Stp CCNA

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Chng trnh tr gip thit k mng Campus

Ph lc 3: SPANNING TREE PROTOCOL

Mt mng mnh m c thit k khng ch em li tnh hiu qu cho vic truyn ccgi hoc frame, m cn phi xem xt lm th no khi phc hot ng ca mng mtcch nhanh chng khi mng xy ra li. Trong mi trng lp 3, cc giao thc nh tuyn sdng con ng d phng n mng ch khi con ng chnh b li th s nhanh chngtn dng con ng th 2. nh tuyn lp 3 cho php nhiu con ng n ch duy trtnh trng hot ng ca mng v cng cho php cn bng ti qua nhiu con ng.

Trong mi trng lp 2 (switching hoc bridging), khng s dng giao thc nh tuynv cng khng cho php cc con ng d phng, thay v bridge cung cp vic truyn dliu gia cc mng hoc cc cng ca switch. Giao thc Spanning Tree cung cp lin ktd phng mng chuyn mch lp 2 c th khi phc t li m khng cn c s can thipkp thi. STP c nh ngha trong chun IEEE 802.1D.

3.1 Spanning Tree l g v ti sao phi s dng n?

Spanning Tree Protocol (STP) l mt giao thc ngn chn s lp vng, cho php ccbridge truyn thng vi nhau pht hin vng lp vt l trong mng. Sau giao thc nys nh r mt thut ton m bridge c th to ra mt cu trc mng logic cha vng lp(loop-free). Ni cch khc STP s to mt cu trc cy ca free-loop gm cc l v ccnhnh ni ton b mng lp 2.

Vng lp xy ra trong mng vi nhiu nguyn nhn. Hu ht cc nguyn nhn thngthng l kt qu ca vic c gng tnh ton cung cp kh nng d phng, trong trnghp ny, mt lin kt hoc switch b hng, cc lin kt hoc switch khc vn tip tc hotng, tuy nhin cc vng lp cng c th xy ra do li. Hnh 3.1 biu din mt mngchuyn mch vi cc vng lp c c dng cung cp kh nng d phng nh th no.

Hnh 3.1: bridging loop trong mng

Hai nguyn nhn chnh gy ra s lp vng tai hi trong mng chuyn mch l dobroadcast v s sai lch ca bng bridge.

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Vng lp broadcast

Vng lp broadcast v vng lp lp 2 l mt s kt hp nguy him. Hnh 3.2 biudin broadcast to ra vng lp phn hi (feedback loop).

Hnh 3.2: khng c STP, broadcast to Feedback loop

Gi s rng, khng c switch no chy STP:

Bc 1: host A gi mt frame bng a ch broadcast (FF-FF-FF-FF-FF-FF).

Bc 2: frame n c hai Cat-1 v Cat-2 qua cng 1/1

Bc 3: Cat-1 s a frame qua cng 1/2.

Bc 4: frame c truyn n tt c cc nt trn on mng Ethernet k ccng 1/2 ca Cat-2.

Bc 5: Cat-2 a frame ny n cng 1/1 ca n.

Bc 6: mt ln na, frame xut hin cng 1/1 ca Cat-1.

Bc 7: Cat-1 s gi frame ny n cng 1/2 ln hai. Nh vy to thnh mtvng lp y.

Ch : frame ny cng trn qua on mng Ethernet v to thnh mt vng lp theohng ngc li, vng lp feedback xy ra c hai hng. Mt kt lun quan trng na

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trong hnh 3.2 l vng lp bridge nguy him hn nhiu so vi vng lp nh tuyn.Hnh3.3 m t nh dng ca mt DIXv2 Ethernet frame.

Hnh 3.3 : nh dng ca mt DIXv2 Ethernet frame

DIXv2 Ethernet Frame ch cha 2 a ch MAC, mt trng Type v mt CRC.Trong IP header cha trng time-to-live (TTL) c thit lp ti host gc v n s cgim i 1 mi khi qua mt router. Gi s b loi b nu TTL = 0, iu ny cho php ccrouter ngn chn cc datagram b run-away. Khng ging nh IP, Ethernet khng ctrng TTL, v vy sau khi mt frame bt u b lp trong mng th n vn tip tc chon khi ai ngt mt trong cc bridge hoc ngt mt kin kt.

