Chuong5 Route Optimization abc

Gio trnh kha hc BSCI Chng 5 Route Optimization

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CHNG 5: ROUTE OPTIMIZATION

Chng ny cp n hai ch chnh l Redistribution v Policy-Based-Routing.

5.1. Redistribution

5.1.1. Tng quan

Nu mt h thng mng chy nhiu hn mt giao thc trong mt cng ty, ngi qun tr

cn mt vi phng thc gi cc routes ca mt giao thc ny vo mt giao thc khc.

Qu trnh gi l redistribution.

Qu trnh redistribution c dng khi mt router nhn thng tin v mt network xa

thng qua cc ngun khc nhau. Mc d tt c cc network c a vo bng nh tuyn v

cc quyt nh nh tuyn c thc hin trn bng ny, mt giao thc ch truyn nhng

network m n hc t chnh bn thn n. Nu khng c vic chia s thng tin mng gia cc

qu trnh nh tuyn, ngi ta gi l ships in the nights (SIN).

Redistribution thng cn thit trong mt network nh mt gii php tm thi. Tuy nhin,

gii php ny khng phi l nhanh chng v d dng. Mc dng cch dng route-

redistribution l mt gii php cu cnh trong nhiu tnh hung, n kh phc tp. Khi mt

giao thc chng hn nh EIGRP c routes c phn phi vo n nh mt qu trnh nh

tuyn, n gi s rng tt c cc routes ny t cc AS khc v l routes ngoi (external routes).

iu ny nh hng n qu trnh chn la ng i v EIGRP thng u tin cho routes

ni.

Trong hnh v di y, bng nh tuyn ca routerB cc cc entry t RIP v OSPF,

khng c entry cho EIGRP bi v y ch l mt mng ring l ni trc tip vo router. Hn

na, routerc ch c cc routes kt ni trc tip trong bng bng nh tuyn. y l v, mc

dng EIGRP c cu hnh, routerC vn ch l mt stub router. Khi c nhng interface

khc c cu hnh v ton b cc mng EIGRP cn li kt nI vo routerC, cc lp mng

ny s c qung b.

Qu trnh redistribution ch din ra cc giao thc lp 3. V vy cc giao thc OSPF, RIP,

IGRP v EIGRP c th phn phi cc routing update gia chnh cc giao thc ny. Tuy nhin

s khng c qu trnh redistribution gia AppleTalk v IPX.


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Bng 5.1 - Cc chnh sch redistribution

Giao thc nh tuyn Chnh sch redistribution

Static Phi cu hnh redistribution bng tay vo cc giao thc khc.

Connected Phi cu hnh redistribution bng tay vo cc giao thc khc.

RIP Cu hnh redistribution bng tay.

IGRP T ng redistribution gia IGRP v EIGRP nu gi tr AS l

ging nhau. nu gi tr AS l khc nhau, phi cu hnh

redistribution bng tay.

EIGRP

S t ng redistribution gia IGRP v EIGRP nu gi tr AS

l ging nhau. nu gi tr AS ny l khc nhau, ta phi cu hnh

redistribution bng tay. EIGRP cho AppleTalk s t ng

redistribution gia EIGRP v RTMP. EIGRP cho IPX s t

ng redistribute gia EIGRP v IPX RIP/SAP. Trong cc

phin bn sau, NLSP c th redistribution bng tay.

OSPF Phi cu hnh redistribution bng tay gia cc OSPF process.

IS-IS Phi cu hnh bng tay gia cc giao thc khc nhau.

BGP Phi cu hnh bng tay gia cc giao thc khc nhau.


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Hnh 5.1 - Hnh v minh ho chnh sch redistribution

Cc nguyn nhn lm cho nhiu giao thc nh tuyn tn ti bn trong mt t chc l:

- H thng ang chuyn t mt giao thc ny sang mt giao thc khc bi v cn ci t

mt giao thc mi phc tp hn.

- Do lch s, h thng mng gm nhiu mng nh hn. Cng ty cn phi chuyn sang

mt giao thc duy nht trong tng lai.

