Giao Thc nh Tuyn RIP, IGRP v OSPFng ngy: 08:43 02-06-2011 Th mc: Vien Thong Cc giao thc cng ni (Interior Routing Protocols) I. Classfull routing protocol Phn A : L thuyt v RIP RIP c pht trin trong nhiu nm, bt u t phin bn 1(RIPv1) v hin nay l phin bn 2 (RIPv2). RIPv1 l mt giao thc nh tuyn theo Distance Vector, s dng s hop lm metric xc nh hng v khong cch cho bt k mt lin kt no trong mng. Qung b ton b bng nh tuyn ca n cho cc router lng ging theo nh k l 30 giy. RIPv1 l giao thc nh tuyn theo lp a ch. Khi RIP router nhn thng tin v mt mng no t mt cng, trong thng tin nh tuyn khng c thng tin v subnet mask i km. Do , router s ly subnet mask ca cng p dng cho a ch mng m n nhn c t cng ny. Nu subnet mask ny khng ph hp th n s ly subnet mask mc nh theo lp a ch p dng cho a ch mng m n nhn c. -a ch lp A c subnet mask mc nh l : 255.0.0.0. -a ch lp B c subnet mask mc nh la : 255.255.0.0. -a ch lp C c subnet mask mc nh la : 255.255.255.0. Khi c mt gi tin chuyn n, nu c nhiu ng dn n mt ch, RIP s chn ng dn c s hop nh nht. Tuy nhin s hop ch l mt metric c dng bi RIP, nn giao thc ny khng phi lc no cng chn chnh xc ng dn n ch. RIP cng khng th nh tuyn cho mt gi vi metric qu 15 hop. RIPv1 yu cu tt c cc thit b trn mng s dng cng subnet mask, v n khng cha thng tin subnet mask trong cc cp nht nh tuyn. iu ny c xem nh Classful Routing. Cc c im chnh ca RIP: -L giao thc nh tuyn theo Distance Vector. -Thng tin nh tuyn l s lng hop. -Nu gi d liu n mng ch c s lng hop ln hn 15 th gi d liu s b hy b. -Chu k cp nht mc nh l 30 giy. RIPv1 l giao thc nh tuyn c s dng ph bin v mi router u c h tr giao thc ny. RIPv1 c ph bin v tnh n gin v tnh tng thch ton cu ca n, n c th chia ti ra ti a l 6 ng c metric bng nhau. RIP trnh nh tuyn loop n v hn bng cch gii hn s lng hop ti a cho php t my gi n my nhn. S lng hop cho mi con ng l 15. i vi cc con ng m router nhn c t thng tin cp nht ca router lng ging, router s tng ch s hop ln 1 v router xem bn thn n cng l mt hop trn ng i. Nu sau khi tng ch s hop ln 1 m ch s ny ln hn 15 th router s xem nh mng ch tng ng vi con ng ny khng n. Ngoi ra, RIP cng c nhiu c tnh tng t nh giao thc nh tuyn khc. V d: RIP cng c split horizon v thi gian holddown trnh cp nht thng tin nh tuyn khng chnh xc.

