Ethernet LANs Chapter 4 Updated January 2009 XU Zhengchuan Fudan University.

Ethernet LANs

Chapter 4Updated January 2009

XU Zhengchuan

Fudan University

4-2

Orientation

• Chapters 2 and 3 Looked at Standards

– Chapter 2: Layered standards (data link to application)

– Chapter 3: Physical layer standards

• Chapters 4-7 Deal With Single Networks

– Chapter 4: Ethernet LANs• Chapter 4a deals with obsolete Token-Ring Networks

– Chapter 5: Wireless LANs

– Chapters 6 and 7: WANs

– Flow is from LANs to WANs

4-3

Figure 4-1: A Short History of Ethernet Standards

• Ethernet

– The dominant wired LAN technology today

– Only “competitor” is wireless LANs (which actually are supplementary)

• The IEEE 802 Committee

– LAN standards development is done primarily by the Institute for Electrical and Electronics Engineers (IEEE)

– IEEE created the 802 LAN/MAN Standards Committee for LAN standards (the 802 Committee)

4-4


• The 802 Committee creates working groups for specific types of standards

– 802.1 for general standards

– 802.3 for Ethernet standards

• The terms 802.3 and Ethernet are interchangeable

– 802.11 for wireless LAN standards

– 802.16 for WiMax wireless metropolitan area network standards

4-5


• Ethernet Standards are OSI Standards

– Single networks, including LANs, are governed by physical and data link layer standards

– Layer 1 and Layer 2 standards are almost universally OSI standards

– Ethernet is no exception

– The IEEE makes 802.3 standards; ISO ratifies them

– In practice, when 802.3 finishes standards, vendors begin building compliant products

4-6

• Test Your Understanding

• P 188

• 2

Ethernet Physical Layer Standards

4-8

Figure 4-3: Baseband Versus Broadband Transmission

Baseband Transmission

Source

Signal Transmitted Signal (Same)

Transmission Medium

Signal is injected directly into the transmission medium(wire, optical fiber)

Inexpensive, so dominates wired LAN transmission technology

BASE in standard names means baseband

4-9

Figure 4-3: Baseband Versus Broadband Transmission, Continued

Broadband Transmission

SourceRadioTuner

Modulated Signal

Radio Channel

The radio tuner modulates the signal to a higher frequency. The transceiver then sends the signal in a radio channel.

Expensive but needed for radio-based networks.

Not used in Ethernet, but is used in wireless LANs (discussed in Chapter 5).

4-10

Figure 4-2: Ethernet Physical Layer Standards

UTP PhysicalLayerStandards

MediumRequired

MaximumRun

Length

Speed

100BASE-TX 4-pair Category 5 or higher100 meters100 Mbps

1000BASE-T(GigabitEthernet)

4-pair Category 5 or higher100 meters1,000 Mbps

10BASE-T 4-pair Category 3 or higher100 meters10 Mbps

100BASE-TX dominates access links today,

Although 1000BASE-T is growing in access links today

4-11

Fiber PhysicalLayerStandards

Medium850 nm light (inexpensive)Multimode fiber

MaximumRun

Length

Speed

1000BASE-SX 275 m1 Gbps




Figure 4-2: Ethernet Physical Layer Standards, Continued

62.5microns

160MHz-km

62.5 200

50 400

50 500

The 1000BASE-SX standard dominates trunk links today.

Carriers use 1310 and 1550 nm light and single-mode fiber.

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10 Gbps Ethernet

• 10 Gbps Ethernet usage is small but growing

• Several 10 Gbps fiber standards are defined, but none is dominant

Revised

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10 Gbps Ethernet

• 10 Gbps Ethernet usage is small but growing

• Several 10 Gbps 10GBASE-x fiber standards are defined, but none is dominant

• Copper is cheaper than fiber but cannot go as far– 10GBASE-CX4 (shielded Infiniband cable) up to 15 m

– UTP

• Category 6: 55 meters maximum (UTP)

• Category 6A: 100 meters (UTP)

• Category 7: 100 meters (shielded twisted pair, STP, which has metal shielding around each pair and around the cord)

Revised

4-14

100 Gbps Ethernet

• 100 Gbps has been selected as the next Ethernet speed

• 100 Gbps Ethernet standards development is just getting underway

NewInformation

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• P 193

• 3

4-16

Figure 4-4: Link Aggregation (Trunking or Bonding)

链路聚合（中继或捆绑）

1 GbpsCord1 Gbps

Cord

1000BASE-SX SwitchWe have been looking at single cords

Link aggregation or bondingallows you to bond two or more

cords between two switches

In this example, if you need 1.6 Gbps,two bonded 1 Gbps links willmeet your need at lower cost

than moving to a 10 Gbps switch.

