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© 2008 Cisco Systems, Inc. Al l r ights reserved. Cisco ConfidentialPresentation_ID

Cha!ter " Introd#ction

to S$itched %et$or&s

Routing and Switching



Presentation_ID 2© 2008 Cisco Systems, Inc. Al l r ights reserved. Cisco Confidential

Chapter 1

.0 Introd#ction

. 'A% Design

.2 S$itched (nvironment

.) S#mmary



Presentation_ID )© 2008 Cisco Systems, Inc. Al l r ights reserved. Cisco Confidential

Chapter 1: Objectives

*!on com!letion of this cha!ter, yo# $ill +e a+le to"

Descri+e convergence of data, voice, and video in the contet ofs$itched net$or&s.

Descri+e a s$itched net$or& in a small-to-medi#m-sied +#siness.

(!lain the !rocess of frame for$arding in a s$itched net$or&.

Com!are a collision domain to a +roadcast domain.



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Converged Networks

Growing Complexit o! Networks

#r digital $orld ischanging.

Information m#st

+e accessed fromany$here in the$orld.

%et$or&s m#st +esec#re, relia+le,

and highlyavaila+le.




Converged Networks

"lements o! a Converged Network

Colla+oration is are#irement.

3o s#!!ort colla+oration,net$or&s em!loy

converged sol#tions.

Data services incl#devoice systems, IP !hones,voice gate$ays, videos#!!ort, and video

conferencing. Call control, voice

messaging, mo+ility, anda#tomated attendant arealso common feat#res.




Converged Networks

#ene!its o! a Converged Network

Converged net$or&

5 a single net$or& designed to handle voice, video, and data6

5 an internal net$or& $here the 'ayer ) devices, s#ch as

ro#ters, have a com!lete ro#ting ta+le to +e a+le to

acc#rately and efficiently send data to a remote destination65 a s$itch net$or& that has com!leted calc#lations that res#lt

in a single !ath thro#gh the s$itch net$or&.


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Converged Networks

#ene!its o! a Converged Network

(lements of a Converged %et$or& ...29

5 Call control: 3ele!hone call !rocessing, caller ID, call

transfer, hold, and conference

5 $oice messaging: :oicemail

5 %obilit: ;eceive im!ortant calls $herever yo# are

5 &utomated attendant: Serve c#stomers faster +y ro#ting

calls directly to the right de!artment or individ#al



Converged Networks

#orderless Switched Networks

3he Cisco <orderless %et$or& is a net$or& architect#re thatallo$s organiations to connect anyone, any$here, anytime,and on any device sec#rely, relia+ly, and seamlessly.

Cisco <orderless %et$or& is designed to address I3 and+#siness challenges, s#ch as s#!!orting the convergednet$or& and changing $or& !atterns.


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Converged Networks

'ierarch in the #orderless Switched

Network

<orderless

s$itched net$or&

design g#idelines

are +#ilt #!on the

follo$ing !rinci!les"

>ierarchical

*nderstanding

the role of

each device atevery tier,



Converged Networks


Network

<orderless

s$itched net$or&

design g#idelines

are +#ilt #!on the


;esiliency

- Satisfies #ser

e!ectations for&ee!ing thenet$or& al$ayson


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Converged Networks


Network

<orderless

s$itched net$or&

design g#idelines

are +#ilt #!on the


?lei+ility

- Allo$s intelligent

traffic load

sharing +y #sing

all net$or&

reso#rces



Converged Networks


Network

<orderless

s$itched net$or&

design g#idelines

are +#ilt #!on the


@od#larity

Allo$s seamless

net$or& e!ansionand integratedservice ena+lementon an on-demand+asis


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Converged Networks

Core (istribution access

Access layer

;e!resents the net$or& edge, $here traffic enters or eits thecam!#snet$or&.

3raditionally, the !rimary f#nction of an access layer s$itch is to!rovide net$or& access to the #ser.

Access layer s$itches connect to distri+#tion layer s$itches, $hichim!lement net$or& fo#ndation technologies s#ch as ro#ting, #ality ofservice, and sec#rity.

.


