Bi co co v giao thc nh tuyn OSPF==================================================================GIAO THC NH TUYN OSPFI. Gii thiu v OSPF. Open Shortest Path First (OSPF) c pht trin bi Internet Engineering Task Force (IETF) nh mt s thay th nhng hn ch cng nh nhc im ca RIP. OSPF l mt link state protocol nh tn gi ca mnh n s dng thut ton Dijkstra Shortest Path First (SPF) xy dng routing table v open ni ln tnh ph bin ca n. OSPF c John Moy a ra thong qua mt s RFC, gn y nht l RFC 2328. Ging nh cc link state protocol, OSPF c u im l hi t nhanh, h tr c mng kch thc ln v khng xy ra routing loop. Bn cnh OSPF cn c nhng c trng sau: S dng area gim yu cu v CPU, memery ca OSPF router cng nh lu lng nh tuyn v c th xay dng hierarchical internetwork topologies. L giao thc nh tuyn dng classless nn h tr c VLSM v discontigous network. OSPF s dng a ch multicast 224.0.0.5 (all SPF router) 224.0.0.6 (DR v BDR router) gi cc thong ip Hello v Update. OSPF cn c kh nng h tr chng thc dng plain text v dng MD5. S dng route tagging theo di cc route. OSPF cn c kh nng h tr Type of Sevice.II. Hot ng ca OSPF Cc OSPF-speaking router gi cc Hello packet ra tt c cc OSPF- enable interface. Nu 2 router sau khi trao i Hello packet v tha thun mt s thong s chng s tr thnh neighbor. Adjacency c th c to qua virtual point link hay to qua mt vi neighbor. OSPF nh ngh ra mt s loi network v mt s loi roter. S thit lp mt adjacency c xc nh bi loi trao i Hello v loi network m Hello trao i qua. Mi router gi cc link state advertisement (LSA) qua tt c adjacency. LSA m t tt c cc interface cu router (link) v trng thi ca link. Cc link ny c th l stub network, ti OSPF router khc, ti network trong cng mt area, ti external network. Do c rt nhiu loi link state information cho nn OSPF nh ngha ra n 11 loi LSA. Mi router nhn mt LSA t neighbor vi link state database ca neighbor v gi mt copy cu LSA ti tt c neighbor khc ca n.Bng cch flooding csc LSA ton b mt area, tt c router s xy dng chnh xc link state database.Khi database c hon tt, mi router s dng thut ton SPF xy dng nn SPF tree.Mi router s xy dng nn routing table t SPF tree. 1. Neighbor v AdjacencyTrc khi bt k LSA no c gi, OSPF router phi khm ph neighbor ca chng v thit lp adjacency. Cc neighbor s c ghi li vo trong neighbor table, cng vi link (interface) m trn neighbor c nh v v thng tin cn thit duy tr neighbor.a. Hello protocol. Hello protocol c c trng sau: N l cch thc m neighbor c khm ph v n qung b mt vi thong s m qua 2 router phi ng trc khi chng tr thnh neighbor. Hello packet hot ng ging nh keepalive gia cc neighbor. m bo thong tin 2 chiu gia cc neighbor. Bnh bu DR v BDR i vi mi trng multi-access. OSPF-speaking router u n gi Hello packet ra tt c OSPF-enable interface. Khong thi gian ny gi l Hello Interval, mc nh khong thi gian ny l 10 giy v ta c th thay i n. Nu router khng nhn c Hello t neighbor sau khi ht thi gian RouterDeadInterval (gp 4 ln HelloInterval) n s cng b neighbor b down.b. Network types.OSPF nh ngha 5 loi network: Hello protocol c c trng sau: N l cch thc m neighbor c khm ph v n qung b mt vi thong s m qua 2 router phi ng trc khi chng tr thnh neighbor. Broadcast network: nh l Ethernet, Token Ring v FDDI. Broadcast network l multi-access trong c kh nng kt ni nhiu hn 2 thit b v chng l broadcast c ngh l tt c cc thit b c th nhn c gi tin khi ch c mt gi c truyn mt ln. OSPF router trn broadcast network s bnh bu