Larger Site Networks Chapter 8 Copyright 2001 Prentice Hall Revision 2: July 2001.

Larger Site Networks

Chapter 8

Copyright 2001 Prentice HallRevision 2: July 2001

2Hubs

Single-hub or single-switch LAN 200 meter max distance between

farthest stations with UTP

100 m

100 m

X

Y

200 m

3Multiple Hubs in 10Base-T

Farthest stations 10Base-T can be 5 segments 100 meters per segment Separated by four hubs

100m

100m

100m

100m

100m

500m, 4 hubs

10Base-T hubs


No loops allowed Only one possible path between any two

stations

No LoopsA

B

C1

2

34

5

6AB=1,2,3,4,5AC=1,2,3,4,6BC=5,4,6

No!


Practical Limit in 10Base-T is Number of Stations

Degradation of service beyond 100 stations

Unacceptable service beyond 200 stations

Maximum possible span normally embraces more than 200 stations

6Multiple Hubs in 100Base-TX

Limit of Two Adjacent Hubs in 100Base-TX & one hub with Gigabit Hub Must be within a few meters of each

other Maximum span is 200 meters Shorter maximum span than 10Base-T

100m

100m2 Collocated

Hubs

100Base-TXHubs

~200 m

7Ethernet Switched Networks

No Limit in Number of Switches Between the Farthest Stations (No maximum distance span)

Limit between pairs of switches - UTP 100 m, optical : longer distance

No Limit OnNumber of

Switches or Span

EthernetSwitch

8Hierarchies

Ethernet Switches Must be Hierarchical (no loops) Usually, Fastest Switches are at the Top

(Root)Gigabit

EthernetCampusSwitch

100Base-XBuilding Switch

10Base-TWorkgroupSwitch/Hub

Root

9Hierarchies

Single Possible Path (2,1,3,4) Between 2 Stations Single Points of Failure

Switch or Link (trunk line between switches)Divide the network into pieces

EthernetSwitch

A

1 34

5

B

2

X X

10Hierarchies

802.1D Spanning Tree Allows Redundant Links Automatically deactivated to prevent

loops Reactivated if there is a failure

DeactivatedRedundant

Link

EthernetSwitch

11Hierarchies

Link Aggregation Protocol Allows Multiple Links Between Stations If one link fails, others continue Switch failures or cuts of all links still

fatal

MultipleLinks

EthernetSwitch

12Hierarchies

Single Possible Path & Switch Forwarding Tables Frame MAC address = single possible

output port Switch sends frame out that port Switch forwarding table has only one row

for each MAC address

EthernetSwitch Address

A3..B2..

Port35

13Hierarchies

Ethernet switch only has to find the single row that matches the destination MAC address

Only has to examine half the rows on average; less if the table is alphabetized

Comparison at each row is a simple match of the frame and row MAC addresses; much less work that row comparison in routers

Switches work much faster than routers & are lessexpensive

AddressA3..B2..

Port35

14Switch Learning

Situation: Switch with NIC A1-33-B6-47-DD-65 (A1) on Port 1 NIC BF-78-C1-34-17-F4 (BF) on Port 2 NIC C9-34-78-AB-DF-96 (C9) on Port 5

Switch Forwarding Table is Initially Empty

Address Port

A1 BF C9

EthernetSwitch

At Start

15Switch Learning

A1 on Port 1 Sends to C9 on Port 5 Switch does not know port for C9 Broadcasts the frame, acting as a hub Notes from source address that A1 is on Port 1 Adds this information to switch forwarding

table

AddressA1

Port1

A1 BF C9

EthernetSwitch

After Transmission

16Switch Learning

C9 on Port 5 Sends to A1 on Port 1 Table shows that A1 is on Port 1 Switch only sends out Port 1: No broadcast! Source address shows that C9 is on Port 5 Switch adds this information to forwarding

table

AddressA1C9

Port15

A1 BF C9

EthernetSwitch

After Transmission

17Switch Learning

Every Few Minutes, Forwarding Table is Erased To eliminate obsolete information Relearning is very fast

Address Port

A1 BF C9

EthernetSwitch

Erased

18Switch Learning

Switches Can be in Hierarchy Switches only learn that stations are out

certain ports Do not Learn of switch in Between

A1 BF C9

AddressA1BFC9

Port111

Port1

Switch A

Switch B

19Switch Purchasing Decisions

Maximum Number of MAC address-port entries

Small switches may store many MAC addresses

Unknown addresses require broadcasting

Queue Size Incoming frames are placed in queues if

they cannot be processed immediately Small queues result in lost framesSwitch

MatrixQueues

OutputPorts

InputPorts

Frames


Switching Matrix Receives input from multiple input ports Switches frame to the correct output