- Mt vi phng ban trong cng ty c dng cc gii php host-based, v d nh dng

RIP trn my Unix.

- Sau khi hai cng ty st nhp vi nhau.


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Hnh 5.2 - Qu trnh phn phi route gia cc giao thc khc nhau

Khi cu hnh mng m mt bn dng routing l static, bn kia l dynamic, hoc mt bn l

RIP, mt bn dng IGRP th chng ta phi dng cc lnh redistribute cc Router bin. Lnh

redistribute c dng chia s thng tin nh tuyn gia cc min routing khc nhau. Mt

min nh tuyn (routing domain) c th l RIP; bn cn li c th l IGRP; hoc mt min l

static v mt bn cn li l OSPF

- Nhng nguyn nhn mang tch chnh tr, nhng qun tr mng khc nhau c cc t

tng khc nhau.

- Trong mt mi trng rt l ln, trong cc min khc nhau c cc yu cu khc

nhau. iu ny lm cho mt gii php n l l khng hiu qu.

5.1.2. Routing Metrics v Redistribution

C nhiu giao thc nh tuyn cho IP. Mi giao thc nh tuyn dng cc metric khc

nhau. Nu cc giao thc khc nhau mun chia s thng tin thng qua qu trnh redistribution,

ta phi cu hnh chuyn i metric. Cc vn s ny sinh khi cc metric c redistribute

m khng dng thm cc lnh cu hnh. Cc metric khng c thng tin g tham kho trong

giao thc mi. V d RIP s khng hiu gi tr metric l 786 v RIP mong i gi tr metric

nm trong khong t 0-15. Khi chp nhn nhng network mi, giao thc nh tuyn phi c


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mt im bt u, gi l seed metric. Gi tr seed metric s tng t v tr router khi cc

mng c truyn trong routing domain mi.

Bng 5.2 - Cc gi tr mc nh ca metric khi c redistribute

IP Routing Default Seed Metric Hnh ng

RIP Gi tr l khng xc nh Route khng c a vo bng nh

tuyn.

IGRP Khng xc nh Route khng c a vo bng nh

tuyn.

EIGRP Khng xc nh Route khng c a vo bng nh

tuyn.

ISIS 0 Route c a vo bng nh tuyn.

OSPF

- 20 (loi 2)

- Cc route t BGP c metric

l 1

Route c a vo bng nh tuyn.

BGP MED s ly gi tr metric t

cc IGP Route c a vo bng nh tuyn.

Gi tr metric l phng thc ch yu ca qu trnh chn la bn trong mt giao thc

nh tuyn. V vy cn thit phi nh ngha mt seed metric cho nhng network c chp

nhn t nhng giao thc nh tuyn khc.

5.1.3. Chn la ng i gia cc giao thc nh tuyn

Qu trnh tm ng bn trong mt giao thc nh tuyn c kho st trong cc

chng trc. Phn ny s kho st qu trnh chn la ng i gia cc giao thc nh tuyn

khi c nhiu hn mt giao thc nh tuyn chy trn mng.

Nu mt giao thc c nhiu ng i n cng mt mng xa, qu trnh nh tuyn phi

quyt nh a ng i no vo bng route. V cc metric ca cc giao thc l khc nhau,

qu trnh chn la ng i da trn metric s khng c dng. Thay vo , mt cch thc

khc c nh ngha gii quyt vn , l gi tr AD. S khc nhau gia hai qu trnh

chn la l n gin: gi tr AD s xc nh gia cc giao thc nh tuyn. Gi tr AD v

metric gii quyt hu ht vn trong qu trnh redistribution. Mi vic bt u pht sinh khi

phi phn phi routes gia cc giao thc nh tuyn v khi qu trnh routing bt u gp kh


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khn trong khu xc nh v ngun gc ca thng tin nh tuyn. Khi , cc vn v nh

tuyn khng ti u v routing loop c th xy ra.

V vy cn phi xem xt cc lut sau khi thc hin redistribution gia cc giao thc:

- Nu c nhiu hn mt giao thc nh tuyn ang chy trn mt router, nhng routes

no c gi tr AD tt nht s c a vo bng route.