Trong mt mng phc tp hn mng c m t trong hnh 3.1, v 3.2 th c th gyra vng lp feedback rt nhanh theo t l s m. V c mi frame trn qua nhiu cng caswitch, th tng s frame tng nhanh rt nhiu

Ngoi ra cn phi ch n cn bo broadcast trn ngi dng ca host A v B tronghnh 3.2. Broadcast c x l bi CPU tt c cc thit b trn mng. Trong trng hpny, cc PC u c x lbo broadcast. Nu ta ngt mt trong s cc kt ni, th n trli hot ng bnh thng. Tuy nhin, ngay khi ta kt ni n tr li th broadcast s sdng 100% CPU. Nu ta khng x l iu ny m vn tip tc s dng mng, th s tora vng lp vt l trong mng.

Vic sai lch bng bridge

Nhiu nh qun tr switch/bridge nhn thc vn c bn cabo broadcast, tuynhin ta phi bit rng thm ch cc unicast frame cng c th truyn mi trong mng mcha vng lp. Hnh 3.4 m t iu ny.

Bc 1: host A mun gi gi unicast n host B, tuy nhin host B ri khimng, v ng vi bng bridge ca switch khng c a ch ca host B.

Bc 2: gi s rng c hai switch u khng chy STP, th frame n cng1/1 trn c hai switch.

Bc 3: v host B b down, nn Cat-1 khng c a ch MAC (BB-BB-BB-BB-BB-BB) trong bng bridge, v n trn frame qua cc cng.

Bc 4: Cat-2 nhn c frame trn cng 1/2 . C 2 vn xy ra:

o Bc 5: Cat-2 trn frame v n khng hc a ch MAC BB-BB-BB-BB-BB-BB, iu ny to ra feedback loop v lm down mng.

o Cat-2 ch rng, n ch nhn mt frame trn cng 1/2 vi a chMAC l AA-AA-AA-AA-AA-AA. N thay i a ch MAC ca hostA trong bng bridge dn n sai cng.

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Hnh 3.4: frame unicast cng c th gy ra Bridging Loopv lm sai lnh bng bridge

V frame b lp theo hng ngc li, nn ta thy a ch MAC ca host A b ln giacng 1/1 v 1/2. iu ny khng ch lm mng b trn vi cc gi unicast m cn sa saibng bridge. Nh vy khng ch c broadcast mi lm h hi mng.

3.2 Hai khi nim c bn ca STP

Vic tnh ton Spanning Tree da trn hai khi nim khi to ra vng lp logic trongcu trc mng l: Bridge ID (BID) v chi ph ng i.

Bridge ID (BID)

BID l mt trng c 8 byte, n gm c 2 trng con c miu t nh hnh 3.5 sau:

Hnh 3.5 : hai trng ca BID

Trong :

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a ch MAC: c 6 byte c gn cho switch. Catalyst 5000 v 6000 s dngmt trong s cc a ch MAC t vng 1024 a ch gn cho mi gim st vi n(supervisor) v bng ni a nng (backplane). a ch MAC trong BID sdng nh dng hexa.

Ch : mt vi Catalyst ly a ch MAC t module gim st (nh Catalyst 5000) vly a ch khc t backplane (nh Catalyst 5500 v 6000)

Bridge Priority: l u tin bridge c 2 byte to thnh 216 gi tr t 0 -65.535. u tin bridge c gi tr mc nh l gi tr khong gia(32.768).

Ch : ta ch tp trung vo phin bn IEEE ca giao thc Spanning Tree. Mc d vc bn l nh nhau nhng c mt vi im khc bit gia IEEE STP v DEC STP nhDEC STP s dng 8 bit Bridge priority.

Chi ph ng i

Bridge s dng khi nim chi ph nh gi cc bridge khc. 802.1D nh ngha chiph l 1000 Mbps bng cch chia bng thng ca lin kt. V d nh mt lin kt10BaseT c chi ph l 100 (1000/10), Fast Ethernet v FDDI s dng chi ph l 10(1000/100). Tuy nhin vi vic gia tng ca Gigabit Ethernet v OC-48 ATM (2,4Gbps),th chi ph c lu tr l mt gi tr nguyn m khng phi l phn s. V d nh ktqu OC-48 ATM trong 1000/2400 Mbps= 41667 bps, mt gi tr chi ph khng hp l.Do cc chi ph ln hn hoc bng 1 Gbps th c chi ph l 1, tuy nhin iu ny ngncn STP la chn chnh xc con ng tt nht trong mng Gigabit.

gii quyt tnh trng kh x ny, IEEE quyt nh sa i chi ph s dng tnhco dn khng tuyn tnh.Bng 3.1 cho ta mt danh sch gi tr chi ph mi.