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Converged Networks


Distri+#tion 'ayer

3he distribution layer interfaces +et$een the access layer and thecore layer to !rovide many im!ortant f#nctions, incl#ding"

Aggregating large-scale $iring closet net$or&s Aggregating 'ayer 2 +roadcast domains and 'ayer ) ro#ting

+o#ndaries

Providing intelligent s$itching, ro#ting, and net$or& access !olicyf#nctions to access the rest of the net$or&

Providing high availa+ility thro#gh red#ndant distri+#tion layers$itches to the end-#ser and e#al cost !aths to the core

Providing differentiated services to vario#s classes of servicea!!lications at the edge of the net$or&



Converged Networks


Core 'ayer

3he core layer is the net$or& +ac&+one. It connects several layers ofthe cam!#s net$or&.

3he core layer serves as the aggregator for all of the other cam!#s+loc&s and ties the cam!#s together $ith the rest of the net$or&. 3he!rimary !#r!ose of the core layer is to !rovide fa#lt isolation and high-s!eed +ac&+one connectivity.

?ig#re -4 sho$s a three-tier campus network design fororganiations $here the access, distri+#tion, and core are eachse!arate layers.



Converged Networks

Core) (istribution) &ccess



Switched Networks

Role o! Switched Networks

Switched Networks *1+1+,-

S$itched net$or&s are im!ortant $hen de!loying $ired 'A%s.

A net$or& !rofessional today m#st +e $ell-versed in s$itches and'A% technology in order to add commonly de!loyed devices s#ch

as PCs, !rinters, video cameras, !hones, co!iers, and scanners. Sharing and accessing net$or& devices is common in +oth the

home and +#siness net$or&.



Switched Networks


S$itching technologies are cr#cial to net$or& design.

S$itching allo$ traffic to +e sent only $here it is needed in mostcases, #sing fast methods.

A s$itched 'A%"

Allo$s more flei+ility Allo$s more traffic management

S#!!orts #ality of service, additional sec#rity, $ireless, IP

tele!hony, and mo+ility services


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Switched Networks




Switched Networks


?orm ?actors ..2.29

3here are vario#s ty!es of s$itches #sed in +#siness net$or&s. It isim!ortant to de!loy the a!!ro!riate ty!es of s$itches +ased onnet$or& re#irements.



Switched Networks

.orm .actor

.ixed/lat!orm



Switched Networks

.orm .actor *cont+-

%odular

/lat!orm


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Switched Networks

.orm .actor *cont+-

Stackable

/lat!orm


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.rame .orwarding

Switching as a General Concept

A s$itch ma&es a decision +ased on ingress and a destination

!ort.

A 'A% s$itch &ee!s a ta+le that it #ses to determine ho$ to

for$ard traffic thro#gh the s$itch.

Cisco 'A% s$itches for$ard (thernet frames +ased on thedestination @AC address of the frames.

3he f#ndamental conce!t of s$itching refers to a devicema&ing a decision +ased on t$o criteria"

Ingress !ort Destination address




.rame .orwarding

Switching as a General Concept

3he decision on ho$ a s$itch for$ards traffic is made inrelation to the flo$ of that traffic.

3he term ingress is #sed to descri+e a frame entering a deviceon a s!ecific !ort.

3he term egress is #sed to descri+e frames leaving the devicethro#gh a !artic#lar !ort.

hen a s$itch ma&es a decision, it is +ased on the ingress!ort and the destination address of the message.

A 'A% s$itch maintains a ta+le that it #ses to determine ho$ to

for$ard traffic thro#gh the s$itch.




.rame .orwarding

(namicall /opulating a Switch %&C

&ddress 0able A s$itch m#st first learn $hich devices eist on each !ort +efore it

can transmit a frame.

It +#ilds a ta+le called a @AC address or content addressa+le

memory CA@9 ta+le.

3he ma!!ing device B- !ort is stored in the CA@ ta+le. CA@ is a s!ecial ty!e of memory #sed in high-s!eed searching

a!!lications.