- c redistributed , route phi c trong bng nh tuyn. Ngoi ra, route phi

c hc t giao thc nh tuyn ang c redistribute. Nh vy, nu RIP ang

c redistribute vo EIGRP , bng nh tuyn phi c mt entry cho mng RIP.

- Khi mt route c redistribute, n s tha hng gi tr AD mc nh ca giao thc

mi.

R rng l qu trnh redistribution khng phi l mt thit k mang tnh ti u. Mt thit

k cng n gin v d dng, mng s d qun tr v hi t nhanh. V vy, mt s a ch

IP c thit k cho php mng pht trin, kt hp vi mt giao thc nh tuyn IP s dn

n mt h thng mng nhanh, mnh v tin cy.

Cc vn c th pht sinh khi thc hin redistribution c th rt kh khc phc bi v

vn xut hin c th nm ni khc. Cc vn c th pht sinh bao gm:

- Cc quyt nh nh tuyn l sai, km hiu qu v s khc bit v metric. Vic chn

la ng i sai cn c gi l sub - optimal path.

- Khi mt routing loop xy ra, data s c chuyn bt tn m khng bao gi n ch.

iu ny l do vn route-feedback trong mt router gi thng tin update ra

khi AS li nhn c route gi ngc li vo AS.

- Khong thi gian hi t ca mng s tng bi v s khc nhau ca cc cng ngh. Nu

cc giao thc nh tuyn hi t cc khong thi gian khc nhau, iu ny c th

dn n vn timeouts v mt cc network.

- Qu trnh ra quyt nh v thng tin c gi bn trong mt giao thc c th khng

tng thch vi nhau v khng d dng trao i. iu ny s dn n li v cc cu

hnh phc tp.

5.1.4. Trnh vng lp khi redistribution

Routing loop xy ra khi mt giao thc nh tuyn nhn c cc mng ca chnh n. Cc

giao thc nh tuyn c th tht mt mng trn mt ng i tt mc d ng i ny ch v

chiu ngc li vo mt giao thc nh tuyn khc.


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Hnh 5.3 - Trnh vng lp khi redistribution

Vn ny c gii quyt bng cc cu hnh nh sau:

- Thay i metric.

- Thay i gi tr AD.

- Dng default-route.

- Dng passive interfaces vi nh tuyn tnh.

- Dng distribute-list.

qun l s phc tp ca cc h thng mng ny v gim thiu s phc tp ca routing-

loop, ngi qun tr nn gii hn mt vi thng tin c gi trn cc domain. Cng vic ny

c thc hin thng qua cch dng access-list.

Hnh 5.4 - Thc hin redistribution gia RIP v EIGRP

Gi s rng thi im routerA ang chy RIP v qung b mng 190.10.10.0 n cc

hai routerB v E. Khi routerB nhn cp nht t RIP, n s redistribute network 190.10.10.0


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vo OSPF v qung b route n routerC. RouterC s qung b route n D. Cui cng

routerE nhn mt OSPF update t D, bo rng network 190.10.10.0 thng qua ng i

D,C,B,A. Tuy nhin, routerE c mt ng i trc tip n A thng qua RIP. y mi l

ung i c ng c u tin hn. Trong tnh hung ny, gi tr AD c tc dng. Bi v

OSPF c gi tr AD l 110 v RIP c gi tr AD l 120, ng i c t trong bng nh

tuyn l ng i c qung b bI OSPF thng qua D,C, B v A. Trong tnh hung ny, ta

nn cu hnh bng tay gi tr AD trn routerB v E.

Nu EIGRP chy trn cc router B,C,D v E s khng c vn g. Khi RIP redistribute

vo EIGRP trn RouterB v update c truyn n routerE, bng nh tuyn s chn la

ng i n 190.10.10.0 thng qua routerA. L do l khi network 190.10.10.0 c phn

phi vo EIGRP, n s c nh du nh external route. Nh vy, route s c gi tr AD

l 170 v s b b qua nu so vi gi tr AD=120 ca RIP. Bng nh tuyn cha cc ng i

qua ng RIP v network 190.10.10.0. Khi EIGRP redistribute li vo RIP, bng nh tuyn

khng c route ca EIGRP no n network 190.10.10.0 v khng th redistribute route ny

ngc vo RIP. V phng din l thuyt, mt routing-loop c trnh. Tuy nhin thc t

khng phi l nh vy. Cn phi trnh qu trnh redistribution hai chiu. Cng nn t cc

filter khi thc hin redistribution ngn nga routing-loop.