Link aggregation allowsincremental growthin speed and cost

1000BASE-SX Switch

4-17

• 链路聚合

• IEEE 802.3ad 标准定义了如何将两个以上的千兆位以太网连接组合起来，为高带宽网络连接实现负载共享、负载平衡，以及提供更好的可伸缩性服务。由于在链路聚合技术的支持下，网络传输的数据流被动态地分布到加入链路的各个端口，因此在聚合链路中自动地完成了对实际流经某个端口的数据管理。

• 链路聚合的另一个主要优点是可靠性。链路聚合技术在点到点链路上提供了固有的、自动的冗余性。如果链路使用的多个端口中的一个出现故障，网络传输的数据流可以动态地快速转向链路中其他工作正常的端口进行传输。

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• P 194

• 4

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Figure 4-5: Data Link Using Multiple Switches

OriginalSignal

ReceivedSignal

RegeneratedSignal

Switches regenerate signals before sending them out;this removes propagation effects.

It therefore allows signals to travel farther.

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Figure 4-5: Data Link Using Multiple Switches, Continued

OriginalSignal

ReceivedSignal

ReceivedSignal

ReceivedSignalRegenerated

Signal RegeneratedSignal

Thanks to regeneration, signals can travel far acrossa series of switches

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OriginalSignal

ReceivedSignal

ReceivedSignal


SignalRegenerated

Signal

UTP UTP62.5/125Multimode Fiber

100BASE-TX(100 m maximum)

Physical Link


Physical Link

1000BASE-SX(220 m maximum)

Physical Link

Each trunk line along the way has a distance limit

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Station-to-station data link does not have a maximum distance(420 m maximum distance in this example)

OriginalSignal

ReceivedSignal

ReceivedSignal


Signal RegeneratedSignal

UTP UTP62.5/125Multimode Fiber


Physical Link


Physical Link

1000BASE-SX(220 m maximum)

Physical Link

4-24

• Answer Question (Page 196)

• 5 c

• 5 f

Ethernet Data Link (MAC) Layer Standards

802 Layering

Frame Syntax

Switch Operation

4-26

Figure 4-6: Layering in 802 Networks, Continued

TCP/IP InternetLayer Standards(IP, ARP, etc.)

Other InternetLayer Standards

(IPX, etc.)

802.2

Ethernet 802.3 MAC LayerStandard

Physical Layer

MediaAccessControlLayer

Non-EthernetMAC Standards

(802.5,802.11, etc.)

100BASE-TX

1000Base-

SX…

LogicalLink

ControlLayer

Non-EthernetPhysical

LayerStandards

(802.11, etc.)

DataLink

Layer

Internet LayerThe 802 LAN/MAN Standards Committee

subdivided the data link layer

The media access control (MAC) layerhandles details specific to a

particular technology (Ethernet 802.3,802.11 for wireless LANs, etc.)

The logical link control layerhandles some general functions:

Connection to the internet layer, etc.;Not important to corporatenetworking professionals

4-27

Figure 4-6: Layering in 802 Networks, Continued

TCP/IP InternetLayer Standards(IP, ARP, etc.)

Other InternetLayer Standards

(IPX, etc.)

802.2

Ethernet 802.3 MAC LayerStandard

Physical Layer

MediaAccessControlLayer

Non-EthernetMAC Standards

(802.5,802.11, etc.)

100BASE-TX

1000BASE-

SX…

LogicalLink

ControlLayer

Non-EthernetPhysical

LayerStandards

(802.11, etc.)

