1. S dng cc topology cho php MLS:- Mt vi topology h tr MLS. Theo bn cht ca MLS, ch topology h thng chc chn mi cho php cc gi i din v enable i qua router v switch ng cch. Nu c hai gi i din v enable khng c nhn dng, th khng th hon thnh mc lu tr lung. Cc topology cho php MLS bao gm:

Router on a stick: gm c mt router (internal RSM/MSFC router hoc external router) v mt switch. Router c mt kt ni vo mng, n c gi l stick. (xem hnh 4.7)

Nhiu switch v mt router: n ch c chp nhn nu ch c mt switch kt ni n router v cc switch c kt theo mt ISL trunk.

2. CEF (Cisco Express Forwarding) chuyn tip vi mc ch ring bit ca Cisco.- Catalyst 3550 v 4000 thnh thong c m t nh l MLS. Tht vy, 3550 c qung b h tr chuyn mch a lp da trn CEF. CEF khng ging nh s thc thi MLS khc, n khng lu tr nh trc y na. Cache a ra mt s cn thit nh a ch. V d nh, mt cache ng trong bao lu? cache cho php ln thm bao nhiu? V lm th no ta gii quyt s thay i topology nh tuyn m khng da vo mc lu tr.- sn sng thy s tin trin t nh tuyn n chuyn mch lp 3, hy xem xt qu trnh chuyn mch, chuyn mch nhanh, v chuyn mch trong iu kin tt nht.

2.1 Qu trnh chuyn mch (Process Switching)- Khi cc gi c chuyn mch, th gi c chuyn tip qua cu trc bn trong n b x l nh tuyn. y l tri tim ca router v l ni bn rn, thng truy cp theo hai bus, l Cbus v bus h thng. Cc gi i khp router v c chuyn tip. B x l nh tuyn s chuyn tip qua interface v ghi li thng tin MAC header. Tuy c tr ng k, nhng cng c mt vi thun li nh nu bng nh tuyn gi nhiu ng i n ch vi chi ph nh nhau, th vic cn bng ti c thc hin.- Process Switching c biu din trong hnh 4.9, s ny m t bn cht ca Process Switching, mt gi phi chuyn qua b x l nh tuyn ca router, v kt qu l chuyn tip chm.

2.2 Chuyn mch nhanh (Fast Switching):- Ging nh Process Switching, Fast Switching c sn trn nn Cisco, k c Catalyst 2500. Fast Switching s dng cache trn b x l nh tuyn bo qun thng tin chuyn tip. Gi u tin trong mt cuc ni chuyn c chuyn n b x l nh tuyn, nh x da vo ng i, v x l chuyn mch. Cache ca Fast Switching c cp nht v cc gi theo sau ch nh x thng tin header vo trong cache. Kt qu l cuc ni chuyn c chuyn tip m khng qua b x l nh tuyn.- Thng tin chuyn tip c lu tr trong mt cy nh phn, cho php quyt nh bit-by-bit thc hin i vi chng k tip. Cy nh phn ny i hi 32 mc so snh nh x hon ton mt ng i, v c xem nh l mt k thut truy tm rt hiu qu.- Cc mc trong cache ca Fast Switching c to khi bt u mt cuc ni chuyn, v v vy phi chu ng vn tn ti ca cache l lm th no cp nht thng tin khc, nh ARP cache, nh hng n thng tin lu tr? V cu tr li l n khng cho php thay i trong ARP cache m n ch b cache khi qu gii hn thi gian v thng tin khng cn ng na. Trong trng hp ny, cache phi c to li. Vn th 2 vi Fast Switching l cache ch xy dng mt con ng n ch, v vy vic chia tai phi da trn cuc ni chuyn vi mt mc lu tr cho mi cuc ni chuyn.- Tuy nhin, Fast Switching c l nhanh hn gp 10 ln so vi tin trnh chuyn mch v c s dng rng ri.- Fast Switching Tree c biu din trong hnh 4.10. Mi bit trong a ch ch c so snh vi bng v v v ch c 2 kh nng l 1 hoc 0, nn mt nh x thnh cng vi mt con ng.2.3 Chuyn mch trong iu kin tt nht (Optimum Switching):- Optimum Switching cng da trn vic lu tr, nhng c mt s khc nhau quan trng vi Fast Switching. im khc nhau u tin l hot ng ca cy. Thay v dng cy nh phn vi mi mc l mt so snh, th n s dng 256 cch (multi-way tree). Mi mc cho php chn mt octet trong a ch ch, kt qu l tm c bt k a ch ch no.- Optimum Switching rt nhanh, nhng vn chu ng cng vn ca cache v v vy cn c c khong thi gian ngt cc lung optimum trong khi cache c xy dng li t cc yu cu n b x l nh tuyn ln na.- Cy ca Optimum Switching c biu din hnh 4.11. Mi octet trong a ch chm thp phn 32 bit c nh x ring l, v kt qu l qu trnh truy tm nhanh hn.

2.4 Qu trnh chuyn tip CEF (CEF Forwarding Process):- CEF duy tr 2 bng ring bit nhng c quan h vi nhau, l bng chuyn tip (forwading table) v bng k tip (adjacency table). Bng chuyn tip cha thng tin nh tuyn v bng k tip cha a ch chng k tip (lp 2). CEF s dng mt trie thay cho cy. Trie l mt con tr s dng vi cu trc d liu nhng trn thc t khng cha d liu. N cho php d liu c lu tr thch hp nht vi kiu d liu c lu tr, v qu trnh truy tm c tch ring ra t d liu.- S tch ring trong cu trc d liu c ngha l qu trnh truy tm c quy, cho php cc con ng i khc nhau c la chn cho cc gi k tip, v vy chia ti c cc gi enable. Ngoi ra, nu thng tin trong cache thay i, v vic tuy tm c thc hin ring bit mi ln, nn hu ht thng tin up-to-date lun c s dng.- Qu trnh chuyn tip CEF c biu din trong hnh 4.12. S ny m t vic truy tm nhanh hn v cu trc d liu 256-way ny l hiu qu nht ca tt c cc phng php truy tm, v c kt hp trc tip vi bng k tip.

- Kt qu ca qu trnh chuyn tip l thng lng cao hn nhiu. S tng thm tc do cu trc ring bn trong switch hoc router, bao gm vic s dng tng thm ASIC (Application-Specific Integrated Circuit), cc bus chuyn dng v s sp xp b nh. Nhng n cng ng vi cc gi khng cn chuyn tip qua cc bus bn internal n b x l nh tuyn na. Ngoi ra cn c nhiu li ch khc t CEF nh kh nng h tr chia ti m khng th t c khi s dng mc lu tr trong Fast Switching v Optimum Switching.