Routing Information Protocol Version 2 (RIPv2) * c im Ca RIPv2: RIPv2 cung cp nh tuyn c nh, truyn thng tin c nh v truyn thng tin subnet mask trong cc cp nht nh tuyn. iu ny cng c gi l Classless Routing. Vi cc giao thc nh tuyn Classless, cc mng con khc nhau trong cng mt mng c th c cc subnet mask khc nhau, iu ny c gi l thao tc subnet mask c chiu di thay i VLSM (Variable-Length Subnet Masking). RIPv2 c pht trin t RIPv1 nn vn gi cc c im nh RIPv1: -L mt giao thc theo Distance Vector, s dng s lng hop lm thng s nh tuyn. -S dng thi gian holddown chng loop vi thi gian mc nh l 180 giy. -S dng c ch split horizon chng loop. -S hop ti a l 16. Tuy nhin, vi phin bn RIPv2 th RIP tr thnh giao thc nh tuyn khng theo lp a ch. RIPv2 c h tr vic xc minh thng tin nh tuyn. Chng ta c th cu hnh cho RIPv2 gi v nhn thng tin xc minh trn cng giao tip ca router bng m ha MD5 hay khng m ha. RIPv2 gi thng tin nh tuyn theo a ch multicast 224.0.0.9. So Snh RIPv1 v RIPv2: RIP s dng thut ton nh tuyn theo Distance Vector. Nu c nhiu ng n cng mt ch th RIP s chn ng c s hop t nht. V vy, i khi con ng m RIP chn cha hn l nhanh nht n ch. RIPv1 cho php cc router cp nht bng nh tuyn ca chng theo chu k mc nh l 30 giy. Vic gi thng tin nh tuyn cp nht lin tc nh vy gip cho mng c xy dng nhanh chng. trnh loop v tn, RIP gii hn s hop ti a chuyn gi tin l 16 hop. Nu mt mng ch nhiu hn 16 hop th mng xem nh khng ti c v gi tin n s b hy. iu ny lm gii hn kh nng m rng ca RIP. RIPv1 s dng c ch split horizon chng lp vng. Vi c ch ny, khi gi thng tin nh tuyn ra mt cng, RIPv1 router khng gi ngc tr li cc thng tin nh tuyn m n hc c t chnh cng . RIPv1 cn s dng thi gian holdown. Trong sut khong thi gian holdown, router s khng cp nht tt c cc thng tin c s nh tuyn xu hn v mng . RIPv2 c pht trin t RIPv1 nn cng tha hng nhng c tnh trn. Mt s im khc nhau gia RIPv1 v RIPv2: RIPv1 nh tuyn theo lp a ch. Khng gi thng tin v subnet-mask trong thng tin nh tuyn. Khng h tr VLSM. V vy tt c cc mng trong h thng RIPv1 phi cng subnet mask. Khng c c ch xc minh thng tin nh tuyn. Gi qun b theo a ch 255.255.255.255

RIPv2 nh tuyn khng theo lp a ch. C gi thng tin v subnet mask trong thng tin nh tuyn. C h tr VLSM. Nn cc mng trong h thng RIPv2 c th c chiu di subnet mask khc nhau. C c ch xc minh thng tin nh tuyn. Gi qun b theo a ch 224.0.0.9 nn hiu qu hn. Vi C Ch Ca RIP : + Split Horizon : c ch ny dng chng loop bng cch, gi s router A nhn thng tin nh tuyn t router B v mng X, th sau khi a vo bng routing table, router A s khng broadcast thng tin nh tuyn ca mng X v li cho router B na. + Route Poisoning : gi s mng X kt ni trc tip vi router B v thng tin nh tuyn v mng X c router B gi cho router A. Nu nh mng X b disconect th ngay lp tc router B s gi ngay thng tin nh tuyn cho router A v mng X vi metric l 16. + Poison Reverse : c ch ny s gn lin vi c ch Route Poisoning, khi router A nhn c thng tin nh tuyn t router B v mng X vi metric l 16 th router A s gi li thng tin nh tuyn v mng X cho router B vi metric l 16 chc chn rng mng X b disconect. (lu l khi c ch Route Poisoning v Poison Reverse hot ng th c ch Split Horizon s c tm dng, y l trng hp c bit v metric = 16). * Qu trnh gi v nhn thng tin nh tuyn M hnh minh ha

Lc gi thng tin nh tuyn : Trc khi gi update( v ng mng 131.108 v 131.99) cho router 2 th router 1 phi check rng ng mng 131.108.5.0/24 c cng major net vi 131.108.2.0/24 hay khng? Trong case ny l c, Router 1 mi check tp 131.108.5.0 v 131.108.2.0 c cng subnet mask hay khng? Nu trng, Router 1 s qung b ng mng ny. Nu k trng , router 1 s drop packet k thng tic ng mng 137.99.88.0/24 c cng major net vi 131.108.2.0/24 hay khng? Nu khng th router 1 s lm ng tc l tng hp (summarize) 137.99.88.0/24 ti major net boundary thnh 137.99.0.0 v qung b n. Trong m hnh ny th ta nhn c kt qu nh th ny trong khi thi hnh lnh debug ip rip

