Larger Site Networks

Part 1

2

• Small Site– Single-hub or Single-

Switch Ethernet LANs

• Large Site– Multi-hub Ethernet

LANs

– Ethernet Switched Site Networks

– Congestion, Latency, and solutions

– ATM LANs

– Routers, Layer 3, and Layer 4 Switches

Multi-hub LANs•Multiple hubs

•Multiple hubs in 10Base-T

•Multiple hubs in 100Base-TX

•Multiple hubs in Gigabit Ethernet

4

Hubs

• Small LANs– Single-hub or single-switch LAN– 200 meter maximum distance span between

farthest two stations with UTP

100 m

100 m

X

Y

200 m

5

Hubs

• Large LANs– Multiple-hub LANs– To increase maximum

distance span

100 m

100 m

100 m

6

Two Hubs in 802.3 10Base-T

• 1. Station X transmits bit to Hub A

– Hub operates at the physical layer (one bit at a time)

• 2. Hub A broadcasts bit out all ports

A

B

X

Y

7

Two Hubs in 802.3 10Base-T

• 3. Uplink Port sends bit to Hub B

– Uplink ports aremarked by an “X”

• 4. Hub B broadcasts bit to all attached stations, including Y

• Note that all stations on both hubs receive the bit broadcast almost simultaneously

A

B

X

Y3

UplinkPort

8

Multiple Hubs in 10Base-T

• Farthest stations in 10Base-T can be five segments (500 meters apart)

– 100 meters per segment

– Separated by four hubs

100m

100m

100m

100m

100m

500m, 4 hubs

10Base-T hubs

9

Multiple Hubs in 10Base-T

• No loops allowed– Only one possible path between any two

stations

No LoopsA

B

C1

2

3 4

5

6AB=1,2,3,4,5AC=1,2,3,4,6BC=5,4,6First two havetoo many hubs

No!

10

Multiple Hubs in 10Base-T

• No loops allowed– If hub or link fails, network is divided

No LoopsA

B

C1

2

3 4

5

6

No!

11

Multiple Hubs in 10Base-T

• 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

– In 10Base-T, the number of stations is the real limit to distance spans

– Still, it is possible to have a LAN with more than a 200 meter maximum span

12

Multiple Hubs in 100Base-TX

• Limit of Two Hubs in 100Base-TX– Must be within a few meters of each other– Maximum span is 200 meters– Shorter maximum span than 10Base-T

100m

100m2 CollocatedHubs

100Base-TXHubs

~200 m

13

Multiple Hubs with 1000Base-T

• Limit of One Hub in Gigabit Ethernet

– Maximum span is 200 meters

– Same limit as 100Base-TX

– Shorter maximum span than 10Base-T

100m

100m

14

Multiple Hubs in Perspective

• 10Base-T Hubs– 500 meter maximum distance span with UTP– Farther with some optical fiber links– However 10Base-T is limited by the number of

stations it can support– So the maximum practical distance span is really

much smaller

• 100Base-TX Hubs and Gigabit Ethernet Hubs– 200 meter maximum distance span

Switched Ethernet Site Networks

•No Maximum Distance Spans

•Hierarchies and Single Possible Paths

•High Speeds and Low Prices

16

Ethernet Switched Networks

• There are Distance Limits Between Pairs of Switches– 100 meters with UTP– Longer with optical fiber

• But There is No Limit on the Number of Switches Between the Farthest Stations– So there is no maximum distance span

Maximum Separation100 m with UTP

Longer with optical fiberEthernetSwitch

17

Hierarchies

• Ethernet Switches Must be Arranged in a Hierarchy– Root is the top-level

• Usually, Fastest Switches are at the Top (Root)– Sizes given are only examples

GigabitEthernet

CampusSwitch

100Base-XBuilding Switch

10Base-TWorkgroup

Switch

Root

18

Hierarchies

• Only a Single Possible Path (2,1,3,4) Between Any Two Stations

Single PossiblePathEthernet

Switch

A

13 4

5

B

2

19

Hierarchies

• Vulnerable to Single Points of Failure– Switch or Link (trunk line between switches)– Divide the network into pieces

X XEthernetSwitch

20

Hierarchies

• 802.1D Spanning Tree Allows Redundant Links– Automatically deactivated to prevent loops

DeactivatedRedundant

Link

EthernetSwitch

21

X

Hierarchies

• 802.1D Spanning Tree Allows Redundant Links– Automatically reactivated in case of failure– Slow and not completely effective

ReactivatedRedundant

Link

EthernetSwitch

Failure

22

Hierarchies

• 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

23

Hierarchies

• Single Possible Path Simplifies Switch Forwarding Decisions– When frame arrives, only one possible output

port (no multiple alternative routes to select among)

– Switch sends frame out that port

SimpleForwarding

Decision EthernetSwitch

24

Hierarchies

• Switches allow only a single path for each MAC destination address– Associated with a single port on each switch– So switch forwarding table has one and only

one row for each MAC address

EthernetSwitch

AddressA3..B2..

Port35

25

Hierarchies

• 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

– Overall, this is much less work than routers must do

AddressA3..B2..

Port35

26

Hierarchies

• Overall, then, Ethernet switch forwarding is much simpler than router forwarding – So Ethernet switches are both cheaper and

faster than routers

SimpleForwarding

DecisionEthernetSwitch

27

Hierarchies

• Router networks are meshes, allowing multiple alternative routes to the destination host

– Each alternative route is represented by a row in the router forwarding table

– Router must evaluate each row for each packet

– For each row, may have to compute match length, and metric

– After looking at all rows, must choose the best alternative route

More on Switched Ethernet•Switch Learning

•Purchase Considerations

•VLANs

•Intelligent Switched Network Design

29

Switch Learning

• Switch Forwarding Table has Address-Port Pairs

• Manual Entry is Too Time Consuming– Many addresses– Addresses change

• Solution: Learn addresses automatically

AddressA3..B2..

Port35

30

Switch 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

31

Switch 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

32

Switch 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: Acts like a switch!– Source address shows that C9 is on Port 5– Switch adds this information to forwarding table

AddressA1C9

Port15

A1 BF C9

EthernetSwitch

After Transmission

33

Switch Learning

• Every Few Minutes, Switch Erases Switch Forwarding Table– To eliminate obsolete information– Relearning is very fast

Address Port

A1 BF C9

EthernetSwitch

Erased

34

Switch 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

35

Switch Purchasing Decision

• Hub Purchases are Simple– Number of Ports and Port Speeds

• Switch Purchases are More Complex– Port speed– Number of ports– Maximum number of MAC-Port pairs in forwarding

table– Queue sizes– Switching matrix aggregate throughput

• Blocking or nonblocking

– Reliability– Manageability