Tóm tắt Spanning - Tree Protocols

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 cc gi hoc frame, m cn phi xem xt lm th no khi phc hot ng ca mng mt cch nhanh chng khi mng xy ra li. Trong mi trng lp 3, cc giao thc nh tuyn s dng con ng d phng n mng ch khi con ng chnh b li th s nhanh chng tn dng con ng th 2. nh tuyn lp 3 cho php nhiu con ng n ch duy tr tnh 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 tuyn v cng khng cho php cc con ng d phng, thay v bridge cung cp vic truyn d liu gia cc mng hoc cc cng ca switch. Giao thc Spanning Tree cung cp lin kt d phng mng chuyn mch lp 2 c th khi phc t li m khng cn c s can thip kp 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 cc bridge truyn thng vi nhau pht hin vng lp vt l trong mng. Sau giao thc ny s 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 cc nhnh ni ton b mng lp 2. Vng lp xy ra trong mng vi nhiu nguyn nhn. Hu ht cc nguyn nhn thng thng l kt qu ca vic c gng tnh ton cung cp kh nng d phng, trong trng hp ny, mt lin kt hoc switch b hng, cc lin kt hoc switch khc vn tip tc hot ng, tuy nhin cc vng lp cng c th xy ra do li. Hnh 3.1 biu din mt mng chuyn 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 do broadcast v s sai lch ca bng bridge.

Chng trnh tr gip thit k mng Campus

Vng lp broadcast Vng lp broadcast v vng lp lp 2 l mt s kt hp nguy him. Hnh 3.2 biu din 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-FFFF). 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 c cng 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 mt vng lp y. Ch : frame ny cng trn qua on mng Ethernet v to thnh mt vng lp theo hng ngc li, vng lp feedback xy ra c hai hng. Mt kt lun quan trng na

Chng trnh tr gip thit k mng Campus trong hnh 3.2 l vng lp bridge nguy him hn nhiu so vi vng lp nh tuyn. Hnh 3.3 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 c gim i 1 mi khi qua mt router. Gi s b loi b nu TTL = 0, iu ny cho php cc router ngn chn cc datagram b run-away. Khng ging nh IP, Ethernet khng c trng TTL, v vy sau khi mt frame bt u b lp trong mng th n vn tip tc cho n 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 gy ra vng lp feedback rt nhanh theo t l s m. V c mi frame trn qua nhiu cng ca switch, 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 trong hnh 3.2. Broadcast c x l bi CPU tt c cc thit b trn mng. Trong trng hp ny, cc PC u c x l bo broadcast. Nu ta ngt mt trong s cc kt ni, th n tr li hot ng bnh thng. Tuy nhin, ngay khi ta kt ni n tr li th broadcast s s dng 100% CPU. Nu ta khng x l iu ny m vn tip tc s dng mng, th s to ra vng lp vt l trong mng. Vic sai lch bng bridge Nhiu nh qun tr switch/bridge nhn thc vn c bn ca bo broadcast, tuy nhin ta phi bit rng thm ch cc unicast frame cng c th truyn mi trong mng m cha 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 khi mng, 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 cng 1/1 trn c hai switch. Bc 3: v host B b down, nn Cat-1 khng c a ch MAC (BB-BB-BBBB-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-BBBB-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 ch MAC l AA-AA-AA-AA-AA-AA. N thay i a ch MAC ca host A trong bng bridge dn n sai cng.


Hnh 3.4: frame unicast cng c th gy ra Bridging Loop v lm sai lnh bng bridge V frame b lp theo hng ngc li, nn ta thy a ch MAC ca host A b ln gia cng 1/1 v 1/2. iu ny khng ch lm mng b trn vi cc gi unicast m cn sa sai bng 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 trong cu 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 :

Chng trnh tr gip thit k mng Campus a ch MAC: c 6 byte c gn cho switch. Catalyst 5000 v 6000 s dng mt trong s cc a ch MAC t vng 1024 a ch gn cho mi gim st vin (supervisor) v bng ni a nng (backplane). a ch MAC trong BID s dng nh dng hexa.