Nhn update : Lc ny nhanh tay debug ip rip ngay trn router 2 th ta thy nh th ny

Router 2 s check xem nn apply mask no cho ng mng 131 v 137 ny y 131.108.5.0 v 131.108.2.0( xt trn interface m nhn update vo) c cng 1 major net k? Nu c th apply thng mask ca interface m n nhn update, trong trng hp ny l apply /24). Nu mng c qung b tc 131.108 m /32 th router 2 s apply /32 v tp tc qung b cho cc router khc l /32( iu ny n khc vi IGRP nh) 131.108.5.0 v 137.99.0.0 c cng major net k? Nu khng xt tp , trong bng nh tuyn c subnet no hay mng con ca major net ny m n hc t cc interface khc khng? Nu khng th router 2 s apply thng classful subnet mask l /16 lun v 137 l mng lp B. Ch y n s apply host mask nu nh gia 2 router l 1 unnumbered link v cha thng tin v subnet ( tc l khi cc bit trong phn portion ca network c set). Ngc li th router s ignore thng tin nh tuyn ny i Lc ny show ip route th xem

Lab

Gi s ta c m hnh nh hnh trn. nh tuyn 3 router trn bng giao thc RIP, ta lm nh sau: i vi router R1: Router >en Router # config terminal Router (config) # hostname R1 R1 (config) # interface loopback 0 R1 (config-if) # ip address 192.168.1.1 255.255.255.0 R1 (config-if) # exit R1 (config) # interface s1/0 R1 (config-if) # ip address 192.168.12.1 255.255.255.0 R1 (config-if) # no shutdown R1 (config-if) # exit R1 (config) # router rip R1 (config-router) # network 192.168.1.0 R1 (config-router) # network 192.168.12.0 i vi router R2: Router >en Router # config terminal Router (config) # hostname R2 R2 (config) # interface loopback 0

R2 (config-if) # ip address 192.168.2.1 255.255.255.0 R2 (config-if) # exit R2 (config) # interface s1/0 R2 (config-if) # ip address 192.168.12.2 255.255.255.0 R2 (config-if) # no shutdown R2 (config-if) # exit R2 (config) # interface s1/1 R2 (config-if) # ip address 192.168.23.2 255.255.255.0 R2 (config-if) # no shutdown R2 (config-if) # exit R2 (config) # router rip R2 (config-router) # network 192.168.2.0 R2 (config-router) # network 192.168.12.0 R2 (config-router) # network 192.168.23.0 i vi router R3: Router >en Router # config terminal Router (config) # hostname R3 R1 (config) # interface loopback 0 R1 (config-if) # ip address 192.168.3.1 255.255.255.0 R1 (config-if) # exit R1 (config) # interface s1/0 R1 (config-if) # ip address 192.168.23.3 255.255.255.0 R1 (config-if) # no shutdown R1 (config-if) # exit R1 (config) # router rip R1 (config-router) # network 192.168.3.0 R1 (config-router) # network 192.168.23.0 Xong vic cu hnh cho 3 router. kim tra cc mng c th thy nhau hay khng, ta c th dng lnh ping kim tra hoc dng lnh show ip route hoc show ip route rip kim tra bng nh tuyn ca cc router I.2. Interior Gateway Routing Protocol (IGRP) 1. nh ngha Trc nhng nhc im vn c ca RIP nh: metric l hop count, kch thc mng ti a l 15 hop. Cisco pht trin mt giao thc c quyn ca ring mnh l IGRP khc phc nhng nhc im . C th l metric ca IGRP l s t hp ca 5 yu