C php tng qut ca lnh redistribute nh sau:

Router(config-router)#

C php:

redistribute protocol [process-id] [metric metric-value] [metric-type type-

value] [match {internal | external 1 | external 2}] [tag tag-value] [route-

map map-tag] [weight weight] [subnets]

xa lnh ny, c th dng:

no redistribute protocol [process-id] [metric metric-value] [metric-type

type-value] [match {internal | external 1 | external 2}] [tag tag-value]

[route-map map-tag] [weight weight] [subnets]

M t c php:

- protocol: Giao thc nh tuyn ang c "y vo"/ang c phn phi vo mt

giao thc nh tuyn khc.


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V d 1: a RIP routes vo OSPF:

Router(config)#router ospf 1

Router(config-router)#redistribute rip

Cc t kha c th dng vi lnh redistribute l: bgp, egp, eigrp, igrp, iso-igrp, isis, odr,

ospf, mobile, static, connected v rip. T kha static c dng vi tuyn ng tnh. T

kha connected c dng ch ra nhng route c kt ni trc tip. i vi nhng giao

thc nh tuyn nh OSPF, cc route c phn phi vo s l loi external.

- process-id: Mc chn ny c dng cho cc giao thc nh tuyn c dng AS

number. Gi tr ny s ch ra process-id hoc routing process. i vi RIP th khng

cn dng.

- metric metric-value: Metric c dng cho nhng route c phn phi vo. Nu gi

tr ny khng c ch ra, v nu khng c gi tr mc nh no c ch ra trong lnh

default-metric, gi tr mc nh uc dng l 0 (chnh xc hn l ty thuc vo giao

thc nh tuyn). Bn nn dng mt gi tr nht qun cho gi tr metric ny.

Lnh trn s phn phi cc IGRP route vo OSPF domain. Cc routes external s c

metric l 100.

Router(config)#router ospf 109

Router(config-router)#redistribute igrp 108 metric 100 subnets

Router(config-router)#redistribute rip metric 200 subnets

Gi tr metric c ch ra trong redistribute command s c u tin cao hn gi tr

metric c ch ra bi lnh default-metric. Khi phn phi mt giao thc nh tuyn vo mt

giao thc nh tuyn khc, bn cn phi gn cc metric ph hp cho cc routes mi ny. Cc

giao thc nh tuyn khc nhau dng cc metric rt khc nhau. Cc gi tr thch hp ph thuc

vo giao thc nh tuyn trong tng trng hp c th. trnh hin tng route li nhiu

ln, ta c th dng cc c ch route-filtering nh distribute-list, route-map, distance, prefix-

list. Trong thc t th c mt s trng hp thng dng redistribution l gia cc nh cung

cp dch v Internet. Cc ISP dng IGP cho mng ca mnh v dng BGP kt ni vi cc

ISP khc. Thng th cc internal routers s c default gateway l cc bgp routers v trn bgp

routers s phn phi cc prefixes c hc t IGP y ra cc bgp routers ca ISP khc.

Mt trng hp khc c th cn n route redistribution l vic st nhp cc cng ty. Cng

ty A (ang dng EIGRP) mua li cng ty B (ang dng OSPF), khi kt ni mng li vi nhau


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m cha kp thay i ton b h thng mng th h s dng redistribution trn mt router no

, cn gi l mutual redistribution.

5.1.5. Trnh vn nh tuyn khng ti u khi thc hin redistribution

Nh cp trong cc mc trc, vn suboptimal thng thong s b to ra bi cc

qu trnh redistribution. V d gi tr AD s chn la ng i km ti u khi mt ng i

kt ni trc tip li c dng nh mt ng i d phng.