Bng thng Chi ph STP4 Mbps 250

10 Mbps 100

16 Mbps 62

45 Mbps 39

100 Mbps 19

155 Mbps 14

622 Mbps 6

1 Gbps 4

10 Gbps 2

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Bng 3.1 Danh sch chi ph mi

Gi tr trong bng 3.1 c chn cn thn s hot ng c v mi c tc linkt nhanh nh hin nay. Mt im ch l gi tr chi ph STP cng thp cng tt.

3.3 Cc bc ra quyt nh ca STP

Khi to ra cu trc mng logic cha vng lp (loop-free) th Spanning Tree lun dngtrnh t bn bc sau:

BID gc (Root BID) thp nht.

Chi ph ng i n Bridge gc thp nht.

BID ca ngi gi thp nht.

ID ca cng (PortID) thp nht.

Bridge trao i thng tin Spanning Tree vi nhau, s dng frame xc nh l n v

d liu giao thc bridge (Bridge Protocol Data Unit - BPDU). Mt bridge s dng trnht bn bc ny lu mt bn sao ca BPDU tt nht trn mi cng. Khi nh gi, nxem tt c BPDU nhn c trn cng cng nh BPDU gi i trn cng . Mi BPDUn u c kim tra theo trnh t bn bc ny, nu tt hn BPDU hin ti th n clu li cng v thay th gi tr c.

Ch : cc bridge s gi BPDU cu hnh cho n khi nhn nhiu hn mt BPDU tt.

Thm vo , qu trnh lu li BPDU tt nht cng iu khin vic gi cc BPDU.Khi mt bridge ln u tin hot ng, th tt c cc cng ca n c gi BPDU 2s mtln (y l gi tr mc nh ca b nh thi). Tuy nhin, nu mt cng lng nghe mtBPDU t mt bridge khc tt hn BPDU m n gi, th cng s ngng gi BPDU. Nu

BPDU ny t mt ln cn ngng n trong mt khong thi gian (20 s l mc nh) thcng tip tc gi BPDU li ln na.

Ch : c 2 loi BPDU l BPDU cu hnh v BPDU thng bo thay i cu trcmng (TCN).

3.4 S hi t STP ban u (Initial STP Convergence)

Phn ny ta s xem xt thut ton m STP s dng hi t ln u tin trn cu trcmng logic cha vng lp (loop-free). Mc d c nhiu kha cnh STP, nhng s hi tban u c phn nh thnh ba bc sau:

Quyt nh mt bridge gc (Root Bridge).

Quyt nh cng gc (Root Port).

Quyt nh cng c ch nh (Designated Port).

Khi mt mng khi ng ln u, tt c cc bridge thng bo thng tin BPDU mtcch ln xn. Tuy nhin, cc bridge ny s lp tc p dng trnh t bn bc ( phn3.1.3). Mt bridge gc c quyt nh hot ng nh l trung tm ca vn vt ivi mng. Tt c cc bridge cn li tnh ton vic thit lp cc cng gc v cc cng ch

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nh xy dng cu trc mng cha loop-free. Kt qu l bridge gc ging nh mt hubvi cc ng i loop-free ra bn ngoi. Khi mng c trng thi n nh, th bridge gc sgi cc BPDU n mi on mng.

Sau khi mng hi t trn cu trc mng loop-free, nu c thm s thay i th s sdng qu trnh thay i cu trc mng.

Hnh 3.6l m hnh ca mt mng switch/bridge. Mng ny gm c ba bridge kt nithnh mt vng lp. Mi cu ni c gn mt a ch MAC khng c tht tng ng vitn thit b (v d nh Cat-A s dng a ch MAC l AA-AA-AA-AA-AA-AA).

Hnh 3.6: m hnh mng s dng STP

Bc 1: quyt nh mtbridge gc.

u tin cc switch cn chn mt bridge gc bng cch tm bridge c BID thp nht.