DataLink

Layer

Internet LayerEthernet only has a single MAC standard(The 802.3 MAC Layer Standard)

Ethernet has many physical layer standards (Fig. 4-2)

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Figure 4-7: The Ethernet MAC Layer Frame

Preamble (7 Octets)10101010 …

Start of Frame Delimiter (1 Octet)10101011

Destination MAC Address (48 bits)

Source MAC Address (48 bits)

Field

Preamble and Start ofFrame Delimiter

Strong repeating 10…pattern. Synchronizesreceiver’s clock withsender’s clock

Like quarterbackcalling out “Hut 1,Hut 2, Hut 3 …” tosynchronize the team

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Figure 4-7: The Ethernet MAC-Layer Frame, Continued

Preamble (7 Octets)10101010 …

Start of Frame Delimiter (1 Octet)10101011

Destination MAC Address (48 bits)

Source MAC Address (48 bits)

Field

Computers use raw48-bit MAC addresses;Humans useHexadecimal notation(A1-23-9C-AB-33-53),which is discussednext.

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Figure 4-8: Hexadecimal Notation

4 Bits(Base 2)*

Decimal(Base 10)

Hexadecimal(Base 16)Symbol

0000 0 0 hex

0001 1 1 hex

0010 2 2 hex

•With 4 bits, there are 24=16 possible symbols.•For example, 01-34-CD-7B-DF hex begins with 00000001 for 01.

0011 3 3 hex

0100 4 4 hex

0101 5 5 hex

0110 6 6 hex

0111 7 7 hex

BeginCounting atZero

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Figure 4-8: Hexadecimal Notation, Continued

4 Bits(Base 2)

Decimal(Base 10)

Hexadecimal(Base 16)Symbol

1000 8 8 hex

1001 9 9 hex

1010 10 A hex

1011 11 B hex

1100 12 C hex

1101 13 D hex

1110 14 E hex

1111 15 F hex

After 9,Count A

Through F

4-32

Figure 4-8: Hexadecimal Notation, Continued

• Converting 48-Bit MAC Addresses to Hex– Start with the 48-bit MAC Address

• 1010000110111011 …

– Break the MAC address into twelve 4-bit “nibbles”• 1010 0001 1101 1101 …

– Convert each nibble to a hex symbol• A 1 D D

– Write the hex symbols in pairs (each pair is an octet) and put a dash between each pair

• A1-DD-3C-D7-23-FF

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Figure 4-7: The Ethernet MAC Layer Frame, Continued

Length (2 Octets)

PAD

Field

Packet(VariableLength)

LLC Subheader(Usually 8

Octets)Data Field

(VariableLength)

Frame Check Sequence (4 Octets)

Data field containsA packet of variablelength

Packet is preceded inthe data field by anLLC subheader thatdescribes the typeof packet (IP, IPX, etc.)

Length field givesthe length of thedata field in octets

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Length (2 Octets)

PAD

Field



Octets)Data Field

(VariableLength)


A PAD is added if the data field is less than 46octets; length is set tomake the data field plusPAD field 46 octets;

A PAD field is notadded if data fieldis greater than 46octets long.

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Length (2 Octets)

PAD

Field



Octets)Data Field

(VariableLength)


Sender computes theframe check sequencefield value based on thebits in the other fields.

The receiver redoes thecomputation. If it getsa different results, theframe must have atransmission error.

The receiver discardsthe frame. There isno error correction.Ethernet is not reliable.

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• Answer Question (Page 201)

• 7 a

• 7 h

4-37

Figure 4-9: Multiswitch Ethernet LAN

Switch 2

Switch 1 Switch 3

Port 5 on Switch 1to Port 3 on Switch 2


A1-44-D5-1F-AA-4CSwitch 1, Port 2

E5-BB-47-21-D3-56Switch 3, Port 6

D5-47-55-C4-B6-9FSwitch 3, Port 2

B2-CD-13-5B-E4-65Switch 1, Port 7

The Situation:A1… Sends to E5…

Frame must go through3 switches along the way

(1, 2, and then 3)

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Figure 4-9: Multiswitch Ethernet LAN, Continued

Switching Table Switch 1Port Station

2 A1-45-D5-1F-AA-4C7 B2-CD-13-5B-E4-655 D5-47-55-C4-B6-9F5 E5-BB-47-21-D3-56

Switch 2

Switch 1



B2-CD-13-5B-E4-65Switch 1, Port 7


On Switch 1

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Switch 2

Switch 1 Switch 3






On Switch 2

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Switch 2

Switch 3



D5-47-55-C4-B6-9FSwitch 3, Port 2




On Switch 3

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Figure 4-10: Hierarchical Ethernet LAN