Hy tun theo cc nguyn tc sau y khi thit k mng trnh routing-loop:

1. C mt kin thc tt v s mng, Routing domain, dng traffic.

2. Khng cho cc giao thc chy cht chng ln nhau. Mi vic s d dng hn nu cc

giao thc khc nhau c th c phn chia r rng vo cc domain ring l trong

router hot ng nh cc router ranh gii. y cn c gi l core v edge

protocol.

3. Ch ra cc router ranh gii m trn phi cu hnh redistribution.

4. Xc nh giao thc no l core, giao thc no l edge.

5. Xc nh chiu ca qu trnh redistribution, trong giao thc no s c

redistribute. Ly v d, RIP s redistribute vo EIGRP nh EIGRP s khng

redistribute vo RIP. iu ny nhm trnh cc mng s b phn hi ngc li vo

domain ban u. Hy dng default-route hoc qu trnh redistribution mt chiu nu

cn thit.

6. Nu qu trnh redistribution hai chiu l khng th trnh khi, hy dng c ch sau:

Cu hnh metric bng tay, cu hnh gi tr AD bng tay, dng distribution access-list.

5.1.6. Trnh cc vn vi hi t mng khi redistribution

duy tr tnh nht qun gia cc giao thc nh tuyn khc nhau, ta phi xem xt nhiu

cng ngh. Mt mi quan tm ln l qu trnh tnh ton ca bng nh tuyn v khong thi

gian hi t trong bao lu. EIGRP c tc hi t kh nhanh trong khi RIP th hi tu chm

hn. Vic chia s thng tin gia hai cng ngh c th gy ra vi vn . V d mng s hi t

tc ca giao thc chm hn. mt vi thi im, iu ny s to ra timeout v kh nng

routing loops. iu chnh timers s gii quyt vn nhng bt c mt giao thc no cng

phi c cu hnh vi mt kin thc vng chc v ton b h thng mng. Cc thng s thi

gian timers thng yu cu l cu hnh ging nhau trong tt c cc routers sao cho timer c

cng gi tr.


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Kim sot routing update khi redistribution

Kim sot routing update th hu ch trong nhiu trng hp. Cc l do kim sot cc

routing update gm:

- Che du mt vi network no ra khi phn cn li.

- Ngn nga routing loop.

- Kim sot ph tn ca traffic trn mng, cho php mng c kh nng m rng.

- V cc l do bo mt.

Cc phng thc khc nhau kim sot bao gm cc phng thc sau:

- Dng passive interface.

- Dng nh tuyn tnh.

- Dng default routes.

- Dng null interface.

- Dng distribute-lists.

- Dng route-map.

Cng giao tip b ng trong qu trnh nh tuyn (Passive Interfaces)

Mt cng b ng s khng tham gia vo qu trnh nh tuyn. Trong RIP v IGRP, qu

trnh ny s lng nghe cc cp nht m khng gi cc thng tin cp nht nh tuyn. Trong

OSPF v EIGRP, cc qu trnh ny s khng lng nghe hay gi cc cp nht v cc router

khng gi i cc gi hello v v vy cc quan h lng ging gia cc router khng th hnh

thnh. Cc cng giao tip ca router tham gia vo cc qu trnh nh tuyn c kim sot bi

cu hnh. Trong khi cu hnh, qu trnh nh tuyn s dng lnh network ch ra cng giao

tip no ca router tham gia vo qu trnh nh tuyn. Cu hnh passive-interface s ngn

nga cc cp nht i vo cc min nh tuyn khc v c th ngn nga routing loop.

nh tuyn tnh

nh tuyn tnh l mt route c cu hnh bng tay. nh tuyn tnh c u tin cao

hn cc routes c hc thng qua mt giao thc ng v n c gi tr AD thp hn. Nu

khng c giao thc nh tuyn no c cu hnh, nh tuyn tnh c th c cu hnh. Cch

dng nh tuyn tnh khng ph hp vi cc h thng mng ln trong thc t bi v bng nh

tuyn s khng cp nht cc thay i mt cch t ng. Trong nhng mi trng nh hn

hoc nhng stub-network, cch dng nh tuyn tnh l mt gii php hiu qu. c bit trong

tnh hung c nhiu giao thc c cu hnh trn router, thay v redistribution ton b bng


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nh tuyn gia cc giao thc, nh tuyn tnh s c nh ngha v redistribution. iu ny

c bit hu ch nu bn cn cung cp nhiu thng tin hn l dng default-route. Cch dng

nh tuyn tnh trong redistribution l mt cch dng tiu biu.