DR v BDR s c cp trong phn sau. NBMA network: nh l X.25, Frame Relay v ATM. Chng c kh nng kt ni nhiu hn 2 router nhng khng c kh nng broadcast. C ngha l mt packet c gi bi mt router s khng th c nhn bi tt c cc router khc. Cc OSPF router nn mng NBMA c bnh bu DR v DBR nhng tt c OSPF packet u l unicast. Point-to-multipoint network: n l mt trng hp c bit trong cu hnh ca NBMA network Router trn cc mng ny khng c qu trnh bnh bu DR v BDR v cc OSPF packet c gi di dng multicast. Virtual link: l trng hp c bit trong cu hnh. OSPF packet c gi di dng unicast qua virtual link.c. Bnh bu DR v BDR c kch hot bi interface state machine, qu trnh bnh bu dc thc hin th mt s iu kin sau phi tn ti: Mi interface ca router m ni vo multi-access network c mt Router priority, l mt s nguyn t 0 n 255. i vi cc Cisco router thng s ny c gi tr mc nh l 1. Router vi priority l 0 s b loi khi qu trnh bnh bu DR v BDR. Hello packet phi c trng cho router gi xc nh router priority v IP address ca interface ca router bnh bu DR v BDR. Khi mt interface ln u tr thnh active trn multi-access network, n thit lp trng DR v BDR c gi tr 0.0.0.0. V n cng thit lp wait timer cng vi gi tr Router DeadInterval. Tn ti interface trn multi-access network ghi li address ca DR v BDR trong interface data structure. Qu trnh bnh bu DR v BDR din ra theo th t sau: Sau khi 2-way state c thit lp vi mt hay nhiu neighbor , trng Priority, DR v BDR s c xem xt trong Hello ca neighbor. Danh sch tt c router t cch tham gia bnh bu c thit lp (router c priority ln hn 0 v neighbor ca n trng thi 2-way state); tt c router cng b chng l DR (interface address ca chng c lu trong trng DR ca Hello packet); v tt c cc router cng b chng l BDR (interface address ca chng c lu trong trng BDR ca Hello packet). T danh sch nhng router t cch, n s to mt subnet nhng router khng i hi l DR. Nu mt hoc nhiu hn neighbor trong danh sch ny cha interface address ca n trong trng BDR, neighbor vi highest priority s cng b l BDR. Nu priority bng nhau th neighbor vi highest Router ID s c chn l DR. Nu khng c router cng b l DR th BDR s tr thnh DR. Nu router thc hin tnh ton l DR hay BDR mi c bu chn hay cha bnh bu c DR, BDR th thc hin repeat t bc 2 n bc 6, ch : khi mt OSPF router tr thnh active v khm ph neighbor ca n, n s kim tra hiu lc ca DR v BDR. Nu DR v BDR tn ti th router s chp nhn n. Nu BDR khng tn ti, qu trnh bnh bu BDR s din ra v router vi highest priority s tr thnh BDR. Nu priority bng nhau th router c highest router ID s tr thnh BDR. Nu khng c active DR thig BDR tng cp lm DR v qu trnh bnh bu BDR mi bt u.d. Neighbor States Down: khng c Hello packet c nhn t neighbor. Init: mt Hello packet c nhn t neighbor nhng local router khng nhn thy n trong Hello packet. Bi-directional communication cha c thit lp. Attempt: neighbor phi cu hnh bng tay cho trng thi ny. N ch p dng cho NBA network connection v cho bit rng khng c thng gn y nhn c t neighbor. 2-way: hello packet c nhn t neighbor v cha ng Router ID trong trng Neighbor. Bi-directional communication c thit lp. ExStart: Quan h Master/Slave c thit lp bng cch trao i Database Description (DD) packet. Router vi highest router ID s tr thnh Master. Exchange: thng tin nh tuyn c trao i thng qua DD v LSR packet. Loading: Link-State Request packet dduwocjguiwr ti neighbor yu cu cho bt k LSA mi c tm thy state Exchange. Full: tt c LSA c ng b gia cc adjacency.e.Xy dng mt Adjacency.