t, mc nh l bandwidth v delay: Bandwidth Delay Load Reliability Maximum transfer unit (MTU) IGRP khng s dng hop count trong metric ca mnh, tuy nhin n vn theo di c hop count. Mt mng ci t IGRP th kch thc mng c th nn ti 255 hop. u im na ca IGRP so vi RIP l n h tr c unequal-cost load sharing v thi gian update lu hn RIP gp 3 ln. Tuy nhin bn cnh nhng u im ca mnh so vi RIP, IGRP cng c nhng nhc im l giao thc c quyn ca Cisco. 2. Hot ng v c trng IGRP c rt nhiu im chung vi RIP, chng cng l classfull distance vector protocol cng nh nh k gi ton b routing table ra tt c active interface. Cng ging nh RIP, IGRP cng broadcasd Request packet ra tt c cc active interface khi khi ng v cn thn check cc paket update nhn c xem source address ca packet c cng subnet m update c nhn. Ging nh RIP n cng khng gi subnetmask trong thng tin nh tuyn. Nu nh RIP dng port 520 ca giao thc UDP trao i thng tin nh tuyn, th IGRP th lm iu ny trc tip trong gi tin IP vi trng Protocol number l 9. IGRP s dng khi nim Autonomous System (AS), mt IGRP AS l mt IGRP process domain_tp hp cc router c chung routing protocol l mt IGRP process. Cho php multiple IGRP AS tn ti bn trong mt AS c ngha l ngi qun tr c phn on mng tt hn. Ngi qun tr c th to mt IGRP AS cho mi routing domain, gip cho vic iu khin thng tin gia cc mng tng tc tt hn. IGRP tha nhn 3 loi tuyn ng trong thng tin update: Interior route: mng ni trc tip vi router. System route: l ng ti a ch mng m b summary bi network border router. Exterior route: l ng hc qua IGRP t IGRP AS khc, n cung cp thng tin s dng bi default route. a/ IGRP Timer Chu k update ca IGRP l 90 giy, IGRP c s dng nhn t random 20% ngn chn s ng b update timer. Khong thi gian gia 2 ln update bin i t 72 n 90 giy.

Khi mt tuyn ng u tin c hc, invalid timer cho tuyn l 270 giy hay l gp 3 ln update timer. Flush timer c thit lp vi gi tr l 630 giy_ gp 7 ln update timer. Mi ln tuyn c c update th nhng thng s thi gian ny c khi ng li. Nu nh invalid timer tri qua m tuyn ng khng nhn c mt update th tuyn ng s b nh du l khng n c. Tuyn ng s c gi trong routing table v qung b vi thng tin l tuyn khng n c cho n khi flush timer tri qua, tuyn s c xo khi routing table. Update timer ca IGRP gp 3 ln RIP, iu chng t IGRP tn it bng thng hn cho vic gi update. Nhng thi gian hi t ca IGRP s ln hn RIP. b/ IGRP Metrics Metric ca IGRP l t hp ca cc thnh phn sau: bandwidth, delay, load, reliability. Mc nh ca metric l bandwidth v delay, bn hy tng tng lin kt d liu (data link) nh l mt ci ng th bandwidth nh l chiu rng ca ng cn delay nh l chiu di ca ng. Ni cch khc bandwidth l thc o kh nng mang thng tin v delay tr cn thit mt bt truyn n ch. Bandwidth: c bit din vi n v l kbps, l mt thng s c s dng IGRP s dng chy thut ton Bellman-Ford. N l mt thng s tnh c th thay i bi ngi qun tr khng lin quan g n bandwidth tht ca ng truyn. BW = [10000000/(bandwidth in Kbps)] Delay: ging nh bandwidth l mt thng s tnh c th c cu hnh bng tay. Delay = [Delay in 10s of microseconds] Reliability: l mt thng s ng, c biu din bi mt s 8bit. c tnh s lng gi tin n ch m khng b hng. Reliability c gi tr 255 c ngha l 100% gi tin khng b hng, gi tr nh nht l 1. Load: l mt phn bng thng s dng trn ng truyn, c biu din bi mt s 8 bit. Load c gi tr l 255 ngha l s dng 100%, 1 l gi tr nh nht. Mc nh: K1 = 1, K2 = 0, K3 = 1, K4 = 0, K5 = 0. Cc hng s trn c th c thay tu theo mc ch ca ngi qun tr. c/ IGRP Packet Format nh dng gi tin IGRP c biu din nh hnh sau: http://chuyenviet.com/html/articles/routingdetail_files/image004.jpg Nh ta thy bn tin IGRP update mang nhiu thng tin hn so vi RIP. Mi bn tin IGRP update c th cha ti a 104 muc nhp (entry) vi mi mc nhp c kch thc 14octet v header ca IGRP update l 12 octet. Ta c maximum ca IGRP packet l 12 + 104*14 = 1468 byte. Cc trng ca IGRP c ngha l: Version: lun lun c gi tr bng 1. Opcode: c gi tr l 1 cho IGRP Request packet v c gi tr l 2 cho IGRP Update packet. Ch Request packet khng cha mc nhp (entry). Edition: gi tr c tng ln bi ni gi bt c khi no c mt thay i v thng tin nh tuyn. Gi tr ny gip cho router trnh update nhm thng tin update c sau khi nhn thng tin update mi. Autonomous System Number: l ID number ca IGRP process. Thng s ny cho php