Ch : nhiu ti liu s dng tnh u tin cao nht khi ni n kt qu ca qu trnhchn bridge gc. Tuy nhin, bridge vi tnh u tin cao nht thc t c gi tr thp nht. trnh nhm ln, ti liu ny lun cp n gi tr thp nht.

Nh ni n phn trn BID l mt nh danh 8 byte c chia thnh 2 trngcon l Bridge Priority v a ch MAC t ngi gim st (supervisor) hoc backplane.Tr li hnh 3.6, ta thy Cat-A c BID mc nh l 32.768 v a ch MAC l AA-AA-AA-AA-AA-AA. Cat-B l (32.768, BB-BB-BB-BB-BB-BB) v Cat-C l (32.768, CC-CC-CC-CC-CC-CC). V c ba bridge u s dng Bridge Priority l 32.678 nn a chMAC thp nht l AA-AA-AA-AA-AA-AA v Cat-A tr thnh Bridge gc.Hnh 3.7mt qu trnh ny.

Ch : gi tr BID cng l thp nht.

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Hnh 3.7: chn Bridge Root

Nhng lm th no cc bridge bit c Cat-A c BID thp nht? l do vic trao

i cc BPDU. Bridge s dng BPDU dnh ring thay i cu trc mng v thng tinSpanning Tree ln nhau. Cc BPDU c gi mc nh 2s mt ln. Cc BPDU l lulng bridge-to-bridge, n khng mang lu lng end-to-end.Hnh 3.8 m t cc phnc bn ca mt BPDU.

Hnh 3.8: cc thnh phn c bn ca BPDU

Mc ch vic chn bridge gc ch lin quan n trng Root BID v Sender BID.Khi mt bridge pht ra mt BPDU 2s mt ln, ngay tc khc n s xc nh bridge gcda vo trng Root BID. Bridge ny lun t BID ca chnh n trong Sender BID.

Ch : Root BID l ID cabridge gc hin ti, trong khi Sender BID l ID ca bridgecc b hoc switch.

Khi bridge khi ng ln u tin, n lun lun t BID trong c hai trng RootBID v Sender BID. Gi s rng, Cat-B khi ng u tin v bt u gi cc BPDU

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thng bo chnh n l Bridge gc 2s mt ln. Mt vi pht sau Cat-C khi ng v thngbo chnh n l Bridge gc. Khi BPDU ca Cat-C n Cat-B, Cat-B s loi b BPDU vn c B-ID thp hn c lu trn cc cng ca n. Ngay lp tc Cat-B truyn BPDU,Cat-C bit c l gi nh ban u ca n l sai. Ti thi im , Cat-C bt u giBPDU vi Root BID l B v Sender BID l C. By gi mng chp nhn B l Bridge gc.

5 pht sau , Cat-A khi ng, n gi s rng n l bridge gc v bt u qung biu ny trong BPDU. Ngay lp tc cc BPDU n Cat-B v C, cc switch ny snhng bridge gc li cho Cat-A. By gi tt c 3 switch u gi cc BPDU thng boCat-A l bridge gc v chnh n l Sender BID.

Bc 2: chn cng gc.

Sau khi xc nh c bridge gc, th cc switch s chuyn qua chn cng gc. Cnggc l mt cng trn bridge cc b. Mi brigde (trbridge gc) phi la chn mt cnggc.

Ch : Mi bridge (trbridge gc) s la chn cng gc.

Bridge s s dng khi nim chi ph xt cng gc. C th l cc bridge theo di chiph ng i gc, chi ph tch ly ca tt c cc lin kt nbridge gc. Hnh 3.9 m tlm th no tnh ton qua nhiu bridge v kt qu ca vic quyt nh cng gc.

(1): khi Cat-A (bridge gc) gi cc BPDU, th n cha chi ph ng i gc l0.

(2): khi B nhn cc BPDU ny, n thm vo chi ph ng i ca cng 1/1vochi ph ng i gc cha trong BPDU nhn. Gi s rng mng ang chyswitch Catalyst 5000 c m ln hn phin bn 2.4 v ba lin kt trong hnh3.9 u l Fast Ethernet. Cat-B nhn chi ph ng i gc l 0 v thm vo chi

ph ca

cng

1/1 l 19. (3): sau Cat-B s dng gi tr 19 v gi BPDU vi chi ph ng i gc l

10 ra cng 1/2.