Client PC 1

EthernetSwitch F

Server YServer X

SinglePossible Path

BetweenClient PC 1

and Server Y

EthernetSwitch E

EthernetSwitch D

EthernetSwitch B

EthernetSwitch A

EthernetSwitch C

4-42

Figure 4-10: Hierarchical Ethernet LAN, Continued

• With only one possible path between stations…

– Therefore there is only one possible port on a switch to send the frame back out

– Therefore only one row per MAC address in switching table

– Switch can find the one row quickly

– This makes Ethernet switches inexpensive per frame

– Low cost has ledto Ethernet’sLAN dominance

Port Station 2 A1-44-D5-1F-AA-4C7 B2-CD-13-5B-E4-655 E5-BB-47-21-D3-56

4-43

Figure 4-10: Hierarchical Ethernet LAN, Continued

Workgroup EthernetSwitch F

CoreSwitches

WorkgroupEthernetSwitch E

WorkgroupEthernetSwitch D

Core EthernetSwitch B

Core EthernetSwitch A

Core EthernetSwitch C

Core

Workgroup Switch

As noted in Chapter 3, thereare workgroup and core switches.

Core switches need more capacity.

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Figure 4-11: Single Point of Failure in a Switch Hierarchy

No CommunicationNo Communication

Switch 1

Switch 2

Switch 3

Switch Fails

A1-44-D5-1F-AA-4C

B2-CD-13-5B-E4-65 D4-47-55-C4-B6-9F

E5-BB-47-21-D3-56

4-45

Figure 4-12: 802.1D Spanning Tree Protocol (STP)生成树协议

Switch 1

Switch 2

Switch 3

A1-44-D5-1F-AA-4C

B2-CD-13-5B-E4-65 D4-47-55-C4-B6-9F

E5-BB-47-21-D3-56

Activated

Activated

Deactivated

Normal OperationLoop, but Spanning Tree ProtocolDeactivates One Link

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Figure 4-12: 802.1D Spanning Tree Protocol (STP), Continued

Switch 1

Switch 2

Switch 3

A1-44-D5-1F-AA-4C

B2-CD-13-5B-E4-65

C3-2D-55-3B-A9-4F

D4-47-55-C4-B6-9F

E5-BB-47-21-D3-56

Deactivated Deactivated

Reactivated

Switch 2 Fails

4-47

Figure 4-12: 802.1D (STP), Continued

• Spanning Tree Protocol (STP)

– Works but when there is a break in the hierarchy, the network converges to a new hierarchy too slowly

• Rapid Spanning Tree Protocol (RSTP)

– Newer algorithm that converges very quickly

Virtual LANs (VLANs)

4-49

Figure 4-13: Virtual LAN (VLAN) with Ethernet Switches

Client A

Client B

Client C

Server D Server E

ServerBroadcast

Server Broadcasting without VLANS

Servers SometimesBroadcast; GoesTo All Stations;Latency Results

4-50

Figure 4-13: Virtual LAN (VLAN) with Ethernet Switches, Continued

Server Broadcasting with VLANS

Client Aon VLAN1

Client Bon VLAN2

Client Con VLAN1

Server Don VLAN2

Server Eon VLAN1

ServerBroadcast

NoNo

With VLANs,Broadcasts Only GoTo a Server’s VLAN

Clients; LessLatency

4-51

Figure 4-13: Virtual LAN (VLAN) with Ethernet Switches, Continued

• VLANs primarily reduce congestion due to latency

– They can also be used for security

• Only people on a server’s VLAN can reach it

– This provides some degree of security

– Not sufficient by itself, but it can help

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Figure 4-14: Tagged Ethernet Frame (Governed By 802.1Q)

Destination Address(6 Octets)

Destination Address(6 Octets)

Source Address (6 Octets)

Length (2 Octets)Length of Data Field in

Octets1,500 (Decimal) Maximum

Tag Protocol ID (2 Octets)1000000100000000

81-00 hex; 33,024 decimal.Larger than 1,500, So not

a Length Field

By lookingat the value

in the 2octets after

theaddresses,the switchcan tell ifthis frameis a basic

frame(value lessthan 1,500)or a tagged(value is 33,024).

