of switching, bridging, androuting on network throughput FastEthernet technology and its benefits



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CSMA/CD prevents multiple devices from

transmitting at the same time.



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The Ethernet/802.3

Interface Ethernet is known as a shared-

medium technology ² all the devicesare connected to the same deliverymedia.

Ethernet media uses a data frame

broadcast method of transmittingand receiving data to all nodes on theshared media.



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Performance of a shared mediaEthernet/802.3 LAN can benegatively effected by severalfactors.

y The data frame broadcast delivery nature of

Ethernet/802.3 LANsy CSMA/CD access methods allow only one station

to transmit at a time.

y Network congestion due to increased bandwidthdemands from multimedia applications such as

video and the Internet.y Normal latency (propagation delay) of frames as

they travel across the LAN layer 1 media and passthrough layer 1, 2 and 3 networking devices.

y Extending the distances of the Ethernet/802.3

LANs using Layer 1 repeaters.



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Half-Duplex Design

Loopback

TxTx

RxRx

Collision

Detection

Ethernet

Controller

TxTx

RxRx

Collision

Detection

Ethernet

Controller

Transmit

Receive

Loopback

Ethernet NIC Ethernet NIC

Ethernet physical connector provides several circuits

Most important are receive (RX), transmit (TX), andcollision detection



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Half-Duplex Ethernet

Design (Standard Ethernet) The most important of these circuits

are the receive (RX), transmit (TX)and collision detection.

The transmit (TX) circuit is active atthe transmitting station.

The receive (RX) circuit is active atthe receiving station.



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To the network this appears as a single one

way bridge. Both devices are contending for the rightto use the single shared medium.

The collision detection circuit on each

node contends for the use of the networkwhen the two nodes attempt to transmit atthe same time.

When a collision occurs, a host will first

listen to see if the network is in usebefore trying to retransmit. It willresume transmitting based on the back-off algorithm.



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Congestion and Bandwidth To relieve network congestion more

bandwidth is needed or the availablebandwidth must be used more

efficiently. ´Throwing bandwidth at the problem.µ

Attacking the symptom and not

always the problem (illness), i.e. Couldbe broadcasts, chatty protocols,applications traffic, etc.



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Propagation Delay

Latency is also known as propagation delay.

Propagation delay is the time a frame orpacket of data takes to travel from thesource station or node to its finaldestination on the network.

The greater the number of devices the

greater the latency or propagation delay adding hosts simply increases collisions,

increases jam signals, and throughput willdecrease



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Ethernet Transmission

Times Each Ethernet bit has a 100ns

window for transmission.

A byte is equal to eight bits.

Therefore, one byte takes a minimumof 800ns to transmit (8 bits at 100nsper equals 800ns).



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A 64 byte frame takes 51,200ns or

51.2 microseconds to transmit (64bytes at 800ns equals 51,200ns,51,200ns/1000 equals 51.2microseconds).



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Extending Shared Media

LANs using Repeaters Signal attenuation ² the signal

weakens as it travels through thenetwork from the resistance found inthe medium.

An Ethernet repeater to extend the

distance of a LAN is that a singlenetwork can cover a greater distanceand more users can share that same

network. (Coverage Area)



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Improving LANPerformance

The performance of a network can beimproved in a shared media LAN such

as Ethernet by using one or more ofthe following solutions:

y Segmenting the network using Bridges,Routers, or LAN Switches

y Move to full duplex transmitting

y Upgrade to the Fast Ethernet Standard



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Why Segment LANs?

A Cisco Segment A network can be divided in smaller

units called segments. Each segment

uses the (CSMA/CD) protocol andmaintains traffic between users onthe segment. By using segments in anetwork less users/devices are

sharing the same 10Mbps whencommunicating to one another withinthe segment. Each segment is

considered its own collision domain.



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Why Segment LANs? In a segmented Ethernet LAN data

passed between segments is

transmitted on the backbone of thenetwork using a bridge, switch, orrouter.

The backbone network is its owncollision domain and uses CSMA/CD toprovide a best effort delivery service

between segments.



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Segmentation with Bridges

Bridges are different than routersbecause they are Layer 2 devices,independent of Layer 3 protocols ²

they pass on data frames regardlessof which Layer 3 protocol is beingused and are transparent to theother devices on the network.

Bridges increase the latency (delay)ina network by 10-30%.

Why?



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Segmentation with Routers Routers operate at the network layer

and base all of their forwarding

decisions between segments on theLayer 3 protocol address.

Because routers perform more

functions than bridges they operatewith a higher rate of latency.(Higher than other internetworkingdevices.)



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Routers: Segment broadcast domains

Forward packets based on destinationnetwork layer addresses

Segment collision domains



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More collision domains,but more bandwidthfor each user



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Segmentation with LAN

Switches A switch segments a LAN into

microsegments creating collision free

domains from one larger collisiondomain, not broadcast domains.