(4): khi Cat-C nhn BPDU ny t B, th n tng chi ph ng i gc thnh 38(19+19).

(5): tuy nhin Cat-C cng nhn BPDU t bridge gc trn cng 1/1. Cat-C sthm vo cng 1/1 vi chi ph l 0, v ngay lp tc n tng chi ph ln 19.

(6): Cat-C thy chi ph ng i gc l 19 trn cng 1/1 v 38 trn cng 1/2,n quyt nh cng 1/1 l cng gc (chn gi tr nh nht).

(7): sau Cat-C bt u qung b chi ph ng i gc vi gi tr 19 n ccswitch xui dng.

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Hnh 3.9 : chn Root Port

Hnh 3.9biu din Cat-B tnh ton v chn ra cng 1/1 l cng gc vi chi ph l 19,v ch l khi mt cng nhn BPDU th chi ph s tng dn.

Ch :

Chi ph STP c tng khi mt cng nhn BPDU, ch khng phi v n cgi ra khi cng. V d nh, cc BPDU n trn cng 1/1 ca Cat-B vi chiph l 0 v tng ln 19 bn trong Cat-B.

S khc nhau gia chi ph ng i v chi ph ng i gc.

Chi ph ng i l gi tr c gn cho mi cng, n c thm vo ccBPDU c nhn trn cng tnh ton chi pha ng i gc.

Chi ph ng i gc l chi ph tch ly n bridge gc. Trong BPDU, y lgi tr ca trng chi ph. i vi mt bridge, gi tr ny c tnh bng cchcng cc chi ph ng i ca cc cng nhn vi gi tr cha trong BPDU.

Bc 3: quyt nh cng c ch nh.

Mi on mng trn mt bridge c mt cng c ch nh, cng ny c chc nngnhn v gi lu lng n on mng kia v bridge gc. Nu ch c mt cng nm gilu lng trn mi lin kt, th tt c vng lp b ph b. Bridge cha cng c ch nhc gi l designated bridge cho on mng .

Vic la chn cng c ch nh cng da trn chi ph tch ly ca ng i gcn bridge gc (hnh 3.10).

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Hnh 3.10: chn Designated Port

xc nh cng c ch nh, ta hy nhn vo mi on mng. u tin l on 1,lin kt gia Cat-A v B c 2 cng l Cat-A: cng 1/1, v Cat-B: cng 1/1. Cng 1/1 caCat-A c chi ph ng i gc l 0, v cng 1/1 ca B l 19 (gi tr 0 c nhn trongBPDU t A cng vi chi ph ng i c gn cho cng 1/1 ca B). V cng 1/1 ca Ac chi ph ng i thp hn nn n tr thnh cng c ch nh i vi lin kt ny.

i vi on mng 2 (kin kt gia Cat-A v C), tng t cng 1/2 ca A tr thnhcng c ch nh. Ch l mi cng hot ng trn bridge gc u tr thnh cngc ch nh.

By gi hy xem on 3 (lin kt gia Cat-B v C), c hai cng 1/2 ca B v 1/2 caC u c chi ph ng i gc l 19. y l mt s hn ch, v STP thng s dng trnht bn bc quyt nh:

B-ID gc thp nht.

Chi phi ng i n bridge gc thp nht.

Sender BID thp nht.

ID ca cng thp nht.Trong v d hnh 3.10, tt c cc bridge u tn thnh Cat-A l Bridge gc, c B v

C u c chi ph l 19, nn ta s ly yu t BID quyt nh. BID ca B l (32.768.BB-BB-BB-BB-BB-BB) v ca C l (32.768.CC-CC-CC-CC-CC-CC), do cng 1/2 ca Bl Cng c ch nh cho on 3.

V d trong mt mng cha 15 switch v c 146 on mng (mi cng l mt onmng duy nht), s thnh phn STP hin c l

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Cc thnh phn STP S

Bridge gc 1

Cng gc 14

Cng c ch nh 146

Bng 3.2: Cc thnh phn STP trongmng c 15 switch v 146 on mng

Tt c cc quyt nh STP u da trn mt trnh t nh cp:

BID gc thp nht.

Chi ph ng i n bridge gc thp nht.

Sender BID thp nht.

ID ca cng thp nht.