Ch : mt vi Catalyst ly a ch MAC t module gim st (nh Catalyst 5000) v ly 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 v c bn l nh nhau nhng c mt vi im khc bit gia IEEE STP v DEC STP nh DEC 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 chi ph l 1000 Mbps bng cch chia bng thng ca lin kt. V d nh mt lin kt 10BaseT 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 kt qu 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 ngn cn 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 tnh co dn khng tuyn tnh. Bng 3.1 cho ta mt danh sch gi tr chi ph mi. Bng thng 4 Mbps 10 Mbps 16 Mbps 45 Mbps 100 Mbps 155 Mbps 622 Mbps 1 Gbps 10 Gbps Chi ph STP 250 100 62 39 19 14 6 4 2

Chng trnh tr gip thit k mng Campus 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 lin kt 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 dng trnh 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 trnh t bn bc ny lu mt bn sao ca BPDU tt nht trn mi cng. Khi nh gi, n xem tt c BPDU nhn c trn cng cng nh BPDU gi i trn cng . Mi BPDU n u c kim tra theo trnh t bn bc ny, nu tt hn BPDU hin ti th n c lu 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 mt ln (y l gi tr mc nh ca b nh thi). Tuy nhin, nu mt cng lng nghe mt BPDU 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) th cng tip tc gi BPDU li ln na. Ch : c 2 loi BPDU l BPDU cu hnh v BPDU thng bo thay i cu trc mng (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 trc mng logic cha vng lp (loop-free). Mc d c nhiu kha cnh STP, nhng s hi t ban 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 mt cch ln xn. Tuy nhin, cc bridge ny s lp tc p dng trnh t bn bc ( phn 3.1.3). Mt bridge gc c quyt nh hot ng nh l trung tm ca vn vt i vi mng. Tt c cc bridge cn li tnh ton vic thit lp cc cng gc v cc cng ch

Chng trnh tr gip thit k mng Campus nh xy dng cu trc mng cha loop-free. Kt qu l bridge gc ging nh mt hub vi cc ng i loop-free ra bn ngoi. Khi mng c trng thi n nh, th bridge gc s gi 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 s dng qu trnh thay i cu trc mng. Hnh 3.6 l m hnh ca mt mng switch/bridge. Mng ny gm c ba bridge kt ni thnh mt vng lp. Mi cu ni c gn mt a ch MAC khng c tht tng ng vi tn 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 mt bridge 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 trnh chn 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 trng con 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-AAAA-AA-AA-AA. Cat-B l (32.768, BB-BB-BB-BB-BB-BB) v Cat-C l (32.768, CCCC-CC-CC-CC-CC). V c ba bridge u s dng Bridge Priority l 32.678 nn a ch MAC thp nht l AA-AA-AA-AA-AA-AA v Cat-A tr thnh Bridge gc. Hnh 3.7 m t qu trnh ny. Ch : gi tr BID cng l thp nht.


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 tin Spanning Tree ln nhau. Cc BPDU c gi mc nh 2s mt ln. Cc BPDU l lu lng bridge-to-bridge, n khng mang lu lng end-to-end. Hnh 3.8 m t cc phn c 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 gc da vo trng Root BID. Bridge ny lun t BID ca chnh n trong Sender BID. Ch : Root BID l ID ca bridge gc hin ti, trong khi Sender BID l ID ca bridge cc b hoc switch. Khi bridge khi ng ln u tin, n lun lun t BID trong c hai trng Root BID v Sender BID. Gi s rng, Cat-B khi ng u tin v bt u gi cc BPDU thng bo chnh n l Bridge gc 2s mt ln. Mt vi pht sau Cat-C khi ng v thng

Chng trnh tr gip thit k mng Campus bo chnh n l Bridge gc. Khi BPDU ca Cat-C n Cat-B, Cat-B s loi b BPDU v n 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 gi BPDU 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 b iu ny trong BPDU. Ngay lp tc cc BPDU n Cat-B v C, cc switch ny s nhng bridge gc li cho Cat-A. By gi tt c 3 switch u gi cc BPDU thng bo Cat-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. Cng gc l mt cng trn bridge cc b. Mi brigde (tr bridge gc) phi la chn mt cng gc. Ch : Mi bridge (tr bridge gc) s la chn cng gc. Bridge s s dng khi nim chi ph xt cng gc. C th l cc bridge theo di chi ph ng i gc, chi ph tch ly ca tt c cc lin kt n bridge gc. Hnh 3.9 m t lm 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 l 0. (2): khi B nhn cc BPDU ny, n thm vo chi ph ng i ca cng 1/1vo chi ph ng i gc cha trong BPDU nhn. Gi s rng mng ang chy switch Catalyst 5000 c m ln hn phin bn 2.4 v ba lin kt trong hnh 3.9 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 s thm 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 cc switch xui dng.


Hnh 3.9 : chn Root Port Hnh 3.9 biu 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 c gi ra khi cng. V d nh, cc BPDU n trn cng 1/1 ca Cat-B vi chi ph 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 cc BPDU 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 l gi tr ca trng chi ph. i vi mt bridge, gi tr ny c tnh bng cch cng 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 nng nhn v gi lu lng n on mng kia v bridge gc. Nu ch c mt cng nm gi lu lng trn mi lin kt, th tt c vng lp b ph b. Bridge cha cng c ch nh c gi l designated bridge cho on mng .