Neighbor trn point-to-point, point-to-multipoint, v virtual link network lun lun tr thnh adjacency tr phi nhng thng s trong Hello packet khng sao khp. Trn Broadcast v NBMA network, th DR v BDR s tr thnh adjacency vi tt c neighbor nhng khng c adjacency gia cc Drother.Qu trnh xy dng Adjacency s dng 3 loi OSPF packet: Database Description pace packet (type 2), Link State Request packet (type 3).Database Description packet c vai tr c bit quan trng trong qu trnh xy dng adjacency. Nh tn gi ca mnh, n mang thng tin m t tm tt ca mi LSA trong Link State database ca router gi. Nhng thng tin m t ny khng phi l cc LSA trn vn m ch n thun l header ca chng trong DD packet c 3 tng flag iu khin qu trnh xy dng adjacency. Bit I (Initial), n c thit lp cho bit DD packet u tin c gi.Bit M (Me), n c thit lp cho bit rng khng ch l DD packet cui cng c gi.Bit MS (Master/Slave), n c thit lp cho bit DD packet c gi bi Master router.2. LSA Flooding mi node a cc router mt cch thch hp chnh xc qua mng, lin mng th mi node phi c mt topology database ca ton mng.Database ny bao gm tt c cc LSA m router nhn c. bt c mt s thay i mng no u c th hin trong cc LSA. Flooding l qu trnh khi mt s thay i suy ra th cc LSA mi c gi qua mng m bo rng database ca mi node c update v ging y ht cc database ca node cn li khc.Qu trnh flooding c to bi 2 loi gi sau: Link State Update packets (type 4), Link State Acknowledgment packets (type 5).Trn point-to-point network, Link State Update packet c gi bng a ch multicast l 224.0.0.5.Trn point-to-point network v virtual link network, Link State Update packet c gi di dng unicast ti interface address ca adjacency ca n.Trn broadcast network, Drother ch l adjacency vi DR v BDR. Do update packet c gi ti DR v BDR vi a ch 224.0.0.6. Sau ch c DR router gi update di dng multicast vi a ch 224.0.0.5 ti tt c cc DRother router. Tip cc DR, BDR router, DRother router flood LSA ra tt c cc interface cn li.Trn mng NBMA network (full), qu trnh trn cng tng t nh vy tr im sau l cc LSA c gi di dng unicast.Mi mt LSA ring l c truyn u phi c bo nhn. Diu ny c thc hin bng mt trong cc cch sau: Implicit acknowledgment: neighbor thc hin bo nhn cho mt LSA bng cch gi li mt Link States Acknowledgment v ni gi. Implicit acknowledgment: neighbor thc hin bo nhn cho mt LSA bng cch gi mt copy ca LSA v ni gi.