Khi mt cng nhn BPDU n s so snh vi cc BPDU nhn c trn cc cng khc(cng nh BPDU c gi trn cng ). Ch BPDU tt nht mi c lu li. Tt nht y c ngha l gi tr thp nht (v d nh BID thp nht tr thnh Bridge gc, gi trthp nht cng c s dng chn cng gc v cng c ch nh). Mt cng sngng truyn BPDU nu n nghe c mt BPDU tt hn BPDU ca n.

3.5 Cc trng thi ca STP

Sau khi bridge phn chia c cc cng nh cng gc, cng c ch nh v cng

khng c ch nh, th vic to ra cu trc mng cha loop-free khng phc tp lm,cng gc v cCng c ch nh chuyn tip lu lng, trong khi cng khng c chnh th kha lu lng. Vic chuyn tip v kha ch l 2 trng thi thng thng trongmng, bng 3.3 m t 5 trng thi ca STP.

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Trng thi Mc ch

Chuyn tip (forwading) Gi v nhn d liu ngi dng

Hc hi (learning) Xy dng bng bridge

Lng nghe (listening) Xy dng cu trc mng active

Kha (blocking) Ch nhn cc BPDU

V hiu ha (disable) Cc cng b down

Bng 3.3: Cc trng thi ca STP

Trng thi disable cho php ngi qun tr mng qun l vic ngng hot ng ca

mt cng. Sau khi khi to, cc cng bt u trong trng thi blocking lng nghe ccBPDU.

Do s a dng ca cc s kin m bridge truyn trong trng thi listening (v dnh mt bridge ngh n l bridge gc ngay sau khi khi ng). trng thi ny, khngc d liu ngi dng c truyn qua, tc l cng ang gi v nhn cc BPDU cgng to cu trc mng hot ng. Trong trng thi listening s s dng ba bc hi t ni trn, cc cng b mt quyn cng c ch nh s tr thnh cng khng cch nh v tr li trng thi blocking.

Cc cng c ch nh v cng gc sau 15s (gi tr mc nh ca b nh thi) schuyn qua trng thi learning. Trong khong 15s khc, bridge vn khng chuyn cc

frame ca ngi dng qua, m xy dng bng bridge ca n. Khi bridge nhn frame, na a ch MAC v cng vo bng bridge. Trng thi learning s gim bt s lngtrn ngp khi vic chuyn tip d liu bt u.

Ch : Trong vic lu tr a ch MAC v thng tin cng, cc Catalyst hc cc thngtin nh VLAN ngun.

Nu mt cng vn l cng c ch nh hay cng gc khong thi gian cui catrng thi learning, th cng chuyn qua trng thi forwading. trng thi ny, nbt u gi v nhn cc frame ca ngi dng. Hnh 3.11 m t trng thi cc cng vvic chuyn trng thi.

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Hnh 3.11: trng thi cc cng v hot ng chuyn trng thi

Hnh 3.12biu din mng vi s phn chia cng v danh sch cc trng thi. Ch ltt c cc cng u chuyn tip tr cng 1/2 ca Cat-C.

Hnh 3.12: s mng vi cc cng c nh danh

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Trng thi/cng K hiu

Blocking B

Forwading F

Cng c ch nh DP

Cng gc RP

Cng khng c chnh

NDP

Bng 3.4 : Cc trng thi STP vcc k hiu cng

3.6 B nh thi gian STP

Mt bridge tri qua 15s mi trng thi listening v learning. STP c iukhin bi bab m thi gian (timer) nh trong bng 3.5.

Timer Mc chGi tr

mc nh

Hello Timer Khong thi gian gi cc BPDUcu hnh gi bi Bridge gc

2s

Forward Delay Thi hn trng thi Listeningv Learning

15s

Max Age Thi gian lu tr BPDU 20s

Bng3.5: STP Timer

V d: gi s rng lin kt on 3 trong hnh 3.12 s dng mt hub v cng 1/2 caCat-B truyn ra ngoi. Cat-C khng thng bo li lin v n vn ang nhn lin ktEthernet t hub. Cat-C ch thng bo l cc BPDU ngng n. Sau 20s (Max Age), thcng 1/2 ca Cat-C ly thng tin BPDU c vi cng 1/2 ca Cat-B l cng c ch nhcho on mng 3. iu ny lm cho cng 1/2 ca Cat-C truyn trong trng thilistening c gng tr thnh cng c ch nh. V vy cng 1/2 ca Cat-C cungcp truy cp tt nht t bridge gc n lin kt ny, nn n chuyn sang trng thiforwarding. Nh vy, Cat-C mt 50s (20s Max Age + 15s Listenning + 15sForwarding) vt qua sau khi cng 1/2 trn Cat-B b li.