Cc l do dng nh tuyn tnh c tm tt nh sau:

- Ngn nga vic phi chy mt giao thc nh tuyn ng chy trn mt mng, gim

chi ph mng v zero.

- Nu c hai AS khng cn trao i ton b bng nh tuyn m ch cn bit v mt vi

routes.

- Khng c mt giao thc nh tuyn ng no c dng, v d mng stub.

- thay i gi tr netmask ca mng. V d nh trong BGP, ta c th nh ngha mt

supernet v redistribution route vo BGP process. Tc v ny cng c dng khi

redistribute mt giao thc h tr cho VLSM vo mt giao thc khng h tr VLSM.

Default Routes

Mt default route c dng nu khng c mt entry no trong bng nh tuyn cho mt

mng ch. Nu qu trnh tm kim bng nh tuyn khng tm thy entry no trong bng

routing cho mng ch v default-route khng c cu hnh, gi d liu v a ch mng

s b loi b. Nu qu trnh nh tuyn b t chi quyn gi cc cp nht, cc router

downstream s c mt kin thc gii hn v mng do khng nhn thng tin. gii quyt

vn ny, default route s c dng. Cc default route s gim ph tn cho h thng v n

gin ha cng tc qun tr, c bit l c th xa routing loop khi c dng thay cho qu

trnh redistribution. Mt giao thc nh tuyn c th dng mt default route n nhng giao

thc nh tuyn khc. Mt v d tiu biu l mt IGP s ch mt default route n mt router

BGP nm ranh gii ca AS. Mt trng hp khc cn cu hnh default route l cho mt

mng stub kt ni n mt h thng mng ln hn.


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Hnh 5.5 - Cch dng default-route (1)

Hnh 5.6 - Cch dng default-route (2)

Null Interface

Null interface l mt interface o c nh ngha nh l next hop trong mt dng nh

tuyn tnh. Tt c cc traffic n mt mng xa s c route vo mt l en. iu ny c tc

dng rt tt trong qu trnh redistribution v n thng c dng loi b route.

Distribute Lists

Distribute list l nhng access list p dng vo qu trnh nh tuyn, xc nh network no

s c chp nhn vo qu trnh routing. Khi giao tip vi qu trnh nh tuyn khc thng

qua qu trnh redistribution, vic kim sot cc thng tin gi n cc qu trnh khc l quan

trng. Vic kim sot ny l nhm mc ch bo mt. Access list l cng c tt xc nh

traffic trn mt network.

Route Maps

Route map l cc access-list phc tp cho php lp trnh thm cc tu chn. Nu mt gi

hoc mt route match vi tiu chun nh ngha trong pht biu match, cc hnh ng hoc

thay i c nh ngha trong lnh set s c thc hin trn packet. Route map th c


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dng trong qu trnh redistribution trong cng mt cch nh distribute list nhng cho php

kim sot nhiu hn trong cc tiu chun a ra.

Hnh 5.7 - Cch dng route-map

Trong hnh v trn, router A c mt distribute list trong t chi qu trnh truyn ca

mng 140.100.32.0 i ra khi E3. Network 140.100.32.0 c th c vi nguyn nhn bo mt

nn khng th cho cc router ni v routerB thy network ny. S0 v S1 c cu hnh nh

tuyn tnh. Trong trng hp S0, y l kt ni i vo Internet v nh tuyn tnh c cu

hnh bI ISP. iu ny cho php h thng mng trn kt ni n ISP m khng nhn nhng

routing update ng t ISP. Cc routing update t ISP cha cc bng nh tuyn rt ln. Mng

c mt default-route c thit lp. Trn S1, cc cng giao tip ca router c cu hnh dng

nh tuyn tnh sao cho cc router u kia khng cn thit chy mt giao thc ng. Router

u xa s dng mt default route c cu hnh v y l m stub network. iu ny m

bo rng routerC c mt cu hnh n gin vi mt vi yu cu t ra.