With switched ethernet

implementation the availablebandwidth can reach closer to 100%.



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LAN Switch Latency Each switch used on an Ethernet LAN

adds latency to the network.

However, the type of switching usedcan help overcome the built in latencyof some switches.



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Full-Duplex Ethernet

Overview Full duplex Ethernet allows the

transmission of a packet and the

reception of a different packet atthe same time.

Requires two pairs of conductors

and a switched connection betweeneach node



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Simultaneous transmission and

reception of frames is calledbidirectional traffic (both directions)and yields 20Mbps of throughput.

The network interface cards (NICs)on both ends need to have full duplexcapabilities.



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Full-Duplex Ethernet Design

Transmit circuit connects directly to receive circuit

No collisions Significant performance improvement

Eliminates contention on Ethernet point-to-point link

Uses a single port for each full-duplexconnection

TX

Full

Duplex

Ethernet

Controller

Loopback

TxTx

RxRx

Collision

DetectionRX

Full

Duplex

Ethernet

Controller

Loopback

TxTx

RxRx

Collision

Detection



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Using Full Duplex

Node must

² Be directly attached to a dedicatedswitched port

² Have installed network interface card thatsupportsfull duplex

Full Duplex

Half Duplex

HUB



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Full-Duplex Ethernet

Design Standard Ethernet normally can onlyuse 50-60% of the 10Mbps availablebandwidth.

This is due to collisions and latency.

Full duplex Ethernet offers 100% ofthe bandwidth in both directions.

This produces a potential 20Mbpsthroughput ² 10Mbps TX and 10MbpsRX.



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This virtual network circuit exists

only when two nodes need tocommunicate.

This is why it is called a virtual circuit

² it exists only when needed and isestablished within the switch.

Allows multiple users to communicatein parallel via these virtual circuits.



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Source MACaddress is used

to build thistable



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How a LAN Switch Learns

Addresses This means that as new addresses are

read they are learned and stored in

Content Address Memory (CAM). Each time an address is stored it is

time stamped.

This allows addresses to be storedfor a set period of time.



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But more domains



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Benefits of Switching A LAN switch allows many users to

communicate in parallel through the

use of virtual circuits and dedicatednetwork segments in a collision freeenvironment.

Cost effective.



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Symmetric Switching A symmetric switch is optimized

through even distribution of network

traffic across the entire network .



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before forwarding



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Asymmetric Switching Asymmetric switching is optimized

for client-server network traffic

flows where multiple clients aresimultaneously communicating with aserver, requiring more bandwidth

dedicated to the switch port that theserver is connected to in order toprevent a bottleneck at that port.



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CCNA version 3

Memory Buffering The area of memory where the switch

stores the destination and

transmission data is called thememory buffer.

This memory buffer can make use of

two methods for forwarding packets² port based memory buffering orshared memory buffering.



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Port based memory buffering

packets are stored in queues that arelinked to specific incoming ports.

² Problem: One port may fill while anotheris empty.

Shared memory buffering depositsall packets into a common memorybuffer that is shared by all the ports

on the switch. (Better!)



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3 frame transmission modes

in a switch (+ 1 variation)



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Three Switching Methods

Store and Forward - the entireframe is received before anyforwarding takes place.

Latency occurs while the frame isbeing received; the latency is greaterwith larger frames because the entireframe takes longer to read.

Error detection is high because ofthe time available to the switch tocheck for errors while waiting for the

entire frame to be received.



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Cut-through the switch reads the

destination address before receivingthe entire frame.

The frame is then forwarded before

the entire frame arrives. This mode decreases the latency of

the transmission and has poor error

detection.



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Fragment-Free Switching

Switch reads the 1st 64bytes of the incomingframe beforeforwarding it to the

destination port



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Means theswitch is in cutthrough mode



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Adaptive Cut Through

Combines cut through and store andforward

The switch uses cut-through untilthere are a given number of errors

Then the switch will change to store

and forward method

Emerging Trends:



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Emerging Trends:The Network Evolution

Shared to Switched

The

New

Wiring

Closet

VLAN

System

LAN

Campus

Switch

The New Backbone

The

Old

Wiring

Closet

HUB

HUB

HUB

HUB

HUB



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Benefits of Switching

Number of collisions reduced

Simultaneous, multiple communications

High-speed uplinks

Improved network response

Increased user productivity



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Switching? Routing?

What·s the difference? In a switching network, you'll find the

intermediate devices keeping track of - or

remembering - qualities of the connection. In a pure routing network, the

intermediate devices will be indifferent toanything but handing off packets to the

next device, and they will not be distractedby any other information, upstream ordownstream.



Module 4

Switching Concepts

Download - Ccna 3mod4 Switching Concepts

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