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Trong trng hp ny, cc bridge c th pht hin s thay i cu trc mng trn cclin kt kt ni trc tip v ngay lp tc chuyn sang trng thi listening m khng cnch thi gian Max Age. Xem v d trong hnh 3.13.

Hnh 3.13: li xy ra trn lin kt gia Root Bridge vRoot Port ca Cat-C

Trong trng hp ny, cng 1/1 ca Cat-C b li, v lin kt trn cng gc cng b linn ngay lp tc cng 1/2 ca Cat-C chuyn sang trng thi learning tr thnh cnggc mi thay v ch 20s ri mi ly thng tin c. iu ny lm cho thi gian hi t STP

gim t 50s xung 30s (15s listening + 14s learning).Ch : thi gian hi t STP l t 30s n 50s.

Hai im quan trng cn nh khi s dng b nh thi STP l:

Th nht: khng thay i gi tr thi gian mc nh khi khng c s cn nhccn thn.

Th hai: ta ch c sa thi gian t bridge gc.

3.7 Hai loi BPDU

C hai loi BPDU l :

BPDU cu hnh.

BPDU thng bo thay i cu trc mng TCN BPDU (Topology ChangeNotification BPDU).

BPDU cu hnh c bt u bi bridge gc v pht ra trn cc con ng hot ngt bridge gc, cn TCN BPDU hng v bridge gc cnh bo vi bridge gc l cutrc mng mng c s thay i.

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BPDU cu hnh : cc trng trong BPDU cu hnh c tm tt trong bng 3.6

TrngChiu di

(octet) ngha

Protocol ID 2 Lun bng 0

Version 1 Lun bng 0

Type 1 Cho bit kiu BPDU

BPDU cu hnh = 0

Flag 1 LSB = C thay i cu trc mng

MSB = C xc nhn thay i cu trcmng

Root ID 8 BID ca bridge gc hin ti

Root Path Cost

(chi ph ng igc)

4 Chi ph tch ly n bridge gc

Sender BID 8 BID ca bridge hin ti

Cng ID 2 ID ca cng gi BPDU ny

Message Age 2 Khong thi gian t khi bridge gc toBPDU n khi pht BPDU i.

Max Age 2 Khong thi gian lu thng tin BPDU

Hello Time 2 Khong thi gian gia cc BPDU

Forward Delay 2 Thi gian trong trng thi listening vlearning

Bng 3.6 : Cc trng trong BPDU cu hnh

TCN BPDU (Topology Change Notification BPDU) :

TCN BPDU n gin hn BPDU cu hnh v ch gm c ba trng, ging nh batrng u tin ca BPDU cu hnh nhng trng Type th thay i vi gi tr nh sau :

0x00 (0000 0000): BPDU cu hnh.

0x80 (1000 0000): TCN BPDU.

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Ch : TCN BPDU khng mang bt c thng tin b sung no.

3.8 Qu trnh thay i cu trc mng

Nu TCP BPDU n gin th lm th no n th hin c vai tr quan trng ca n?Ta hy xem xt s thay i cu trc mng trong hnh 3.14.

Host D ang lin lc vi host E qua hai bc:

(1): lu lng t host D qua Cat-B lin lc vi host E.

(2): gi s b thu pht trn cng 1/2 ca Cat-B b hng.

Hnh 3.14: TCN BPDUc dng cp nht bng Bridge nhanh hn

Nh tho lun, cng 1/2 ca Cat-C mt 50s tr thnh cng c ch nh. Tuynhin nu khng c TCN BPDU th n tip tc b ngt khong 250s. Trong khong thigian li, c ba switch u cha a ch MAC ca host E trong bng Bridge nh bng 3.7.

Bng BridgeCng lin quan n a

ch MAC ca host ECat-A Cng 1/1

Cat-B Cng 1/2

Cat-C Cng 1/1

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Bng 3.7: Gi tr bng Bridge trc khic s thay icu trc mng

TCN BPDU l mt phng php n gin ci tin thi gian hi t, v n lm viccht ch vi BPDU cu hnh nh sau:

1. Mt bridge bt u mt TCN BPDU khi:

N chuyn mt cng sang trng thi forwarding v n c t nht mtcng c ch nh.