3. Tnh ton SPF tree.Shortest Path First (SPF) l nhng tuyn ng qua mng ti bt k destination no. C 2 loi destination c tha nhn trong OSPF:-Network Router: l cc area border router (ABR) v autnomous syste boundary router (ASBR). Ch cn mt ln sau khi tt c cc OSPF router ng b c link state database, mi router s tnh ton SPF tree cho mi destination m n bit. s tnh ton nay XAHFGDCB

c thc hin bi thut thc hin bi thut ton Dijkstra.EBCEXAHFGD

Linhk-state database Shortest path

Dijkstras(SPF) algorithm

Adjacency database Forwarding databaseAssume all links are Ethernet, with an OSPF cost of 10-Metric ca OSPF: OSPF cp n metric l cost. Cost ca ton tuyn l tng ca cost ca cc outgoing interface dc theo tuyn ng . Cch tnh cost c IETF a ra trong RFC 2328. Cisco thc thi cch tnh cost ca ring mnh nh sau: 108/bandwidth vi gi tr bandwidth c cu hnh cho mi interface.III. OSPF vi Multi-Area.Nh ta bit khi kch thc mng cng ln th s lng cc LSA cng ln, kch thc database s rt lnChnh nhng iu s lm tng yu cu v CPU cng nh memory ca OSPF router. gii quyt vn trn OSPF a ra k thut Multi-Area. 1.u im ca Multi-Area. Mi router phi chia s mt link state database ging ht nhau ch vi router trong cng area vi chnh n ch khng phi ton mng. Do gim c kch thc ca database dn ti gim yu cu ti phn cng ca router nh: memory.Gim kch thc link state database c ngh l gim s lng LSA phi x l v do lm gim tc ng trn CPU.Bi v link state database ch phi duy tr database trong mt area cho nn hu ht flooding ch gii hn rong mt area. 2.Mt s laoij Area trong OSPF (OSPF Area Types). a) Stub Area. Mt stub l mt area m cc External LSA khng c flood vo trong area . Trong stub area s khng c LSA loi 4 v 5 hay nhng LSA b block. ABR ti cnh ca stub area s s dng Network Summary qung b mt default router vo trong area. Bt c destination ca Internal router khng th match ti mt intra hay inter area, router s c match vi default router. Bi v default router c mang bi LSA loi 3, n s khng c qung b ra ngi area.S thc thi ca router trong stub area c ci thin, memory c bo tn v gim kch thc database ca chng. Tt nhin s ci thin ny cng r rng trong internetword vi rt nhiu LSA loi 5.Bn cnh n vn mang nhng nhc im ca mnh: Nh bt k area no, tt c router trong stub area phi c mt link state database ging ht nhau. m bo iu kin ny, tt c cc stub router s thit lp mt flag (bit_E) trong Hello packet l 0. Chng s khng chp nhn bt c Hello packet no c bt_E l 1, kt qu l adjacency khng c thit lp vi bt c router no khng c cu hnh l stub router. Virtual link khng c cu hnh trong stub area. Khng c routr no trong stub area c th l ASBR. V trong stub area khng c LSA loi 5. Mmotj stub area c th c hn mt ABR nhng bi v s dng defaul router, internal router khng th xc nh c router no s l gateway ti u ti ASBR. b)Totally Stubby Areas.Totally stubby area: s dng default khng ch cho destination external ti autonomous system m cn cho destination external ti area. ABR ca totally stubby area s khng ch block AS External LSA m cn block tt c Summary LSA tr LSA loi 3 no qung b default router. c)Not-So-Stubby Area. Not-so-stubby area(NSSA): cho php external router c qung b vo trong OSPF autonomous systemtrong khi d li nhng c tnh cn li ca stub area. C th l ASBR trong mt NSSA s sinh ra LSA loi 7 qung b external destination. Nhng External LSA c flood khp NSSA area nhng chng b block li ABR. Tm li ta c bng tng kt sau:

IV. nh dng gi tin OSPF. OSPF packet c ng gi trong IP packet tng ng vi trng Protocol number l 89, do vy maximum ca OSPF packet l 1500 octet. OOSSPF packet header l ging i vi cc loi OSPF packet khc nhau nhng OSPF packet dt th bin i ty theo loi OSPF packet.

Ch : IP packet vi protocol number =89 th trng TTL lun lun bng 1 m bo rng packet khng bao gi i qu mt hop.1. The Packet Header.Tt c cc OSPF packet u c chung 1 dng nh sau:

Trong : Version: l phin bn OSPF, phin bn gn y nht l 2. Type : xc nh ra loi OSPF packet. C 5 loi OSPF packet nh sau:

Packet lengh: l di ca OSPF packet gm c header( n v l octet). Router ID: l ID ca router gi. Area ID: l area m t packet c gi. Nu packet c gi qua virutal link, Area ID s l 0.0.0.0 ( backbone Area ID) bi v virutal link lun c gn vi backbone. Checksum : kim tra ton b packet k ca header. Autype : xc nh loi nn thc c s dng. Bng sau l cc loi nhn thc c th:

a.The Hello Packet. Hello packet c dng thit lp v duy tr adjiacecy. Hello packet mang nhng thng s m neighbor phi ng tr thnh adjacency. Netword mask : l a ch mask ca interface m packet c gi t . Nu mask ny khng match vi interface m packet c nhn thig packet s b drop. Hello Interval: l chu k gi bn tin Hello, c tnh bng giy. Nu router gi v nhn khng c cng thng s ny th n s khng thit lp quan h neighbor. Options : trng ny trong Hello packet m bo rng neighbor c kh nng tng thch. Router c th t chi mt neighbor nu kh nng ny l khng tng thch. Router Priority: c s dng bnh bu DR v BDR. Nu n c thit lp gi tr 0 th s loi khi qu trnh bnh bu DR v BDR. Router Dead Interval: l s giy mag router gi i mt Hello packet t neighbor trc khi cng b neighbor dead. Nu thng sso ny trong Hello u khng ging vi thng s ca n th packet s b drop. Designated Router: l a ch IP ca interface ca DR trn mng. Backup DR: l a ch IP ca interface ca BDR trn mng. Neighbor: cha danh sch tt c neighbor trn mng m router gi nhn t cc Hello howpk l.b.The Database Description Packet. Database Description packet: n c s dng khi mottj adjacency c thit lp. Mc ch chnh ca D packet l m t mt vi hay tt c LSA trong database cho n khi no c th xc nh l match LSA trong database ca n. Interface MTU: l kch thc ln nht ca IP packet ( n v l octet) m packet c th c gi i m khng b phn mnh> trng ny c thit lp l 0.0000 khi packet c gi qua virutal link. Option: l trng ty chn. router s khng chuyn tip LSA nu khng tha mn iu kin trong trng Option. C 5 bit khng s dng v c gi tr l: 00000b. Ba bt I, M v MS gii thiu trong phn building adjacency. DD Sequence Number: trng ny m bo rng DD packet c nhn ng th t trong qa trnh ng b database. Thng s ny lun lun c thit lp bi master cho DD packet u tin v tng dn ln trong cc DD packet gi sau. LSA Header: danh sch ca mt vi hay tt c LSA header trong link state database ca router gi.c.The Link State Request Packet. Trong qu trnh ng b database khi router nhn cc DD packet, router s kim tra xem LSA header trong DD packet nu khng c trong database ca n th nhng LSA ny ghi li vo Link State Request list. Router s gi mt hay vi link state request packet hi neighbor v LSA . Link State Type: xc nh laoij LSA. Link State ID: xc nh ra LSA header.Advertising Router: l router ID ca router m gi LSA.d.The Link State Update Packet. N c s dng khi flood LSA v gi LSA tr li cho Link State Request Packet.

Number of LSAs: xc nh s LSA trong packet ny.LSAs: l full LSA ( header + data). Mi update c th mang nhiu LSA ti maximum kch thc ca packet cho php trn link.e.The Link State Acknowledgment Packet. c s dng to qu trnh flood cc LSA mt cch tin cy ( reliable).

Options: Trng ny trong Hello packet m bo rng Neighbor c kh nng tng thch. Router c th t chi mt neighbor nu kh nng ny l khng tng thch. Router Priority: c s dng bu chn DR v BDR. Nu n c thit lp gi tr l 0 th s loi khi qu trnh bnh chn DR v BDR. Router Dead Interval: l s giy m router gi i mt Hello packet t neighbor trc khi cng b neighbor dead. Nu thng s ny trong Hello n khng ging vi thng s ca n th packet s b drop. Designated Route: l IP address ca interface ca DR trn mng (khng phi l Router ID ca n). Backup DR: l IP address ca interface ca BDR trn mng. Neighbor: cha danh sch tt c neighbor trn mng m router gi nhn t cc Hello hp l. b) The Database Description Packet Database Description Packet: n c s dng khi mt adjacency c thit lp. Mc ch ca DD packet l m t vi hay tt c cc LSA trong database cho n khi no c th xc nh l match LSA trong database ca n. Interface MTU: l kch thc ln nht ca IP packet m packet c th c gi i m khng b phn mnh. Trng ny c thit lp l 0x0000 khi packet khi packet c gi qua virtual link. Option: l trng ty chn, router s khng chuyn tip LSA nu khoong tha mn iu kin trong trng Option. C 5 bit khng s dng v c gi tr l 00000b. Ba bit I,M v MS gii thiu trong phn building adjacency. DD Sequence Number: Trng ny m bo rng DD packet c nhn ng th t trong qu trnh ng b database. Thng s ny lun lun c thit lp bi master cho DD packet u tin v tang dn ln trong cc DD packet gi sau. LSA Header: danh sch ca mt vi hay tt c LSA header trong link state database ca router gi. c) The Link Request Packet Trong qu trnh ng b database khi router nhn cc DD packet, router s kim tra xem LSA header trong DD packet nu khng c trong database ca n th nhng LSA ny ghi li vo Link State Request list. Router s gi mt hay mt vi Link State Request packet hi neighbor v LSA . Link State Type: xc nh loi LSA Link State ID: xc nh ra LSA header Advertising Router: l router ID ca router m gi LSA. d) The Link State Update Packet N c s dng khi flood LSA v gi LSA tr li Cho Link State Request packet.