Basic 802.3 MAC Frame Tagged 802.3 MAC Frame

Start-of-Frame Delimiter(1 Octet)

Preamble (7 octets)

Start-of-Frame Delimiter(1 Octet)

Preamble (7 octets)

Source Address (6 Octets)

4-53

Figure 4-14: Tagged Ethernet Frame (Governed By 802.1Q), Continued

Tag Control Information(2 Octets) Priority Level (0-7)

(3 bits); VLAN ID (12 bits)1 other bit

Basic 802.3 MAC Frame Tagged 802.3 MAC Frame

Length (2 Octets)

Data Field (variable)

Data Field (variable)

PAD (If Needed)

Frame Check Sequence(4 Octets)

PAD (If Needed)

Frame Check Sequence(4 Octets)

4-54

Figure 4-15: Handling Momentary Traffic Peaks with Overprovisioning and Priority

Traffic

Network Capacity

Momentary Traffic Peak:Congestion and Latency

Time

Momentary Traffic Peak:Congestion and Latency

Momentary traffic peaks usually last onlya fraction of a second;

They occasionally exceed the network’s capacity.When they do, frames will be delayed, even dropped.

4-55

Figure 4-15: Handling Momentary Traffic Peaks with Overprovisioning and Priority, Continued

Traffic

Overprovisioned Network Capacity Momentary Peak:No Congestion

Time

Overprovisioned Traffic Capacity in Ethernet

Overprovisioning:Build high capacity than will rarely if ever be exceeded.

This wastes capacity.But cheaper than using priority (next)

4-56

Figure 4-15: Handling Momentary Traffic Peaks with Overprovisioning and Priority, Continued

Traffic

Network Capacity

MomentaryPeak

Time

Priority in Ethernet

High-Priority Traffic GoesLow-Priority Waits

Priority:During momentary peaks, give priority to

traffic that is intolerant of latency (delay), such as voice.No need to overprovision, but expensive to implement.

Ongoing management is very expensive.

4-57

• Answer Question

• 13 c, page 210

• 14 c d e f, page 213

Box:Hubs and Switches

4-59

Figure 4-16: Hub Versus Switch Operation

A B C D

EthernetSwitch

An Ethernet SwitchSends Frame Out One Port

If A Is Transmitting to C,B Can Transmit to D

Simultaneously

Box

Today, All Corporations Use Ethernet Switches

4-60

Figure 4-16: Hub Versus Switch Operation, Continued

A B C D

EthernetHub

A Hub Broadcasts Each BitOut All Other Ports.Simple and Cheap

---But If A Is Transmitting,B Must Wait to Transmit

---In Large Hub Networks,Delays Are Intolerable

X Box

Years Ago, Corporations Used Ethernet Hubs

4-61

Figure 4-16: Hub Versus Switch Operation, Continued

• Hubs Need Media Access Control– This limits when a station may transmit– Ethernet NICs must use CSMA/CD with hubs

• Carrier Sense Multiple Access (CSMA)– Only transmit if no other station is transmitting– Otherwise, wait

• With Collision Detection (CD)– If two NICs transmit at the same time, this is a collision

– Both will stop, wait a random amount of time, and the go back to CSMA to send again

Box

4-62

Figure 4.11: Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

载波侦听多路访问 /冲突检测 • 1. Carrier Sense Multiple Access (CSMA)

– If a NIC wishes to transmit, it must listen for traffic

• If there is no traffic, the NIC may transmit

• If there is traffic, the NIC must wait to transmit until no traffic is being transmitted; then it may send

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• 2. Collision Detection (CD)

– If there is a collision (by two or more stations transmitting at the same time),

• All NICs stop transmitting and wait for a random amount of time

• The first NIC that finishes its wait my transmit– but only if there is no traffic!

– If there is traffic, the NIC must wait until there is no traffic

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• 3. Collision Detection (CD)

– If there are multiple collisions,

• The random wait is increased each time

• After 16 collisions, the sending NIC discards the frame

4-65


• CSMA/CD Recap

– Hubs do not implement it when talking to stations

– Switches do not implement it when talking to stations

– NICs implement it

CSMA/CD

4-66


• Recap: Three basic elements for describing CSMA/CD

– CSMA—transmit only if line is clear; waiting otherwise

– CD—handling a collision

– CD—Handling of multiple collisions

test

4-67

Network Interface Cards (NICs)

PC Card NICInternal NIC

4-68

Network Interface Card (NIC)

RJ-45Jack

PCI Connector Pins

4-69

Motherboard

• Intel D850MV Desktop BoardPCI Slots

For Expansion Boards(NICs, etc.)