N chuyn mt cng t trng thi forwarding hoc learning sangblocking.

S thay i cu trc mng i hi phi gi thng bo n bridge gc, gi s rngbridge hin ti khng phi l bridge gc, th n bt u qu trnh thng bo bng cch giTCN BPDU ra cng gc ca n. N tip tc gi TCN BPDU cho n khi thng ipTCN c xc nhn.

2. Bridge upstream s nhn TCN BPDU. Mc d, mt vi bridge nghe cTCN BPDU (v n kt ni trc tip vo cng gc ca on mng) nhng chc cng c ch nh chp nhn v x l TCN BPDU.

3. Bridge upstream s thit lp c xc nhn thay i cu trc mng TCA(Topology Change Acknowledgement) trong BPDU cu hnh k tip c gingc li(ra cng c ch nh). C ny dng xc nhn vi bridge khiu n ngng pht TCN BPDU.

4. Bridge upstream s truyn TCN BPDU ra cng gc ca n.

5. Tip tc bc 2 n bc 4 cho n khi bridge gc nhn TCN BPDU.

6. Sau bridge gc s thit lp c xc nhn thay i cu trc mng TCA (xc nhn vi bridge trc ), v c thay i cu trc mng TC (TopologyChange) trong BPDU cu hnh m n s gi i.

7. Bridge gc tip tc thit lp c thay i cu trc mng TC trong tt c ccBPDU cu hnh m n gi ra ngoi vi tng thi gian l 35s (Forward Delay+ Max Age). C ny s thu ngn gi tr 300s xung 15s (tc tr chuyntip - Forward Delay).

Hnh 1.15 tm tt cc bc trong qu trnh thay i cu trc mng.

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Hnh 3.15: trnh t cc bc trong qu trnh thay i cu trc mng

Da vo hnh 3.15 ta c th bit c qu trnh thay i cu trc mng cho hnh 3.14nh sau: bc 1 Cat-B v C gi TCN BPDU ra cng 1/1. V bridge upstream cng lbridge gc nn b qua bc 3 v 4. Sau bc 2 v 5 xy ra ng thi. Trong BPDUcu hnh k tip m bridge gc gi i, c TCN ACKs c thit lp xc nhn l nhn TCN ca hai brigde downstream. Tip theo l bc 6 v 7, Cat-A cng thit lp cTA trong 35s (Forward Delay + Max Age) cp nht bng bridge nhanh hn. Nh vyc ba switch u nhn c c TA v khong thi gian cho bng bridge l 15s.

Ch l khong thi gian ngn 15s ny khng bt buc cho ton b bng, n ch lmqu trnh ny nhanh hn thi. Cc thit b tip tc ni sut 15s ny m khng cho bngbridge ngh. Tuy nhin, nu host D c gng gi mt frame n host E trong 20s (gi shost E khng ni g ht), th frame s c trn n tt c cc on mng v a ch EE-EE-EE-EE-EE-EE khng cn c trong bng bridge na. Ngay khi frame n host E vhost E tr li, th switch hc c gi tr bng bridge mi tng ng vi cu trc mngmi.

Bng 3.8biu din ton b bng bridge cho a ch MAC ca E trn c ba switch saukhi cu trc mng mi hi t v lu lng li tip tc.

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Bng BridgeCng lin quan n a

ch MAC ca host E

Cat-A Cng 1/2

Cat-B Cng 1/1

Cat-C Cng 1/2

Bng 3.8: Gi tr bng Bridge sau khithay icu trc mng

Ti thi im ny, kt ni gia host D v E c thit lp li v lu lng li tiptc. Ch l TCN BPDU gim thi gian li t 300s (5ph) xung 50s.

Hnh 3.16m t trng c trong BPDU cu hnh, c hai c TCA v TA u c lutr trong cng mt octet ca BPDU cu hnh.

Hnh 3.16: trng c trong BPDU cu hnh

Nh tho lun, c TCN c thit lp bi bridge upstream ni cho cc bridgedowstream ngng gi TCN BPDU. Cn c TC c thit lp bi bridge gc gimkhong thi gian li t 300s xung 15s (Forward Delay).

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