Chng trnh tr gip thit k mng Campus Vic la chn cng c ch nh cng da trn chi ph tch ly ca ng i gc n bridge gc (hnh 3.10).

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 ca Cat-A c chi ph ng i gc l 0, v cng 1/1 ca B l 19 (gi tr 0 c nhn trong BPDU t A cng vi chi ph ng i c gn cho cng 1/1 ca B). V cng 1/1 ca A c 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 thnh cng c ch nh. Ch l mi cng hot ng trn bridge gc u tr thnh cng c 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 ca C u c chi ph ng i gc l 19. y l mt s hn ch, v STP thng s dng trnh t 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.BBBB-BB-BB-BB-BB) v ca C l (32.768.CC-CC-CC-CC-CC-CC), do cng 1/2 ca B l Cng c ch nh cho on 3.

Chng trnh tr gip thit k mng Campus V d trong mt mng cha 15 switch v c 146 on mng (mi cng l mt on mng duy nht), s thnh phn STP hin c l Cc thnh phn STP Bridge gc Cng gc Cng c ch nh S 1 14 146

Bng 3.2: Cc thnh phn STP trong mng 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 tr thp nht cng c s dng chn cng gc v cng c ch nh). Mt cng s ngng 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 ch nh th kha lu lng. Vic chuyn tip v kha ch l 2 trng thi thng thng trong mng, bng 3.3 m t 5 trng thi ca STP.


Trng thi Chuyn tip (forwading) Hc hi (learning) Lng nghe (listening) Kha (blocking) V hiu ha (disable)

Mc ch Gi v nhn d liu ngi dng Xy dng bng bridge Xy dng cu trc mng active Ch nhn cc BPDU 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 cc BPDU. Do s a dng ca cc s kin m bridge truyn trong trng thi listening (v d nh mt bridge ngh n l bridge gc ngay sau khi khi ng). trng thi ny, khng c d liu ngi dng c truyn qua, tc l cng ang gi v nhn cc BPDU c gng 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 c ch 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) s chuyn 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, n a a ch MAC v cng vo bng bridge. Trng thi learning s gim bt s lng trn 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 thng tin nh VLAN ngun. Nu mt cng vn l cng c ch nh hay cng gc khong thi gian cui ca trng thi learning, th cng chuyn qua trng thi forwading. trng thi ny, n bt u gi v nhn cc frame ca ngi dng. Hnh 3.11 m t trng thi cc cng v vic chuyn trng thi.


Hnh 3.11: trng thi cc cng v hot ng chuyn trng thi Hnh 3.12 biu din mng vi s phn chia cng v danh sch cc trng thi. Ch l tt c cc cng u chuyn tip tr cng 1/2 ca Cat-C.

Hnh 3.12: s mng vi cc cng c nh danh


Trng thi/cng Blocking Forwading Cng c ch nh Cng gc Cng khng c ch nh

K hiu B F DP RP NDP

Bng 3.4 : Cc trng thi STP v cc k hiu cng 3.6 B nh thi gian STP Mt bridge tri qua 15s mi trng thi listening v learning. STP c iu khin bi ba b m thi gian (timer) nh trong bng 3.5. Timer Hello Timer Forward Delay Max Age Mc ch Khong thi gian gi cc BPDU cu hnh gi bi Bridge gc Thi hn trng thi Listening v Learning Thi gian lu tr BPDU Bng 3.5: STP Timer V d: gi s rng lin kt on 3 trong hnh 3.12 s dng mt hub v cng 1/2 ca Cat-B truyn ra ngoi. Cat-C khng thng bo li lin v n vn ang nhn lin kt Ethernet t hub. Cat-C ch thng bo l cc BPDU ngng n. Sau 20s (Max Age), th cng 1/2 ca Cat-C ly thng tin BPDU c vi cng 1/2 ca Cat-B l cng c ch nh cho on mng 3. iu ny lm cho cng 1/2 ca Cat-C truyn trong trng thi listening c gng tr thnh cng c ch nh. V vy cng 1/2 ca Cat-C cung cp truy cp tt nht t bridge gc n lin kt ny, nn n chuyn sang trng thi forwarding. Nh vy, Cat-C mt 50s (20s Max Age + 15s Listenning + 15s Forwarding) vt qua sau khi cng 1/2 trn Cat-B b li. Gi tr mc nh 2s 15s 20s

Chng trnh tr gip thit k mng Campus Trong trng hp ny, cc bridge c th pht hin s thay i cu trc mng trn cc lin kt kt ni trc tip v ngay lp tc chuyn sang trng thi listening m khng cn ch thi gian Max Age. Xem v d trong hnh 3.13.

