Trc ht chng ta hy nh li format ca BPDU v cch thc m chng so snh vi nhau to ra loop-free topology cho mng switch network

Field Description Number of Bytes

Protocol ID (always 0) 2

Version (always 0) 1

Message Type (Configuration or TCN BPDU) 1

Flags 1

Root Bridge ID 8

Root Path Cost 4

Sender Bridge ID 8

Port ID 2

Message Age (in 256ths of a second) 2

Maximum Age (in 256ths of a second) 2

Hello Time (in 256ths of a second) 2

Forward Delay (in 256ths of a second) 2

Mt BPDU c coi l tt hn BPDU khc khi:

- N c Bridge ID tt hn ( BID) ( gi tr thp hn s tt hn )- Khi BID bng nhau th s so snh tip Root path cost ( lowest)- Khi BID bng nhau , Root path cost cng bng nhau th so snh tip Sender ID ( lowest)- Nu cc gi tr trn u bng nhau thi so snh tip Port ID ( lowest).

Ta s xem xt tip STP s x l nh th no i vi Indirect Link Failure:

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Ba Switch R , B , S ang kt ni full-meshed vi nhau . Gi s STP converged v R ang l Root Bridge , S ang block port P . Tin trnh ng ph ca STP i vi indirect link failure s xy ra nh sau : 1 . Nu Link L1 down , B s nhn thy kt ni trc tip ca mnh va b down v lp tc t nhn mnh l root . N gi BPDU cho S thng bo rng n l newroot . 2 . Khi S nhn c BPDU mi ny , n nhn thy y l mt inferior BPDU , km cht lng hn BPDU n ang lu trn port P nn S s ignore BPDU ny . 3 . Sau khi khong thi gian Max_age ht hn , S s age_out thng tin BPDU cha trong port P . Port P lc ny s chuyn sang trng thi Listening v gi 1 BPDU tt hn cho B . 4 . Khi B nhn c BPDU mi tt hn , n s khng gi ra BPDU ca n na . 5 . Port P chuyn t trng thi Listening + Learning mt 30s , cng thm khong thi gian Max_age l 20s na , nh vy phi mt 50s th mng mi hot ng li bnh thng c . Tnh nng Backbone Fast cho php chng ta tit kim c khong thi gian Max_age 20s bng cch age_out thng tin BPDU trn mt port ngay lp tc nu port nhn c mt inferior BPDU . Khi mt switch gi i mt inferior BPDU , ngha l bridge ny : - Mt kt ni n Root v bt u gi mt inferior BPDU . - Path cost dn n Root tng ln .

Ch trng hp ngoi l sau y i vi Indirect Link Failure

Link Failure xy ra gia R v Hub , B s khng pht hin c kt ni down ngay lp tc m phi i cho n khi Max_age ht hn mi pht hin c . Backbone Fast s x l nh th no i vi trng hp Indirect link failure ? Khi nhn c mt inferior BPDU, thay v switch phi th ng i cho Max_age ht hn , switch s ch ng ping root bng Root Link Query PDU. N cho php switch nhanh chng xc nh c BPDU n lu trn non-designated port l cn gi tr hay l phi discard i.

Khi nhn c mt inferior BPDU t designated bridge , Switch s gi RLQ Request trn tt c cc non-designated port ( block port v root port ), ngoi tr port m n nhn vo v port thuc loi seft-loop. (seft-loop port l port c kt ni vi mt port khc trn cng mt switch v to thnh loop). Khi nhn c RLQ response , nu c tr li l negative , tc l port ny mt kt ni n Root v Switch ngay lp tc Age_out thng tin BPDU lu trn port ny. Thm vo , nu nh switch nhn c RLQ response u l negative ngha l Switch mt Root v bt u tnh ton li STP t u. Nu nhn c RLQ response l ok th Switch ngay lp tc Age_out Port nhn c inferior BPDU ban u ngay. V d : cc port A , B , D , E l cc non-desinagted port ca Switch S , A l root port , B , D , E l block port . Khi port E nhn c Inferior BPDU th backbone fast s kch hot v lm gim thi gian converge xung .

Switch S gi RLQ request trn tt c cc non-designated port ( ngoi tr port E l port nhn inferior BPDU ) .

RLQ reply s ch ra rng , c th n c bng port ny hay khng ? Port D nhn c thng tin khng n c root . Switch S s age_out port D v chuyn sang trng thi Listening , tuy nhin lc ny Port E vn cha b Age_out do Switch S vn cha bit r l mnh cn n c root na hay khng. (phi ch thm cc thng tin reply khc na).

Port A v port B nhn c RLQ response ch ra rng vn cn kt ni ti R . Thng tin BPDU trn port E by gi c th b age_out ngay .

Port E chuyn sang trng thi Listening , khng phi ch n thi gian max_age ht hn na . Thut ton STP s quyt nh port E hay port D l blocking hay forwarding .

Tm tt:

- Nu inferior BPDU c nhn trn mt port ang trng thi Blocking , Switch s d tm ng i n root thng qua cc non-designated port thng qua root port hay cc blocking port khc . - Nu inferior BPDU c nhn trn root port , Switch s tm ng i n root thng qua cc blocking port khc . - Nu inferior c nhn trn root port v switch ny khng c port no l blocking port c th Switch coi nh mt kt ni ti Root v n s bt u li gii thut STP bng cch gi s mnh l root v gi i BPDU . Backbone fast trong trng hp ny cho php loi b i khong thi gian max-age v lp tc chuyn sang trng thi Listening . Ch : R rng Switch phi i nhn c tt c cc RLQ reply, trong c t nht mt RLQ reply n c root th Switch mi age-out thng tin trn port nhn inferior BPDU. Nh vy tit kim c thi gian max-age th switch phi chu mt khong thi gian relay i cc RLQ reply.Tuy nhin, khong thi gian Relay ny rt thp , ch khong vi giy m thi nn vn m bo cho vn tit kim max-age timer. Khi enable backbone fast phi enable trn tt c cc switch, v tt c cc switch phi process cc RLQ PDU. Tnh nng ny c p dng cho Core layer v trng hp indirect link failure .