Computernetze 1 (CN1)

Steffen/Stettler, 03.10.2013, 3-LAN_Design_Basics.ppt 1

Computernetze 1 (CN1)

3 LAN Design Basics

Prof. Dr. Andreas Steffen

Institute for Internet Technologies and Applications


Lesestoff im Ethernet Buch

• Kapitel 6 Ethernet Internals, Seiten 189-2146.1 Längenbeschränkung im Ethernet

• Kapitel 8 Ethernet-Komponenten, Seiten 275-3298.1 Netzwerkkarten8.2 Repeater und Hubs8.3 Switches8.4 Medienwandler

• SelbststudiumErarbeiten Sie als Vorbereitung für die Übung 4selbständig das Thema “Spanning Tree Protocol”mit Hilfe des Cisco Tutorials von Dan DiNicolo unddes Kapitels 4 des CN1 Foliensatzes.


LAN and LAN Devices

• LANs make it possible for businesses that use computer technology to efficiently share such items as files and printers and to make possible communications such as e-mail.

• LANs are designed to do the following: • operate within a limited geographic area or building• allow many users to access high-bandwidth media • provide full time connectivity to local services • connect physically adjacent devices


The Beginning of LAN Design

• Initial idea: shared media LAN• Bus structure; CSMA/CD was access method• Coax cable; transmission rate up to 10 Mbit/s, half-

duplex


Repeater


Repeater

• The purpose of a repeater is to amplify and retime network signals at the bit level.

• Repeaters can be single-port "in" and single-port "out" devices, though more often now, they are stackable (modular), or multi-port repeaters, better known as hubs.

• Repeaters are classified as layer 1 devices, in the OSI model, because they act only on the bit level and look at no other information.


Hub

• Used in 10BASE-T and 100BASE-T networks

• Is equivalent to a multi-port repeater

• Hubs come in three basic types:• passive – no traffic and signal manipulation, used only

toshare the media

• active – amplifies the incoming signal• intelligent – (smart hub) same function as an active

hub,additionally includes a controller chip and diagnostic capabilities


Ethernet StrukturierungCollision Domain

Hub

Collision Domain

Multiport Repeater (Concentrator, Hub)

simple Repeater Collision Domain

Collision Domain

single Segment


Bridge

• The purpose of a bridge is to filter traffic on a LAN, to keep local traffic local, yet allows connectivity to other parts (segments) of the LAN for traffic that has been directed there.

• The bridge makes its decisions based on MAC addresses.

• The bridge is a layer 2 device.


MAC Layer Bridge (Transparent Bridge)

HigherLayers

LLC Sublayer

MAC Sublayer

PhysicalLayer

LAN-Segment “1”

PhysicalLayer

PhysicalLayer

MAC MAC

MAC Layer Bridge

BC

A

ZY

X

HigherLayers

LLC Sublayer

MAC Sublayer

PhysicalLayer

SA

DA

FC

S DATA

Port1 Port2

Table

LAN-Segment “2”

(ForwardingDatabase)

SA

DA

FC

S DATA


Bridge: Flooding or Filtering (destination address)

Error-free frame received on port x

1

Flooding Filtering Discardframe

Forward frameto port y

no yesDestinationaddress found in forwarding

database?

Is destination again on port x?

no yesForward frame

to all ports (except port x)


Bridge: Learning (source address)

Learning

1

end

Add address with portand age value to theforwarding database

no yesSource addressfound in forwarding

database?