3he information in the @AC address ta+le #sed to send frames.

hen a s$itch receives an incoming frame $ith a @AC address thatis not fo#nd in the CA@ ta+le, it floods it to all !orts, +#t the one that

received the frame.

;efer online notes ?ig#re -) to -8




.rame .orwarding

Switch .orwarding %ethods *1+,+1+- Commonly, in earlier net$or&s, as they gre$, enter!rises +egan to

e!erience slo$er net$or& !erformance.

(thernet +ridges an early version of a s$itch9 $ere added

to net$or&s to limit the sie of the collision domains. In the ==0s,

advancements in integrated circ#it technologies allo$ed for 'A% s$itches to re!lace

(thernet

+ridges. 3hese 'A% s$itches $ere a+le to move the 'ayer 2for$arding decisions

from soft$are to application-specific-integrated circuits (ASICs). ASICs red#ce

the !ac&et-handling time $ithin the device, and allo$ the device tohandle an

increased n#m+er of !orts $itho#t degrading !erformance. 3hismethod of for$arding




.rame .orwarding

Switch .orwarding %ethods

&dd a header



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.rame .orwarding

Store2and2.orward Switching

Allo$s the s$itch to" Chec& for errors via ?CS chec&9

3he ?CS is an error chec&ing !rocess that hel!s to ens#re that the frame is free of !hysical andata-lin& errors. If the frame is error-free, the s$itch for$ards the frame. ther$ise, the frame idro!!ed.

A#tomatic <#ffering

3he ingress !ort +#ffering !rocess #sed +y store-and-for$ard s$itches !rovides the

flei+ility to s#!!ort any mi of (thernet s!eeds. ?or eam!le, handling an incoming

frame traveling into a 00 @+s (thernet !ort that m#st +e sent o#t a E+s

interface $o#ld re#ire #sing the store-and-for$ard method. ith any mismatch in

s!eeds +et$een the ingress and egress !orts, the s$itch stores the entire frame in a



Presentation_ID )0© 2008 Cisco Systems, Inc. Al l r ights reserved. Cisco Confidential

.rame .orwarding

Store2and2.orward Switching

Allo$s the s$itch to" Perform a#tomatic +#ffering

3he ingress !ort +#ffering !rocess #sed +y store-and-for$ard s$itches !rovides the flei+ility ts#!!ort any mi of (thernet s!eeds. ?or eam!le, handling an incoming frame traveling into a00 @+s (thernet !ort that m#st +e sent o#t a E+s interface $o#ld re#ire #sing the store-and-for$ard method.

Store-and-for$ard s$itching is CiscoFs !rimary 'A% s$itchingmethod.



Presentation_ID )© 2008 Cisco Systems, Inc. Al l r ights reserved. Cisco Confidential

.rame .orwarding

Store2and2.orward Switching *cont+-




.rame .orwarding

Cut20hrough Switching

An advantage to c#t-thro#gh s$itching is the ca!a+ility of the s$itchto start for$arding a frame earlier than store-and-for$ard s$itching.

3here are t$o !rimary characteristics of c#t-thro#gh s$itching" ra!idframe for$arding and invalid frame !rocessing.

Allo$s the s$itch to start for$arding in a+o#t 0 microseconds

%o ?CS chec&

%o a#tomatic +#ffering



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.rame .orwarding


;a!id ?rame ?or$arding As indicated in ?ig#re -20, a s$itch #sing the c#t-thro#gh method

can ma&e a for$arding decision as soon as it has loo&ed #! thedestination @AC address of the frame in its @AC address ta+le.

3he s$itch does not have to $ait for the rest of the frame to enter the

ingress !ort +efore ma&ing its for$arding decision.

3he c#t-thro#gh s$itching method does not dro! most invalidframes. ?rames $ith errors are for$arded to other segments of thenet$or&. If there is a high error rate invalid frames9 in the net$or&,c#t-thro#gh s$itching can have a negative im!act on +and$idth6

th#s, clogging #! +and$idth $ith damaged and invalid frames.