Slots for RAM

Slot forMicroprocessor

(Pentium 4)

4-70

Internal NICs Fit into Motherboard Slots

4-71

Figure 4.12: Logical Link Control (LLC) Layer

逻辑链路控制层• 802.2 LLC Standard is Used in All 802 LANs

– All MAC layer standards interact only with 802.2 at the layer above them

– All internet layer standards interact only with 802.2 at the layer below them

LLC 802.2

Ethernet MAC Wireless MAC

IP IPX

4-72

Figure 4.12: Logical Link Control (LLC) Layer

• Optional Error Correction

– Error detection, and retransmission

– Almost never used

– NICs rarely allow user to control LLC functionality

4-73

Ethernet Frame Organization Including LLC

• Here is how Ethernet frames are organized, including the LLC header

EthernetHeader

802.2LLC

Header

IP or OtherPacket

EthernetTrailer

Ethernet Data Field: LLC Frame

test

Purchasing Switches

4-75

Figure 4-17: Switch Purchasing Considerations

• Number and Speeds of Ports

– Buyers must decide on the number of ports needed and the speed of each

– Buyers often can buy a prebuilt switch with this configuration

4-76

Figure 4-18: Switching Matrix

1

2

3

4

100 Mbps

100 Mbps

100 Mbps

100 Mbps

1 2 3 4

Port 1to

Port 3400 MbpsAggregateCapacity

to BeNonblocking

InputQueue(s)

100BASE-TXInput Ports

100BASE-TXOutput Ports

Any-to-AnySwitching

Matrix

Note: Input Port 1 and Output Port 1 are the same port.Aggregate switching matrix capacity is its total switching speed.

Maximum input for this switch is 400 Mbps (4 x 100 Mbps).400 Mbps aggregate capacity is needed for switch to be nonblocking

4-77

Figure 4-17: Switch Purchasing Considerations, Continued

• Store-and-Forward Versus Cut-Through Switching (see Figure 4-19)

– Store-and-forward Ethernet switches read whole frame before passing the frame on

– Cut-through Ethernet switches read only some fields before starting to pass the frame back out

– Cut-through switches have less latency, but this is rarely important

4-78

Figure 4-19: Store-and-Forward Versus Cut-Through Switching

Preamble

Start-of-Frame Delimiter

Destination Address

Source Address

Tag Fields if Present

Length

Cyclical Redundancy Check

Data (and Perhaps PAD)

2.Cut-Through BasedOn MAC DestinationAddress (14 Octets)

3..Cut-Through forPriority or VLANs(24 Octets)

4.Cut-Through at64 Bytes (Not a Runt)

1.Store-and-ForwardProcessingEnds Here(OftenHundredsOf Bytes)

4-79

Figure 4-20: Managed Switches

Manager

Command to ChangeConfiguration(can fix many

problems remotely)

Get Data

Data Requested

Managed Switch

Managed Switch

Manager can manageall switches remotely

Managed switchescost much morethan unmanaged

switcheds

4-80

Ethernet Security

• Port-Based Access Control (802.1X)

– Attackers on site can walk up to any Ethernet port and plug in a computer, bypassing the firewall

– 802.1X standard

• Computer attaching to a port must first authenticate itself. (More details in Chapter 5) or be rejected.