Update port and age value

Typical address aging value: 300 s


AA

Learning / Flooding

table of bridge 1

table of bridge 2p1 p2

SA DAA F L2data

MAC A

MAC FMAC C

MAC B MAC Gbridge

1bridge

2

p1

p2

p1

p2

LAN 1

LAN 2

LAN 3learn A

flood Flearn A

flood F

A F L2data

A F L2data

p1 p2


F

Learning / Filtering

FDA SAAL2 data

MAC FMAC C

MAC B MAC Gbridge

1bridge

2

p1

p2

p1

p2

LAN 1

LAN 2

LAN 3learn F

filter A

MAC A

A

table of bridge 1 p1 p2

A



F

Learning / Flooding

MAC FMAC C

MAC B MAC Gbridge

1bridge

2

p1

p2

p1

p2

LAN 1

LAN 2

LAN 3

flood G

FGL2 dataDA SA

learn F

flood GMAC

A

FA


A


F G L2data

F G L2data


G G

Learning / Forwarding

MAC FMAC C

MAC B MAC Gbridge

1bridge

2

p1

p2

p1

p2

LAN 1

LAN 2

LAN 3 learn G

forward Flearn G

forward F

GFL2 dataDA SA

MAC A

FA


FA


GFL2 data

G F L2data


• Bridges split a LAN into several workgroups with smaller collision domains -> reduces whole LAN traffic

• One "broadcast domain"

half-duplex collision domains

Segmentation with Bridges


Switch

• The purpose of a switch is to concentrate connectivity, while guaranteeing bandwidth.

• It switches packets from incoming ports (interfaces) to outgoing ports, while providing each port with full bandwidth.

• The switch uses the MAC address to make its switching decisions. You might think of each port on a switch as a micro-bridge, which makes it a layer 2 device.


Ethernet Strukturierung Broadcast Domain (IP-Subnet)

Broadcast Domain

(simple) Bridge

Broadcast DomainMultiport Bridge

Broadcast Domain

Bridges and Backbone


Switch Forwarding Principles

6B

DestinationAddress

6B

SourceAddress

2B

Length(E-Type)

4BFrameCheck

Sequence

0 (46)…1500BLLC PDU

Early-Cut

Cut-Through

Fast-Forward

Store-and-Forward

6B 4,8s=12B 9,6s=

64B 51,2s=

1518B 1,2144ms=

* delays computed for 10 Mbit Ethernet

*


Switch Forwarding Principles

• Early-Cut• only usable when finished

learning• forwards nearly all

faulty frames

• Cut-Through• reads destination and

source address• forwards many

faulty frames

• Fast-Forward (Fragment Free)• detects runts and

collisions

• can filter specific Ethertypes

• Store-and-Forward• detects CRC-errors

• can do filtering based onhigher-layer Information


Switching Fabric

Port 0

Port 3

Port 2

Port 1

cross-connect any two switch ports


Non-Blocking Switching Fabric

10 Gbps

10 Gbps

10 Gbps

10 Gbps

60 Gbps Fabric

Bandwidth of Fabric > Ingress + Egress


Blocking Switching Fabric

10 Gbps

10 Gbps

10 Gbps

10 Gbps

20 Gbps Fabric

Bandwidth of Fabric < Ingress + Egress


CPU Memory

Data/Address/Control Buses

Packet Memory

BuffersQueuesPointersHeader

s

IOS Image/Files

System Buffers

Forwarding Tables

Processor Queues

Inte

rface

Inte

rface

Inte

rface

Inte

rface

CPU

General Purpose CPU (CISC older or RISC newer)

Physical Media Interfaces

(Fixed or Modular)

Shared Memory Architecture

Inte

rface

Inte

rface

Inte

rface


Tx

Rx

Tx

Rx

Tx

Rx

CPU Memory(DRAM)

(C) ForwardingTable

CPU

CPU

InterfaceCard(D) FT

Packet Memory

CPU

InterfaceCard(D) FT

Packet Memory

CPU

InterfaceCard(D) FT

Packet Memory

CPU

InterfaceCard(D) FT

Packet Memory

Cross Bar ASIC

• Multiple conflict free paths• Typically higher bandwidth• Signaling and scheduling

is more complex

Cross Bar Switch Architecture


Router

• The purpose of a router is to examine incoming packets, choose the best path for them through the network, and then switch them to the proper port.

• Routers make their path selection decisions based on layer 3 information - the network addresses therefore they are consideredlayer 3 devices.

• Routers can connect different layer 2 tech-nologies, such as Ethernet, Token-ring, or FDDI.

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