portSwitching Matrix Aggregate Throughput

Maximum number of bits it can switch/second

Nonblocking: aggregate throughput = no. ports X port speed

All ports receive input simultaneously with no delay

QueuesOutputPorts

InputPorts

FramesSwitch Matrix


Reliability through Redundancy Redundant power supplies and cooling fans

May even have redundant switch matrix for backup

Manageability Managed remotely from network administrator’s

deskNetwork administrator can check on status of switch

Network administrator can modify how the switch functions

Remote management greatly reduces labor

22Ethernet Virtual LANs

Hubs versus Switches Hubs broadcast bits out all ports Switches usually send a frame out a one

port

Unicasting: message intended for one destination Switches assume unicasting


Broadcasting

Broadcasting is occasionally required

Ex: servers send advertisement of their presence

Broadcasting with Ethernet Switches Switch turns single-point delivery off Broadcaster sets destination MAC

address to ones Can create congestion

24Ethernet Virtual LANsMulticasting: messages intended some

stations Ex: from a server only to the clients it

serves If Ethernet switches can implement

multicasting, traffic overload would be avoided

MulticastFrame

25Ethernet Virtual LANsEthernet switches do implement

multicasting A server and its clients are treated as

(VLAN) Can only communicate among

themselves

Frame

MarketingVLAN Server

MarketingVLAN Client


VLAN Benefits

VLANs reduce traffic on the switched network

Other benefits

Provide weak security:clients cannot reach all servers (easily defeated but good first line of defense)

Ease of management: user changes organizational membership, VLAN membership is easily changed centrally


VLAN Problems

VLANs have not been standardized

A network of switches from different vendors cannot implement VLANs

Standardization is beginning

Using tagging (Chapter 7)

Tag Control Information field has a 12-bit VLAN ID (VID) number, allowing 212 VLANs to be identified

28When are Frames Forwarded?

Cut-Through Ethernet Switches Forward after seeing only part of a

frameMinimum is destination address May need to see tag fields for priority, VLANMay wait until 46 octets of data plus PAD

Fast operation-less latency

PreSFDDASALenDataPADFCS

Forward the Frame

29When are Frames Forwarded?

Store-and-Forward Ethernet Switches Forwarded only after receiving full

frame Allows error checking (CRC field) Brief latency

Hybrid Ethernet Switches Start in cut-through mode but check

errors If many errors, go to store-and-forward

mode PreSFDDASALenDataPADFCS

Forward the Frame

30Bad Switch Organization

One Server for All Clients All traffic goes to and

from server Bottlenecks: no simultaneous

conversations No major benefits compared

to hub

Multiple Servers for Clients Allows simultaneous

conversations Brings switching’s

main benefit

BottleneckEthernetSwitch

EthernetSwitch

31The Peak Load Problem

Capacity Sufficient Most of the Time Otherwise, get bigger switches and trunk lines!

Brief Traffic Peaks can Exceed Capacity Frames will be delayed in queues or even lost

if queue gets full

CapacityTrafficPeak

32Overprovisioning

Overprovisioning: Install More Capacity than Will be Needed Nearly All of the Time Wasteful of capacity Cheapest solution today because of its

simplicity

Overprovisioned Capacity

TrafficPeak

33Priority

Assign Priorities to Frames High priority for time-sensitive applications

(voice) Low priority for time-insensitive applications

(e-mail) In traffic peaks, high-priority frames still get

through Low-priority applications do not care about a

brief delay for their frames High-PriorityFrame Goes

Low-Priority FrameWaits Briefly

34Priority

Standardizing Priority 802 Tag Fields are standardizing priority for

Ethernet and other 802 LAN technologies Priority being standardized by the IETF for IPv4 &

IPv6 (Diffserv for differentiated services) 802 and IETF are harmonizing efforts for end-to-

end priority

High-PriorityFrame Goes

Low-Priority FrameWaits Briefly

35Full Quality of Service (QoS)

Quality of Service (QoS) Makes Quantitative Promises for service

Reserves capacity Capacity wasted for bursty

transmissionsNon guaranteed traffic will not benefit &

may be lost voice traffic: strong guarantees data traffic: low or no guaranteesHigh Guarantee

Reserved CapacityLow or No Guarantee

36Traffic Shaping

Traffic Shaping recognizes that congestion is beginning, acts to stop it

Switch Tells Some Sources to Slow or Stop if Congestion is Beginning, based on Policies

Source A

Source BNetwork

Slow or Stop

Continue

37ATM Switches

Asynchronous Transfer Mode

Basic Standards Set by ITU-T Partner with ISO in OSI standards ATM standards developed within OSI

architecture

ATM Forum Sets Detailed Standards Group of mostly ATM vendors Moves quickly Also tests for interoperability

38ATM Switches

Cells of fixed length frame (5 octet header, 48 octet payload)

