06 ethernet principle 20090724 a

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Copyright 2006 Huawei Technologies Co., Ltd. All rights reserved.

Ethernet Principle


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Copyright 2006 Huawei Technologies Co., Ltd. All rights reserved. Page2

Objectives

Upon completion of this course, you will be able to:

Know the technical background of the Ethernet and its basic

concepts

Describe the common Ethernet equipment and their workingprinciple

Understand the function of VLAN


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Contents

1. Ethernet Concept

2. Ethernet Port Technology

3. VLAN Basis


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Do you know why IP address isnot fixed as MAC?

00000001 10111011 00111010 10111010 10111110 10101000

It means this is a multicast address.

Ethernet MAC Address

00.e0.fc.39.80.34

MAC address includes 48 bits and it is shown as 12 dotted

hexadecimal notations

MAC address is exclusive globally which is allotted and managed by

IEEE. Every MAC address is composed of two parts. The first 24 bitspart is the vendor code and the other 24 bits part is serial number

If 48 bits are all 1, it means the address is used for broadcast

If the 8th bit is 1, it means the address is used for multicast


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Ethernet_II

DMAC SMAC Length/T DATA/PAD FCS

Length/Type Mean

Length/T > 1500

Length/T


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Ethernet Principle---CSMA/CD

CS: carrier sense

Sense before sending data to ensure the cable is idle and reduce

collision.

MA: multiple access The data from every station can be received by other multiple stations.

CD: collision detection

Detect collision while sending data and stop it when the station find

collision then continue to send after waiting for a random time.


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1 2 3 4 5

IN

OUT OUTOUTOUT

HUB only changes the physical topology of Ethernet,

It is half duplex.

Working Principle of HUB


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HUB

LAN

LAN

LAN

LAN

LAN

HUB only repeats all the signals coming from connected LAN and

all the physical equipment construct a collision domain.

Collision Domain


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Defects of Traditional Ethernet

The transmission efficiency is low in case of many hosts

connected

A lot of collisions

Broadcast storm

No security

Use HUBs to construct the LAN as a sharing Ethernet


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Physical

layer

Data link

layer

(BRIDGE/Ethernet switch/L2) equipment workmode

L2 Work Mode

Application layer

Presentation layer

Conversation layer

Transport layer

Network layer

Data link layer

Physical layerPhysical

layer

L2 switch

Data link

layer

Application layer

Presentation layer

Conversation layer

Transport layer

Network layer

Data link layer

Physical layer


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L2 Work Principle

Learning based on source

Segment 1 Segment 2

A

B

C

PORT

1

PORT

2 D

typical use of the switch

switch

MAC ADD. port

MAC A 1

MAC B 1

MAC C 2

MAC D 2


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L2 Work Principle (Cont.)

Forwarding based on destination address

Port 1

Port 2

L2 Switch

MACD MACA

MACA MACD

MAC ADD. port

MAC A 1

MAC B 1

MAC C 2

MAC D 2


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L2 Switch Principle

Receive all the data frames from the network segment

Save the source MAC addresses of the received frames to establish MAC

address table( self-learning based on source address) and maintain the

address table by aging mechanism

Check the MAC address table to find out the port corresponding to the

destination address. If it is the receiving port, it will take the frame; if it is

not the receiving port, the data will be broadcasted to all the other ports

(except the source port)

Forward broadcast and multicast frames to all other ports (except the

source port)


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Three Switch Modes

Cut-Through Switch forward the frame immediately after it receive destination address

Short time delay

Switch dont check the error

Store-and-Forward

Switch forward the frame after it receive the whole frame

Long time delay

Switch check the error so no error frame will be forwarded

Fragment-free

Switch forward the frame after it receive 64 bytes( the shortest frame length)

Integrate the advantage of the cut-through and store-and-forward mode


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L2 will forward the received frames according to the MACaddress so collision domain is limited to one port. But it can not

limit the broadcast domain.