multiple IGRP process trao i thng tin nh tuyn qua mt lin kt d liu chung. Number of Interior Routes: l s mc nhp trong update, l nhng subnet ca nhng network ni trc tip. Number of System Routes: s tuyn ng ti nhng mng m khng ni trc tip. Hay ni cch khc, l nhng tuyn ng c summary bi router bin. Number of Exterior Routes: l s tuyn ng ti nhng mng m c hc bi default route. Checksum: c tnh trn IGRP header v tt c cc mc nhp. Destination: l trng u tin ca mi mc nhp. C mt ch l trng destination ch c 3 octet trong khi a ch IP c 4 octet. iu ny c thc hin do nhng nguyn nhn sau. Nu mc nhp l mt interior route th t nht octet u tin ca a ch IP lun lun c xc nh t a ch IP ca interface m n nhn c update. Tng t nh vy nu mc nhp l system hay external route th route s b summary v t nht l octet cui cng l ton zero. Do trng destination ch cn biu din 3 octet u l . Delay: trng ny bao gm 24 bit. Bandwidth: trng ny bao gm 24 bit. MTU: l Maximum Transmission Unit nh nht ca bt k link no trong tuyn ng n ch. Mc d y l mt thng s nhng khng bao gi c s dng tnh route. Reliability, Load: c gi tr bin i t 0x01 n 0xFF. Hop Count: c gi tr bin i t 0x01 n 0xFF cho bit s hop ca tuyn ng n ch. d/ Unequal-Cost Load Balancing Load balancing l cch router gi lu lng qua nhiu ng n cng ch. N c s dng gim lu lng qua single path. Khng ging nh RIP, IGRP khng nhng ch h tr equal-cost balancing m cn h tr c unequal-cost balancing. iu ny c thc hin nh s dng thng s variance. Nhng route no c metric nh hn hoc bng metric*variance s c chn l feasiable route (metric tt nht). Thng s Maximum Paths xc nh ti a c bao nhiu route tham gia load balancing. L thuyt v OSPF 1.Tng Quan V OSPF: OSPF l mt giao thc nh tuyn theo trng thi ng lin kt c trin khai da trn cc chun m. OSPF c m t trong nhiu chun ca IETF (Internet Engineering Task Force). Chun m y c ngha l OSPF hon ton m vi cng cng, khng c tnh c quyn. Nu so snh vi RIPv1 v RIPv2 l mt giao thc ni th IGP tt hn v kh nng m rng ca n. RIP ch gii hn trong 15 hop, hi t chm v i khi cn chn ng c tc chm v khi quyt nh chn ng n khng quan tm n cc yu quan trng khc nh bng thng chng hn. OSPF khc phc c cc nhc im ca RIP v n l mt giao thc nh tuyn mnh, c kh nng mi rng, ph hp vi cc h thng mng hin i. OSPF c th cu hnh n vng s dng cho cc mng nh. 2. So Snh OSPF Vi Giao Thc nh Tuyn Theo Distance Vector: Router nh tuyn theo trng thi ng lin kt c mt c s y v cu trc h thng mng. Chng ch thc hin trao i thng tin v trng thi ng lin kt lc khi