5.1.7. Cu hnh Redistribution

Cu hnh redistribution cho mt giao thc s ty thuc vo c im ca tng giao thc.

Tt c cc giao thc yu cu cc bc sau khi thc hin redistribution:

- Bc 1: Cu hnh redistribution.


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- Bc 2: nh ngha gi tr metric mc nh c gn vo bt c network no ang

c redistribute vo cc qu trnh nh tuyn.

Cc lnh cu hnh redistribution c cu hnh nh l cc lnh con ca qu trnh

routing. Lnh redistribute c th ch ra giao thc nh tuyn m t cc updates c chp

nhn. N s ch ra ngun ca updates. Ty thuc vo thit k mng, cc cu hnh thm vo l

cn thit.

5.1.8. Cu hnh Default Metric

Gi tr default metric c th c cu hnh theo vi cch. Cch u tin l bao gm ty

chn metric trong lnh redistribute, nh ngha gi tr metric cho route . Cng c th cu

hnh gi tr default metric vi lnh default-metric trong qu trnh nh tuyn. Dng lnh

default-metric s gim s lnh phi cu hnh v khi ny khng cn phi cu hnh cho mi qu

trnh redistribution ring l.

Cu hnh gi tr metric mc nh cho OSPF, IS-IS, RIP, EGP hoc BGP

C th cu hnh redistribution giao thc nh tuyn, sau dng lnh ring l default-

metric ch ra gi tr default-metric. im thun li l y l cu hnh n gin hn, d

troubleshooting hn. Ngoi ra nu c nhiu giao thc c redistribute vo, gi tr default-

metric s p dng cho tt c cc giao thc ang c redistribute. Ring ISIS khng th nh

ngha gi tr default metric. Gi tr metric phi ch ra khi thc hin redistribute. Nu gi tr

metric khng c ch ra, gi tr mc nh (cost = 0) s c dng v route s b b qua.

cu hnh cc gi tr mc nh cho ospf, rip, egp hoc bgp, hy dng c php sau:

Router(config-router)#default-metric number

Cu hnh gi tr AD

Bn cnh vic ch ra gi tr metric cho cc route trong qu trnh redistribution, ngi qun

tr mng cng cn kim sot qu trnh chn la gia cc giao thc nh tuyn khc nhau.

Trong trng hp ny th ch dng gi tr metric l khng v c nhiu giao thc ang c

redistribute. Vic thay i gi tr AD cho php ng i tt nht c thc hin. m bo

ng i ti u c chn la, i khi cn phi thay i gi tr AD lm cho mt route l

km hn so vi cc route khc. Cu trc lnh l ging nhau cho cc giao thc ngoi tr

EIGRP dng kiu lnh khc. C php di y l dnh cho EIGRP:

Router(config)#distance eigrp internal-distance external-distance

cu hnh gi tr AD cho giao thc IP, dng c php lnh sau:


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Router(config-router)#distance weight [ address mask] [ access-list-number

| name] [ip]


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Cu hnh Passive Interface

Passive interface c dng cho nhng giao thc nh tuyn trong gi update ra tt c

cc interface c a ch c ch ra trong lnh network. Lnh passive interface s lm router

khng gi ra cp nht v hng o. C php nh sau:

Router(config-router)#passive--interface type number

Cu hnh nh tuyn tnh

Router(config)#ip route prefix mask { address | interface} [ distance]

[tag tag] [permanent]

Lnh ny s nh ngha bng cch ch ra next-hop router gi traffic. Cu hnh ny ch

c th dng ch nu a ch phn mng ca next-hop router l c trong bng nh tuyn. Nu

nh tuyn tnh cn phi c qung b n cc router khc, n nn c redistribute. Trong

mt vi phin bn IOS, route ny s t ng redistribute.