Broadcast Domain

SWITCH

LAN

LAN

LAN

LAN

LAN

Broadcast domain

collision domain

collision domaincollision domain

collision domain collision domain


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Contents

1. Ethernet Concept


3. VLAN Basis


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Ethernet Connection Requirements

Ethernet connection must ensure the performance of thenetwork

Bandwidth

Delay

Two methods to increase bandwidth for users:

Increase the overall network bandwidth

Build switching Ethernet, and use bandwidth exclusively.

Increase link rate: 10M--100M--1000M

Decrease the number of equipment attached to the same shared

medium


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Auto-Negotiation

If one device can not support auto-negotiation, the other device

that can support auto-negotiation will work at the default work

method.

Half-Duplex Full-Duplex

10M Support Support

100M Support Support

1000M - Support

10G - Support


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Auto-Negotiation on Optical Fiber

For the optical fiber Ethernet, the conclusion is:

The operation modes at two ends of the link must be manually

configured (speed, duplex modes, flow-control and etc.)

The auto-negotiation of Gigabit Ethernet has been realized


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Flow Control

The function of flow control is to prevent frames from being

lost in case of congestion

Under half duplex mode, the flow control is achieved by

backpressure technology

Under full duplex mode, the flow control generally abides by

standard IEEE 802.3x


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Half Duplex Flow Control

backpressure

Pretend that there iscollision, then you will notconstantly transfer!


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PAUSE Frame

DATA Frame

Full Duplex Flow Control

IEEE802.3x standard defines a new method named PAUSE

frame, to realize the flow control in the full duplex

environment.

PAUSE frame uses a reserved multicast address whichwouldn't be repeated by bridge or switch

Stop Congestion


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Contents

1. Ethernet Concept


3. VLAN Basis


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Cause of VLAN Generation---

Broadcast Storm

broadcast


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Broadcast domain1VLAN 10



Engineering Department

Financial DepartmentMarketing Department

Prevent Broadcast Storm viaVLAN

Prevent Broadcast Storm


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MAC VLAN

MAC A 5MAC B 10

MAC C 5

MAC D 10

MAC A MAC B MAC C MAC D

Switch

Port VLAN

Port 1 5Port 3 10

Port 7 5

Port 10 10

PC A PC B PC C PC D

Switch

Port 1

Port 3Port 7

Port 10

VLAN Based on MAC or Port


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Protocol VLAN

IPX 5UDP 10

IP 2

. 11

IPX IP UDP IP

Switch

IP VLAN

1.1.1.1/24 51.1.2.1/24 10

1.1.3.1/24 2

. 11

1.1.1.1/24 1.1.2.1/24 1.1.3.1/24 2.1.1.1/24

Switch

VLAN Based on Protocol or Subnet


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Frame Format of VLAN

DA SA Type Data CRC

Standard Ethernet Frame

DA SA Type Data CRCtag

TPID Priority CFI VLAN ID

TCI

Ethernet Frame with IEEE802.IQ Flag


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VLAN 2 VLAN 1

VLAN 1 VLAN 2

Ethernet framewith VLAN tag 1

Ethernet framewith VLAN tag 2

Ethernet framewithout VLAN flag

Frame Changes in Network

Communication


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Ethernet Card Port Tag ModePackets

PortTag flag Untag flag

Tag aware (In) Transmit transparently Drop

Tag aware (Out) Transmit transparently

Access (In) DropAttach a Tag flag (default VLAN

ID)

Access (Out)Remove the Tag flag (default

VLAN ID)

Hybrid (In) Transmit transparentlyAttach a Tag flag (default VLAN

ID)

Hybrid (Out)

If VLAN ID is the same, remove

the Tag flag, otherwise transmit

transparently


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Advantage and Disadvantage of VLAN

VLAN technology resolves the broadcast problem and

enhances the communication security.

Disadvantage of VLAN:

Improve the bandwidth utilization rate by dividing a physicallyinterconnected network into several small logic networks. But

what should we do if different VLANs need to communicate

with each other?


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Questions

Please describe the working mechanism of L2 Switch?

What is the advantage of VLAN?


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Summary

Ethernet Concept

Ethernet Port Technology

VLAN Basis


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06 ethernet principle 20090724 a

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