Presentation_ID )/© 2008 Cisco Systems, Inc. Al l r ights reserved. Cisco Confidential

.rame .orwarding


?ragment ?ree ?ragment free s$itching is a modified form of c#t-thro#gh s$itching

in $hich the s$itch $aits for the collision $indo$ 4/ +ytes9 to !ass+efore for$arding the frame.

3his means each frame $ill +e chec&ed into the data field to ma&e

s#re no fragmentation has occ#rred.

?ragment free mode !rovides +etter error chec&ing than c#t-thro#gh,$ith !ractically no increase in latency.

ith a lo$er latency s!eed advantage of c#t-thro#gh s$itching, it ismore a!!ro!riate etremely demanding, high-!erformancecom!#ting >PC9 a!!lications that re#ire !rocess-to-!rocesslatencies of 0 microseconds or less.




.rame .orwarding

Cut20hrough Switching *cont+-




Switching (omains

Collision (omains

A collision domain is the segment $here devices m#st com!ete tocomm#nicate.

All !orts of a h#+ +elong to the same collision domain.

(very !ort of a s$itch is a collision domain on its o$n.

A s$itch +rea& the segment into smaller collision domains, easingdevice com!etition.




Switching (omains

#roadcast (omains

A +roadcast domain is the etend of the net$or& $here a +roadcastframe can +e heard.

S$itches for$ard +roadcast frames to all !orts6 therefore, s$itches

do not +rea& +roadcast domains.

All !orts of a s$itch, $ith its defa#lt config#ration, +elong to thesame +roadcast domain.

If t$o or more s$itches are connected, +roadcasts are for$arded to

all !orts of all s$itches, ece!t for the !ort that originally received

the +roadcast.



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Switching (omains

#roadcast (omains

<roadcasts are sometimes necessary for initially locating otherdevices and net$or& services, +#t they also red#ce net$or&efficiency.

3oo many +roadcasts and a heavy traffic load on a net$or& canres#lt in congestion" a slo$do$n in the net$or& !erformance.

;efer online notes " ?ig#re .2.2



Presentation_ID /0© 2008 Cisco Systems, Inc. Al l r ights reserved. Cisco Confidential

Switching (omains

&lleviating Network Congestion

S$itches hel! alleviating net$or& congestion +y"

?acilitating the segmentation of a 'A% into se!arate collision

domains

Providing f#ll-d#!le comm#nication +et$een devices

3a&ing advantage of their high-!ort density

<#ffering large frames

(m!loying high-s!eed !orts

3a&ing advantage of their fast internal s$itching !rocess

>aving a lo$, !er-!ort cost



Presentation_ID /© 2008 Cisco Systems, Inc. Al l r ights reserved. Cisco Confidential

Chapter 1: Summar

In this cha!ter, yo# learned" 3he trend in net$or&s is to$ards convergence #sing a single set of

$ires and devices to handle voice, video, and data transmission.

3here has +een a dramatic shift in the $ay +#sinesses o!erate.

3here are no !hysical offices or geogra!hic +o#ndaries constraints.;eso#rces m#st no$ +e seamlessly availa+le anytime andany$here.

3he Cisco <orderless %et$or& architect#re ena+les differentelements, from access s$itches to $ireless access !oints, to $or&

together and allo$ #sers to access reso#rces from any !lace at anytime.



Presentation_ID /2© 2008 Cisco Systems, Inc. Al l r ights reserved. Cisco Confidential

Chapter 1: Summar *cont+-

3he traditional, three-layer hierarchical design model divides thenet$or& into core, distri+#tion, and access layers, and allo$s each!ortion of the net$or& to +e o!timied for s!ecific f#nctionality.

It !rovides mod#larity, resiliency, and flei+ility, $hich !rovides afo#ndation that allo$s net$or& designers to overlay sec#rity,

mo+ility, and #nified comm#nication feat#res. S$itches #se either store-and-for$ard or c#t-thro#gh s$itching.

(very !ort on a s$itch forms a se!arate collision domain allo$ing foretremely high-s!eed, f#ll-d#!le comm#nication.

S$itch !orts do not +loc& +roadcasts and connecting s$itches canetend the sie of the +roadcast domain, often res#lting in degradednet$or& !erformance.

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