các loại spanning treet.doc

Bi vit ny nhm phn bit cc loi STP . Tm thi , c th chia ra cc loi sau : - CST - PVST- PVST+- MST ( IEEE 802.1s)- IST- STP ( dng truyn thng 802.1d )- RSTP ( IEEE 802.1w)1 . CST : Common Spanning Tree . Ta bit IEEE 802.1q khng ch nh ngha lm sao m cc vlan c th trunking qua cc switch , n cn ch ra rng ch c mt tin trnh STP duy nht chy trn ton b cc vlan m thi . V tin trnh duy nht ny ngi ta gi tn l CST . Tt c cc CST BPDUs c vn chuyn trn ng trunk bng native vlan vi cc untagged frame . Vic chy duy nht mt tin trnh STP c li ch n khng tiu tn nhiu kh nng x l ca CPU khi phi tnh ton li STP topology , cng nh lm cho cng vic cu hnh ca chng ta tr nn n gin . Tuy nhin , n hn ch ch s khng cho php load balancing c thc hin ( layer 2 s cp cc trang di ) .(loadbalancing 2 . PVST : per VLan Spanning Tree . y l mt version c quyn ca Cisco , cho php hot ng linh hot hn CST . Mi vlan s chy mt tin trnh STP ring bit v khng b ph thuc ln nhau . u im : lm gim kch thc ca STP topology , chia ra lm cc topology nh hn , dn n thi gian converge ca STP gim xung . Thm vo , n cn cung cp thm kh nng load balancing . Khuyt im : s dng nhiu ti nguyn ca CPU trong vic qun l nhiu vlan . PVST s dng ISL trunking nn i vi mt mng tn ti 2 dng STP l PVST v CST th s dn n khng tng thch v mt trunking .( CST xi dot1q ) . Do BPDUs s khng c qung b gia 2 loi STP ny . 3 . PVST+ : per Vlan Spanning Tree Plus Cisco tung ra mt version khc ca STP l PVST+ nhm gii quyt vn tng thch gia CST v PVST . PVST+ m nhim vai tr nh l mt translator gia PVST v CST . PVST+ c th giao tip vi PVST qua kt ni ISL trunking , ngc li PVST+ c th giao tip vi CST qua kt ni dot1q trunking . Ti bin gii ca PVST v PVST+ s din ra vic mapping STP one-to-one . Ti bin gii ca PVST+ v CST s din ra vic mapping gia mt STP ca CST v mt PVST trong PVST+ ( STP trong CST ch c mt m thi ) .

INCLUDEPICTURE "http://tinypic.com/f24o76.jpg" \* MERGEFORMATINET

Trong hnh trn c mt k hiu l MST , nhng MST y khng phi l Multiple Spanning tree m l Mono Spanning tree . Ta c th hiu MST trong trng hp ny ng ngha vi li CST . 4 . MST : Multiple Spanning Tree . Trong hu ht cc network , mt switch u c kt ni d phng ti switch khc . V d mt access-layer switch s c 2 kt ni uplink n 2 distribution / core switch khc nhau . Khi STP hot ng dng CST th ch c mt kt ni l forwarding v kt ni kia ri vo trng thi blocking . Nh vy chng ta lng ph mt mt kt ni . Chng ta s sp xp lm sao s dng ng thi 2 kt ni trn nhm mc ch load balancing . PVST+ c v l mt la chn hp dn , v n cho php cc vlan khc nhau to ra nhng loop-free topology khc nhau v chy trn cc ng uplink khc nhau . Tuy nhin , nu s lng Vlan tng ln , ng ngha vi vic xut hin nhiu STP instance hn s lm cho CPU tiu tn nhiu resource hn . Ngh xa hn , v d chng ta c 100 Vlan tng ng chng ta cng s c 100 loop-free topology . Nhng 100 topology ny c mt im chung l u i qua mt trong 2 kt ni uplink .