Hnh 3.13: li xy ra trn lin kt gia Root Bridge v Root Port ca Cat-C Trong trng hp ny, cng 1/1 ca Cat-C b li, v lin kt trn cng gc cng b li nn ngay lp tc cng 1/2 ca Cat-C chuyn sang trng thi learning tr thnh cng gc 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 nhc cn 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 Change Notification BPDU).

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


BPDU cu hnh : cc trng trong BPDU cu hnh c tm tt trong bng 3.6 Trng Protocol ID Version Type Chiu di (octet) 2 1 1 Lun bng 0 Lun bng 0 Cho bit kiu BPDU BPDU cu hnh = 0 Flag 1 LSB = C thay i cu trc mng MSB = C xc nhn thay i cu trc mng Root ID Root Path Cost (chi ph ng i gc) Sender BID Cng ID Message Age Max Age Hello Time Forward Delay 8 4 BID ca bridge gc hin ti Chi ph tch ly n bridge gc ngha

8 2 2 2 2 2

BID ca bridge hin ti ID ca cng gi BPDU ny Khong thi gian t khi bridge gc to BPDU n khi pht BPDU i. Khong thi gian lu thng tin BPDU Khong thi gian gia cc BPDU Thi gian trong trng thi listening v learning

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 ba trng 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.

Chng trnh tr gip thit k mng Campus 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 BPDU c dng cp nht bng Bridge nhanh hn Nh tho lun, cng 1/2 ca Cat-C mt 50s tr thnh cng c ch nh. Tuy nhin nu khng c TCN BPDU th n tip tc b ngt khong 250s. Trong khong thi gian li, c ba switch u cha a ch MAC ca host E trong bng Bridge nh bng 3.7. Bng Bridge Cat-A Cat-B Cat-C Cng lin quan n a ch MAC ca host E Cng 1/1 Cng 1/2 Cng 1/1

Chng trnh tr gip thit k mng Campus Bng 3.7: Gi tr bng Bridge trc khi c s thay i cu trc mng TCN BPDU l mt phng php n gin ci tin thi gian hi t, v n lm vic cht 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 mt cng c ch nh. N chuyn mt cng t trng thi forwarding hoc learning sang blocking.

S thay i cu trc mng i hi phi gi thng bo n bridge gc, gi s rng bridge hin ti khng phi l bridge gc, th n bt u qu trnh thng bo bng cch gi TCN BPDU ra cng gc ca n. N tip tc gi TCN BPDU cho n khi thng ip TCN c xc nhn. 2. Bridge upstream s nhn TCN BPDU. Mc d, mt vi bridge nghe c TCN BPDU (v n kt ni trc tip vo cng gc ca on mng) nhng ch c 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 gi ngc li(ra cng c ch nh). C ny dng xc nhn vi bridge khi u 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 (Topology Change) 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 cc BPDU 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 chuyn tip - Forward Delay). Hnh 1.15 tm tt cc bc trong qu trnh thay i cu trc mng.


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 3.14 nh sau: bc 1 Cat-B v C gi TCN BPDU ra cng 1/1. V bridge upstream cng l bridge gc nn b qua bc 3 v 4. Sau bc 2 v 5 xy ra ng thi. Trong BPDU cu hnh k tip m bridge gc gi i, c TCN ACK s c thit lp xc nhn l nhn TCN ca hai brigde downstream. Tip theo l bc 6 v 7, Cat-A cng thit lp c TA trong 35s (Forward Delay + Max Age) cp nht bng bridge nhanh hn. Nh vy c 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 lm qu trnh ny nhanh hn thi. Cc thit b tip tc ni sut 15s ny m khng cho bng bridge ngh. Tuy nhin, nu host D c gng gi mt frame n host E trong 20s (gi s host E khng ni g ht), th frame s c trn n tt c cc on mng v a ch EEEE-EE-EE-EE-EE khng cn c trong bng bridge na. Ngay khi frame n host E v host E tr li, th switch hc c gi tr bng bridge mi tng ng vi cu trc mng mi. Bng 3.8 biu din ton b bng bridge cho a ch MAC ca E trn c ba switch sau khi cu trc mng mi hi t v lu lng li tip tc.


Bng Bridge Cat-A Cat-B Cat-C

Cng lin quan n a ch MAC ca host E Cng 1/2 Cng 1/1 Cng 1/2

Bng 3.8: Gi tr bng Bridge sau khi thay i cu trc mng Ti thi im ny, kt ni gia host D v E c thit lp li v lu lng li tip tc. Ch l TCN BPDU gim thi gian li t 300s (5ph) xung 50s. Hnh 3.16 m t trng c trong BPDU cu hnh, c hai c TCA v TA u c lu tr 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 bridge dowstream ngng gi TCN BPDU. Cn c TC c thit lp bi bridge gc gim khong thi gian li t 300s xung 15s (Forward Delay).

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Tóm tắt Spanning - Tree Protocols