Ethernet Switching.ppt

7/25/2019 Ethernet Switching.ppt

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Module 8Ethernet Switching



2Version 3

Ethernet Switching

• Ethernet is a shared media

– One node can transmit data at a time

• More nodes increases the demands on the

available bandwidth – The probability of collisions increases, resulting in

more retransmissions

• A solution to the problem is to segment

• Segmenting creates more collision domains



3Version 3

Shared Media Environment

• Shared media environment –

– multiple hosts have access to the same medium

• Extended shared media environment – – !sing networ"ing devices e#tends the environment

to accommodate multiple access or longer cable

distances

• Point-to-point network environment –

– one device is connected to only one other device$e# dialup networ" connections%



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Shared media environments



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Layer !evices

• &ayer ' devices – repeaters and hubs

• E#tend collision domains

• (rimary function is e#tending cable segments

• Additional hosts increase the amount of traffic

• More traffic ) greater chances of collisions

– "his results in diminished per#ormance



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$epeater $ule

• *our repeater rule+ – o more than four repeaters between any

two computers

– -ontributing *actors• .epeater latency

• (ropagation delay

• /- latency

– &ate collision frames add delay that is

referred to as consumption delay


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%ollision !omains

• -ollision 0omains

– -onnected physical networ" segments

where collisions can occur

• -ollisions cause+

– The networ" to be inefficient – Transmissions to stops for a period of time


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%ollision domains



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%ollision !omains

• The types of devices that interconnect themedia segments define collision domains

• -lassified as OS/ &ayer ', 1 or 2 devices

• &ayer ' devices do not brea" up collisiondomains

• &ayer 1 and &ayer 2 devices brea" up

collision domains – /ncreasing the number of collision domains is

"nown as segmentation



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Segmentation



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&etwork segment



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Layer ' !evices

• &ayer 1 devices

– 3ridges and Switches

– Segments collision domains

– -ontrols frame propagation using the MA-

address

– Trac"s the MA- addresses and segment they are

on



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Layer ' (ridging



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(ridges

• 4as only two ports and divides a collision domaininto two parts

• Entire networ" will share the same logicalbroadcast address space

• -reates more collision domains but will not addbroadcast domains

• All decisions made are based on MA- or &ayer 1addressing

• o effect on the logical or &ayer 2 addressing



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Layer ' Switching



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Switches

• A switch is a fast, multi5port bridge

• Each port creates its own collision domain

• A switch dynamically builds and maintains a -ontent5

Addressable Memory $-AM% table

• The -AM holds all of the necessary MA- information

for each port



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Switch )peration

• Micro5segments consist of the switch port and thehost connected to it

• -ommunication in both directions at once is "nown

as full duple#

• Most switches are capable of supporting full duple#,

as are most networ" interface cards $/-s%

• /n full duple# mode, there is no contention for

the media – A collision domain no longer e#ists

– Theoretically, the bandwidth is doubled when

using full duple#



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Switch Modes

• -ut5through switching

– A switch transfers the frame as soon as the

destination MA- address is received

– lowest latency

– no error chec"ing



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Switch Modes

• Store5and5forward switching

– 4igher latency

– The switch receives the entire frame beforesending it out

– 6erifies the *rame -hec" Sum $*-S%

– /nvalid frames are discarded at the switch



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Switch Modes

• *ragment-#ree switching• + compromise ,etween cut-through and

store-and-#orward switching

• Switching ,egins ,e#ore the entire data #ield

and checksum are read

• $eads the #irst . ,ytes

• /ncluding the #rame header

• Veri#ies the relia,ility o#0

• +ddressing

• Logical Link %ontrol 1LL%2 protocol



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Switch Modes

• Synchronous switching – The source port and destination port must

be operating at the same bit rate

• Asynchronous switching – The bit rates are not the same

– The frame must be stored at one bit ratebefore it is sent out at the other bit rate

– Store5and5forward must be used



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Switch Modes

• Asymmetric switching

– Switched connections between ports of unli"e

bandwidths

– Asymmetric switching is optimi7ed for client8server

– A server re9uires more bandwidth dedicated to the

server port to prevent a bottlenec" at that port



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Spanning "ree Protocol

• Switching loops can lead to broadcast storms that willoverwhelm a networ"

• To counteract loops, switches are provided with the

Spanning5Tree (rotocol $ST(%

• Switches in a &A using ST( – Send 3ridge (rotocol 0ata !nits $3(0!s% out all its

ports

– &ets other switches "now of its e#istence

– Elect a root bridge $switch% for the networ"

– Switches use the Spanning5Tree Algorithm $STA% to

resolve and shut down the redundant paths



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S"P

• Each port using Spanning5Tree (rotocol is inone of the following five states+

– 3loc"ing

– &istening

– &earning

– *orwarding

– 0isabled



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S"P

• A port moves through five states as follows+

– *rom initiali7ation to bloc"ing

– *rom bloc"ing to listening or to disabled

– *rom listening to learning or to disabled

– *rom learning to forwarding or to disabled

– *rom forwarding to disabled

• .esolving and eliminating loops creates a logicalhierarchical tree with no loops

• The alternate paths are available if needed



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Spanning tree protocol



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Layer ' (roadcasts

• Ethernet 3roadcasts

– :hen a node needs to communicate with all

hosts on the networ"

– A broadcast frame with a destination MA-address ;#************ is sent

– The networ" interface card $/-% of everyhost must respond



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Layer ' (roadcasts

• &ayer 1 devices must flood all broadcast andmulticast traffic

• 3roadcast .adiation

– The accumulation of broadcast and multicasttraffic from each device

• 3roadcast storm

– -irculation of broadcast radiation that saturates

the networ"

– There is no bandwidth left for application data



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Layer ' (roadcasts

• The three sources of broadcasts andmulticasts+

– :or"stations

– .outers

– Multicast Applications



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(roadcast %ollision !omain

%ollision !omain

%ollision !omain



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Layer 3 !evices

• &ayer 2 devices

– .outers

– 0o not forward collisions

– 3rea"s up collision domains

– 3roadcast domains are controlled



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(roadcast domain



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(roadcast !omain

• 3roadcast 0omain – A grouping of collision domains

– All the nodes that are a part of that networ" segmentbounded by a layer three device

– 3roadcasts have to be controlled at &ayer 2 devices

– &ayer 1 and &ayer ' devices do not controlbroadcasts



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!ata *low

• &ayer 1 devices filter data frames based on thedestination MA- address

– A &ayer 1 device will forward the frame unless something

prevents it from doing so

• &ayer 2 devices filter data pac"ets based on /(

destination address

– A &ayer 2 device will not forward the frame unless it has to

– &ayer 2 device creates multiple collision and broadcast

domains



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!ata#low



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Latency

The delay between the time a frame leaves the source device

and the time the frame reaches its destination

• The following conditions can cause delays+

– (hysical media – -ircuit delays

• Electronics that process the signal along the path

– Software delays

• 0ecisions that must be made to implement switching andprotocols

– 0elays caused by the content of the frame

• 0estination MA- address has to be read



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Ethernet Switching.ppt