Small cell reduces latency at each switchOverhead = all bits but payload (5 of 53 or

10%) Not efficient use of transmission capacity

Highly Scalable Comparable to Ethernet

Very sophisticated Offers quality of service guarantees Very expensive to purchase and manage

39ATM Switches

Unfortunately, very expensive Usually cheaper to use high-capacity

Ethernet switches with overprovisioning, so that latency does not grow to the point where QoS is critical

40ATM QoS Categories

ATM Offers Varying Levels of QoS

Parameters Peak cell rate (maximum burst speed) Maximum burst size (bits per burst) Sustainable cell rate (always allowed) Cell Delay Variation Tolerance (CDVT):

how exact cell-to-cell timing is; Critical for voice and video

Cell Loss Ratio: Losses during transmission


For Voice and Video ITU-T Class A ATM Forum Service Category: Constant

Bit Rate (CBR) Low latency Low Cell Delay Variation Tolerance


For IP and LAN Data ITU-T Class D Several ATM Forum Service Categories

Available bit rate (ABR): send if capacity is available

Unspecified bit rate (UBR):, but can get almost no share of capacity

Guaranteed frame rate (GFR) gets roughly fair share of capacity during congestion


For Videoconferencing ATM: Class B ATM Forum Service Category: Variable

Bit Rate-Real Time (VBR-RT)Momentary bandwidth increase for burst of

screen motion Needs Low Cell Delay Variation

Tolerance

For Connection-Oriented Data ATM: Class C ATM Forum Service Category: Variable

Bit Rate-Not Real Time (VBR-NRT) Most data not connection-oriented

44ATM Switches: Virtual Circuits

Often Arranged in a Mesh

Virtual Circuits: Single Possible Path between Any Two Stationssimplify switch operation and lower switch

cost

VirtualCircuit

ATMCell

45ATM Switches

Permanent Virtual Circuits (PVCs) Set up once, for each pair of sites Simplest and least expensive

administratively because rarely changed Most widely used form of virtual circuit

Switched Virtual Circuit (SVC) Set up at time of use Flexible but expensive

46ATM Switches

ATM Frame Header

Does NOT have a destination address field

Instead, has two fields that together contain a hierarchical virtual circuit number

Has Virtual Circuit Number & Port in forwarding table

Virtual Circuit NumberATM Header

47ATM Switches

Hierarchical Virtual Circuit Number Virtual Path Identifier

Higher-level number; Often specifies a site Virtual Channel Identifier

Lower-level number; Often specifies a computer at a site

All traffic between two sites can be given the same VPI numberBut difference VCI values

Switch needs only one VPI table entry for all this trafficDramatically reduces number of table entries in

switches between sites and therefore makes lookups very fast

48Switches Versus Routers

Switches

Fast

Inexpensive

No benefits of alternative routing

Routers

Slow

Expensive

benefits of alternative routing

“Switch where you can; route where you must”

49Early Site Networks

Organization LANs (subnets) based on hubs Routers link hubs Hierarchy of Routers

Router

Hub

50The Switching Revolution

Switches Push Routers to the Edge Switches replace most routers in site networks Because switches are cheaper than routers Routing’s sophistication is still needed at the edge

Layer 3 Switches Traditional switches operate at Layer 2; Switch

based on MAC addresses Layer 3 switches: based on internet layer IP

addresses Layer 3 switches are replacing many Layer 2

switchesExternal

Switch

Router


Layer 3 Switches versus Routers Layer 3 switches are much faster than routers

Layer 3 switches cost less than routers

Internet layer: Layer 3 switches normally only support IP and sometimes IPX; Routers route many more internet layer protocols, including those of AppleTalk, SNA, and others

At the data link layer, Layer 3 switches normally support only Ethernet on LANs. Routers support many Layer 2 LAN protocols.


Layer 3 Switches versus Routers Layer 3 switches rarely support Layer 2 WAN

protocols

Routers usually are still needed at the edge of the site network, to communicate with external links

External

Layer 3Switch


Routers

Forward based on IP addresses and other internet layer addresses

Expensive and slow

Handle multiple internet layer protocols

Handle multiple LAN and WAN subnet protocols

Layer 3 Switches

Forward based on IP addresses, sometimes IPX addresses

Inexpensive and Fast

Do not handle multiple internet layer protocols

Do not handle multiple LAN and WAN subnet protocols


Layer 4 Switches Examine port fields in TCP and UDP

These fields describe the application

Therefore, can switch based on application (to give priority by application, etc.)

Layer 4Switch

Larger Site Networks Chapter 8 Copyright 2001 Prentice Hall Revision 2: July 2001.

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Transcript of Larger Site Networks Chapter 8 Copyright 2001 Prentice Hall Revision 2: July 2001.