ng v khi h thng mng c s thay i. Chng khng pht qung b bng nh tuyn theo nh k nh cc router nh tuyn theo distance vector. Do , cc router nh tuyn theo trng thi ng lin kt s dng t bng thng hn cho hot ng duy tr bng nh tuyn. RIP ph hp vi cc mng nh v ng tt nht i vi RIP l ng c s hop t nht. OSPF th ph hp vi mng ln, c kh nng m rng, ng i tt nht ca OSPF c xc nh da trn tc ca ng truyn. RIP cng nh cc giao thc nh tuyn theo distance vector khc u s dng thut ton chn ng n gin. Cn thut ton SPF th phc tp. Do , nu router chy theo giao thc nh tuyn theo distance vector th s t tn b nh v cn nng lc x l thp hn so vi khi chy OSPF. OSPF chn ng da trn chi ph c tnh t tc ca ng truyn. ng truyn c tc cng cao th chi ph OSPF tng ng cng thp. OSPF chn ng tt nht t cy SPF. OSPF bo m khng b nh tuyn lp vng. Cn giao thc nh tuyn theo distance vector vn c th b loop. Nu mt kt ni khng n nh, chp chn, vic pht lin tc cc thng tin v trng thi ca ng kin kt ny s dn n tnh trng cc thng tin qung co khng ng b lm cho kt qu chn ng ca cc router b o ln. *OSPF gii quyt c cc vn sau: -Tc hi t. -H tr VLSM (Variable Length Subnet Mask). -Kch c mng. -Chn ng. -Nhm cc thnh vin. Trong mt h thng mng ln, RIP phi mt t nht vi pht mi c th hi t c v mi router ch trao i bng nh tuyn vi cc router lng ging kt ni trc tip vi mnh m thi. Cn i vi OSPF sau khi hi t vo lc khi ng, khi c thay i th vic hi t s rt nhanh v ch c thng tin v s thay i c pht ra cho mi router trong vng. OSPF c h tr VLSM nn n c xem l mt giao thc nh tuyn khng theo lp a ch. RIPv1 khng h tr VLSM, nhng RIPv2 th c. i vi RIP, mt mng ch cch xa hn 15 router xem nh khng th n c v RIP c s lng hop gii hn l 15. iu ny lm kch thc mng ca RIP b gii hn trong phm vi nh. OSPF th khng gii hn v kch thc mng, n hon ton c th ph hp vi mng va v ln. Khi nhn c t router lng ging cc bo co v s lng hop n mng ch, RIP s cng thm 1 vo thng s hop ny v da vo s lng hop chn ng n mng ch. ng no c khong cch ngn nht hay ni cch khc l c s lng hop t nht s l ng tt nht i vi RIP. Nhn xt thy thut ton chn ng nh vy l rt n gin v khng i hi nhiu b nh v nng lc x l ca router. RIP khng h quan tm n bng thng ng truyn khi quyt nh chn ng. OSPF th chn ng da vo chi ph c tnh t bng thng ca ng truyn. Mi OSPF u c thng tin y v cu trc ca h thng mng v da vo chn ng i tt nht. Do , thut ton chn ng ny rt phc tp, i hi nhiu b nh v nng lc x l ca router cao hn so vi RIP. RIP s dng cu trc mng dng ngang hng. Thng tin nh tuyn c truyn ln lt cho mi router trong cng mt h thng RIP. Cn OSPF s dng khi nim v