Trong hnh trn , c 2 STP instance , mi instance chy cho mt vlan . Topology cho mi instance l c lp v c ch ra nh hnh v . Vlan A c th chn ng uplink bn tri i , v blocking ng uplink bn phi . Tng t , Vlan B c th chn ng uplink bn phi i v blocking ng uplink bn tri . Nh vy c 2 ng uplink u c s dng , load balancing c thc hin . Khng loi tr trng hp c vlan A v Vlan B u tnh ton v chn ra i mt ung uplink duy nht ( v d ng uplink bn phi ) . Tuy nhin chng ta s phi can thip vo qu trnh tnh ton ca chng bng cch cu hnh sao cho c 2 ng u c s dng . Cng vi hnh ny , nu chng ta c 100 vlan th topology ca cc instance tnh ton c cng ch i qua mt trong 2 kt ni uplink m thi . Nh vy vic s dng ti 100 STP instance trong khi topology th ch i qua mt trong 2 ng uplink s lm tiu tn rt nhiu resource ca CPU . Ta s hn ch s instance li bng cch cho instance th nht chy cho 1 range vlan ( v d t 1-50 ) , instance th 2 chy cho range vlan t 51-100 . Chng ta hn ch c s lng instance ng thi vn m bo cho load balancing hot ng . V d cu hnh load balancing :

Ta s cu hnh sao cho range vlan 3-6 s ly ng trunk 2 lm forwarding v trunk 1 l blocking . Trong khi , range vlan 8-10 s ly ng trunk 1 lm forwarding v trunk 2 li l blocking . Nu mt trong 2 kt ni trunk 1 hoc trunk 2 b down , ton b traffice trong c 2 range vlan u dn v mt pha v chy . C th cu hnh bng cch gn priority nh hnh v . on trn m t khi nim ca MST . Nh vy MST c xy dng da trn vic mapping mt hay nhiu vlan vo mt STP instance . Khi trin khai MST , chng ta cn ch nhng iu sau y : - S lng Instance cn phi dng l bao nhiu - Range vlan cn map cho mi instance l g . hiu c cc khi nim k tip , chng ta cn phi nm c MST region l g ? switch c th tham gia vo mt MST instance th n phi c cu hnh vi cc thng s nht qun vi nhau . Cc thng s ny bao gm MST region , revision number , v MST Vlan map . Tp hp cc switch c cu hnh MST ging nhau to thnh MST region . 5 . IST , CIST , CST :CST chy trn ton b domain , CST coi cc MST region nh l mt big switch tham gia vao CST instance . CST lc ny s to ra mt loop-free topology cho cc MST region . Trong mi MST region cng cn phi c cc sub STP instance to ra mt sub loop-free tree . Cc instance chy trong mt MST region gi l cc MSTIs . V cc MSTIs ny c qun l bi mt IST instance . Trong MST , th IST c coi nh l mt locally CST trong MST . Cc MSTIs c nh s t 0-15 . Trong , MSTIs 0 ( instance 0 ) chnh l IST . IST i din cho ton b MST region giao tip vi cc MST region khc . Ch c IST instance l cho php gi v nhn cc MST BPDUs . Thng tin ca cc MSTIs khc c a vo BPDUs vi gi tr M record . Mc d MST region c th c ti 16 MSTIs cng chy nhng ch c mt BPDUs l mang thng tin ca chng i . MSTIs ch c ngha cc b trong mt MST region m thi . Nu MST region 1 c MSTI 2 , MST region 2 cng c mt MSTI c nh s l 2 th cng khng h b nh hng . Nu mt MST region kt ni vi mt PVST+ switch th n s nhn bit ngay c iu ny bng cch kim tra BPDUs . Nu BPDUs c nghe thy t nhiu VLAN , ngha l neighbor ang chy PVST+ . Khi MST region gi BPDUs cho PVST+ switch n se replicate IST BPDUs cho mi Vlan trong PVST+ . CIST l mt tp hp cc MSTIs trong mt MST region . IST trong mt MST region lc converge s to ra mt IST master ( CIST regional Root ) ca region . Sau cc CIST regional root nay s tip tc c gi ra ngoi mt Region tip tc sc vi cc CIST regional root khc . Kt qu ca cc cuc sc ny l to nn c mt CST root .