Number LSAs: Xc nh s LSA trong packet ny.LSAs: l full LSA ( header + data ). Mi update c th mang nhiu LSA ti maximum kch thc ca packet cho php trn link.

e) The Link State Acknowledgment Packet c s dng to qu trnh flood cc LSA mt cch tin cy.

V. u, nhc im: OSPF ra i l hon thin vic nh tuyn v khc phc cc hn ch ca RIP, v vy ta ch yu ni n u im ca OSPF Tc hi t nhanh H tr mng con VLSM C th p dng cho mng ln Chn ng theo trng thi ng link hiu qu hn vector khong cch. ng i linh hot hn H tr xc thc OSPF c th cu hnh theo nhiu vng ( area ), bng cch ny c th gii hn lu thng trong tng vng. Thay i vng ny khng nh hng n vng khc, do vy c kh nng m rng cao.

IV. Cu hnh OSPF1. Enabling OSPFLnh kch hot OSPF l: Lab_A( config )#router ospf ?

Mt gi tr trong min 1-65535 nh danh OSPF process ID, l s duy nht trn router ny m nhm li mt lot cc lnh cu hnh OSPF trong mt tin trnh ang chy xc nh.2. Cu hnh OSPF areas. Sauk hi nh danh tin trnh OSPF, bn cn nh danh interface m bn mun kch hot s lin lc OSPF trn . Vic ny cng s cu hnh mng m bn s cng khai cho nhng ngi khc. OSPF s s dng k t i din trong vic cu hnh.

VD Lnh cu hnh: Lap_A#config t Lap_A (config) router ospf 1 Lap_A (config - router)#network 10.0.0.0.0.255.255.255Area ? OSPF Area ID as a demical valueA.B.C.D OSPF Area ID in IP address formatLap_A ( config - router)#network 10.0.0.0.0.255.255.255Area 0i s cu lnh network l s hiu network ( 10.0.0.0) v k t i din mask (0.255.255.255). S kt hp ca 2 s ny s nh danh interface m OSPF s hot ng trn v cng bao gm cc thng bo OSPF LSA ca n. OSPF s s dng lnh tm interface trn router c cu hnh trong mng 10.0.0.0 v n s thay th interface m n tm thy trong area 0. Lu rng bn c th to hn 4,2 t area, bn cng c th dn nhn cho area theo nh danh IP address. Octet 0 trong wildcard mask biu th vi octet tng ng trong mng phi ph hp mt cch chnh xc. i s cui cng l s area, n biu th vi area m interface c nh danh trong mng v phn wildcard mask thuc s hu. Nh rng router OSPF s ch tr thnh neighbor nu interface ca chng chia s mt mng c cu hnh. nh dng s area l mt trong cc gi tr thp phn t 1 n 4294967295

3. Xc nhn cu hnh OSPF Ta dng lnh: Lap_A#sh ip router Lap_A#sho ip ospf Lap_A# sho ip ospf database Lap_A# sho ip ospf interface Lap_A# sho ip ospf neighbor Lap_A# sho ip ospf protocols

4. Cu hnh looback interface Lap_A# sho ip ospf Lap_A#config t Lap_A# (config)#int lookback 0 Lap_A# (config - if) #ip address 172.16.10.1 255.255.255.0 Lap_A# (config - if) #no shut Lap_A# (config - if) #^Z 5. Xc nhn loopback v RIDs nhn a ch loopback ta s dng lnh: Lap_A#show running - config xc nhn RIDs ta ln lt g cc lnh: Lap_A#sho ip ospf database Lap_A#sho ip ospf interface Lap_A#show ip ospf

2Nhm 3_ Lp KTTT&TT K52