phn vng. Mt mng OSPF c th chia cc router thnh nhiu nhm. Bng cch ny, OSPF c th gii hn lu thng trong tng vng. Thay i trong vng ny khng nh hng n hot ng ca cc vng khc. Cu trc phn lp nh vy cho php h thng mng c kh nng m rng mt cch hiu qu. 3. Thut Ton Chn ng Ngn Nht: Theo thut ton ny, ng tt nht l ng c chi ph thp nht. Thut ton c s dng l Dijkstra, thut ton ny xem h thng mng l mt tp hp cc nodes c kt ni vi nhau bng kt ni point-to-point. Mi kt ni ny c mt chi ph. Mi nodes c mt tn. Mi nodes c y c s d liu v trng thi ca cc ng lin kt. Do , chng c y thng tin v cu trc vt l ca h thng mng. Tt c cc c s d liu ny iu ging nhau cho mi router trong cng mt vng. 4. Cc Loi Mng OSPF: Cc OSPF phi thit lp mi quan h lng ging trao i thng tin nh tuyn. Trong mi mng IP kt ni vo router. N u c gng t nht l tr thnh mt lng ging hoc l mt lng ging thn mt vi mt router khc, router OSPF quyt nh chn router no lm lng ging thn mt l ty thuc vo tng loi mng kt ni vi n. C mt s router c th c gng tr thnh lng ging thn mt vi mi router lng ging khc. C mt s router khc li c th ch c gng tr thnh lng ging thn mt vi mt hoc hai router lng ging thi. Mt khi mi quan h lng ging thn mt c thit lp gia hai lng ging vi nhau th thng tin v trng thi ng lin kt mi c trao i. Giao thc OSPF nhn bit cc loi mng sau: -Mng qung b a truy cp, v d mng Ethernet. -Mng point-to-point. -Mng khng qung b a truy cp (NBMA NonBroadcast Multil-Access), v d Frame Relay. -Mng Point-to-Multipoint c th c nh qun tr mng cu hnh cho mt cng ca router. Trong mng a truy cp khng th bit c l c bao nhiu router s c th c kt ni vo mng. Trong mng point-to-point th ch c hai router c kt ni vi nhau. Trong mng qung b a truy cp c rt nhiu router kt ni vo. Nu mi router u thit lp mi quan h thn mt vi mi router khc v thc hin trao i thng tin v trng thi ng lin kt vi mi router lng ging th s qu ti. Nu c 10 router th s cn 45 mi lin h thn mt, nu c n router th s c n*(n-1)/2 mi quan h lng ging cn thit lp. Gii php cho vn qu ti trn l bu ra mt router lm i din (DR- Designated Router). Router ny s thit lp mi quan h thn mt vi mi router khc trong mng qun b. Mi router cn li s ch gi thng tin v trng thi ng lin kt cho DR. Sau DR s gi cc thng tin ny cho mi router khc trong mng bng a ch multicast 224.0.0.5 DR ng vai tr nh mt ngi pht ngn chung. Vic bu DR rt c hiu qu nhng cng c mt nhc im. DR tr thnh mt tm im nhy cm i vi s c. Do , cn c mt router th hai c bu ra lm i din d phng (BDR Backup Designated Router), router ny s m trch vai tr ca DR nu DR b s c. m bo c DR v BDR u nhn c thng tin v trng thi ng lin kt t mi router khc trong cng mt mng, a ch multicast 224.0.0.6 cho

cc router i din. Trong mng point-to-point ch c 2 router kt ni vi nhau nn khng cn bu ra DR v DBR. Hai router ny s thit lp mi quan h lng ging thn mt vi nhau. Loi Mng Cc c Tnh Bu DR Broadcast Multi-Access Ethernet,ToKen Ring,FI C NonBroadcast Multi-Access Frame Relay,X25,SMDS C Point-to-Point PPP,HDLC Khng Point-to-Multipoint c cu hnh bi Administrator khng 5. Giao Thc OSPF Hello: Khi router bt u khi ng tin trnh nh tuyn OSPF trn mt cng no th n s gi mt gi hello ra cng v tip tc gi hello theo nh k. Giao thc hello a ra cc nguyn tc qun l vic trao i cc gi OSPF hello. lp 3 ca m hnh OSI, gi hello mang a ch multicast 224.0.5.0 a ch ny ch n tt c cc OSPF router. OSPF router s dng gi hello thit lp mt quan h lng ging thn mt mi v xc nh l router lng ging c cn hot ng hay khng. Mc nh hello c gi i 10 giy mt ln trong mng qung b a truy cp v mng Point-to-Point. Trn cng ni vo mng NBMA, v d nh Frame Relay, chu trnh mc nh ca hello l 30 giy. Trong mng a truy cp, giao thc hello tin hnh bu DR v BDR. Mc d gi hello rt nh nhng n cng bao gm c phn header ca gi OSPF. Cu trc ca phn header trong gi OSPF c th hin nh hnh sau. Nu gi hello th trng Type s c gi tr l mt.

Gi hello mang nhng thng tin thng nht gia mi lng ging vi nhau trc khi

c th thit lp mi quan h lng ging thn mt v trao i thng tin v trng thi ng lin kt.

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