No AccessWithout

Authentication

4-81

Ethernet Security

• MAC Security (MACsec) 802.1AE

– Switches must talk to one another for STP, VLANs, and other supervisory protocols

– An attacker on a PC can pretend to be a switch and send false supervisory messages

– 802.1AE MACsec protects supervisory communication, preventing many types of attacks

Stops FakeMessage

PC impersonatinga switch

False SupervisoryMessage

Box: Advanced Switch Purchase Considerations

Physical and Electrical Features

Box

4-83

Figure 4-21: Physical and Electrical Features

• Physical Size

– Switches fit into standard 19-in (48-cm) wide equipment racks

– Switch heights usually are multiples of 1U (1.75 in or 4.4 cm)

19 inches(48 cm)

Box

4-84

Figure 4-21: Physical and Electrical Features, Continued

• Port Flexibility

– Fixed-port switches

• No flexibility: the number of ports is fixed

• 1 or 2 U tall

• Most workgroup switches are fixed-port switches

Box

4-85



– Stackable Switches

• Fixed number of ports

• 1 or 2 U tall

• High-speed interconnect bus connects stacked switches

• When demand increases, firm can simply add a new stackable switch

Box

4-86



– Modular Switches

• 1 or 2 U tall

• Contain one or a few slots for modules

• Each module usually contains 1 to 4 ports

Box

Module

4-87



– Chassis switches

• Several U tall

• Contain several expansion slots

• Each expansion board contains several slots

• Most core switches are chassis switches

Box

4-88


• Switch and NIC Ports

– Normal Ethernet RJ-45 switch ports transmit on Pins 3 and 6 and listen on Pins 1 and 2

– NICs transmit on Pins 1 and 2 and listen on Ports 3 and 6

Box

NormalPC NIC

Port

NormalSwitch

Port

Pins1 & 2 Pins

3 & 6

4-89


• Switch and NIC Ports

– If you connect two normal ports on different switches via UTP cords, BOTH will send on Pins 3 & 6 and neither will listen on Pins 3 & 6

• Communication will be impossible

Box

NormalSwitch

Port

NormalSwitch PortOn Parent

Switch

Pins3 & 6

Pins3 & 6

4-90


• Switch Uplink Ports

– On a growing number of switches, normal ports change automatically to uplink ports if used that way

Box

NormalSwitch

Port


Switch

Pins3 & 6

1.Normally

Transmits on Pins 3 & 6

2.Changes automatically

to Pins 1 & 2

4-91


• Crossover Cables

– Designed to connect ordinary ports on two switches

– Internally, connect Pins 1 & 2 on one machine to Pins 3 & 6 on the other switch

– Do NOT use to connect NICs to switches or a switch uplink port to another switch!

Box /New

NormalSwitch

Port


SwitchPins3 & 6

Pins1 & 2

CrossoverCable

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• Electrical Power

– Under the 802.3af standard, switches can provide electrical power to devices over the UTP cord

– Currently limited to 12.95 watts; sufficient for most wireless access points (Chapter 5) and voice over IP telephones (Chapter 6) but not sufficient for computers

– New slightly higher-power version of the standard is being developed to be able to serve sophisticated access points; still not good enough for computers.

Box

Topics Covered

4-94

Topics Covered

• Ethernet Standards Setting– 802.3 Working Group

– Physical and data link layer standards

– OSI standards

• Physical Layer Standards– BASE means baseband

– 100BASE-TX dominates for access lines

– 10GBASE-SX dominates for trunk lines

– Link aggregation for small capacity increases

– Regeneration to carry signals across multiple switches

4-95

Topics Covered

• Ethernet MAC Layer Standards– Data link layer subdivided into the LLC and MAC layers– The Ethernet MAC Layer Frame

• Preamble and Start of Frame Delimiter fields• Destination and Source MAC addresses fields

– Hexadecimal notation• Length field• Data field

– LLC subheader– Packet– PAD if needed

• Frame Check Sequence field

4-96

Topics Covered

• Ethernet MAC Layer Standards– Switch operation

• Operation of a hierarchy of switches

– Single possible path between any two computers

– Hierarchy gives low price per frame transmitted

– Single points of failure and the Spanning Tree Protocol

• VLANs and frame tagging to reduce broadcasting

• Momentary traffic peaks: addressed by overprovisioning and priority

• Hubs and CSMA/CD

4-97

Topics Covered

• Switch Purchasing Considerations

– Number and speed of ports

– Switching matrix (nonblocking)

– Store-and-forward versus cut-through switches

– Managed switches

– Ethernet security

• 802.1X Port-Based Access Control

• 802.1AE MACsec

Ethernet LANs Chapter 4 Updated January 2009 XU Zhengchuan Fudan University.

Documents

Transcript of Ethernet LANs Chapter 4 Updated January 2009 XU Zhengchuan Fudan University.