Cu hnh default route

Trong nhng h thng mng ln hn, c th cu hnh nhiu tuyn ng tnh. Nu qu

nhiu tuyn ng tnh c cu hnh, bn c th cu hnh mt dng tuyn ng tnh c

bit gi l static default route.

Router(config)#ip route 0.0.0.0 0.0.0.0 s0

Khi thng tin nh tuyn c truyn thng qua mt giao thc nh tuyn ng, khng cn

yu cu mt cu hnh g thm. Trong trng hp ca RIP, ch c th c mt default-route,

network 0.0.0.0. Tuy nhin trong trng hp ca IGRP, vi network c th lm default route

mc d ch mt default-route l c dng.

Nu mt router khng c mt network kt ni trc tip nhng c mt route v n, route

c th xem xt nh candidate default-route. cu hnh mt default-route, dng lnh sau:

Router(config)#ip default-network network-number

Lnh ny s to ra mt default route c gi trong cc update. N s khng to ra mt

default route trn router c cu hnh v n ch to ra mt default route nu route ang

kt ni trc tip. Khi c nhiu default route trong bng nh tuyn, cc default route ny s

c kim tra. ng i mc nh tt nht s c chn la da trn gi tr AD v metric.

a ch gateway ca ng i tt nht s tr thnh gateway of last resort ca router. Thut


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ng ny cn c mt tn gi khc l default-router. Bn c th hin th gateway of last resort

bng lnh:

Router#show ip route

Gi tr default route s xut hin trong bng nh tuyn v c nh du nh mt tuyn

ng tnh vI k hiu S*. Gi tr gateway of last resort s c gn v network ny.

5.1.9. Route Maps

Route map l cc cng c trong cc logic if/then c th c p dng cho mt router.

Cc route-map l cc cng c lp trnh c dng kim sot qu trnh redistribution,

hin thc PBR, kim sot qu trnh NAT hoc hin thc BGP.

C th dng route-map cho cc mc ch sau y:

- kim sot qu trnh redistribution: cc route map cho php kim sot mt mc cao

hn so vi cch dng distribution list. Route-map khng n thun ngn chn hay cho

php mt mng ging nh distribute list m cn c kh nng gn metric cho nhng

route b match.

- kim sot v thay i thng tin nh tuyn: cc route map c dng thay i

thng tin nh tuyn bng cch gn gi tr metric cho cc route.

- nh ngha chnh sch trong PBR: cc route-map ra cc quyt nh da trn a ch

ngun. Khi mt match c tm thy trong access-list, s c cc hnh ng tng ng.

- thm vo mc tinh t trong cu hnh NAT: cc route map nh ngha dy ca

cc a ch public v a ch private. C cc lnh show gim st v kim tra hot

ng ca NAT.

- hin thc BGP: mt trong nhng im mnh ca giao thc BGP l kh nng thc

hin policy based routing. Cc thuc tnh trong BGP c dng nh hng n

ng i cho traffic. Cc thuc tnh ny thng c hin thc dng route maps. Nu

c mt match th p dng thuc tnh ny. Khi ny dng lnh set thc hin. Route

map l phng thc ch yu c dng bi BGP nh ngha chnh sch nh tuyn

BGP.

Route map rt ging ACL. C hai thc hin tc v if/then, trong cc tiu ch c dng

xc nh l packet c c cho php hoc t chi hay khng. S khc nhau c bn l route

map c kh nng thc hin hnh ng thay i thuc tnh n cc gi d liu tha iu kin

match. Trong mt ACL, tiu ch match l ngm nh, trong mt route map, l mt

keyword. iu ny c ngha rng, nu mt gi tha vi mt tiu chun cho trong mt route


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map, mt vi hnh ng phi c thc hin thay i gi, trong khi accesslist ch n gin

cho php hoc deny mt gi.

Cc c im ca route map c tm tt trong danh sch sau:

- Mt route map c mt danh sch cc tiu ch v tiu chun chn la, c lit k vi

pht biu mtch.

- Mt route map c kh nng thay i cc gi hoc cc route b match bng cch dng

lnh set.

- Mt tp hp ca cc pht biu mch c cng tn c xem l cng mt route map.

- Route map s ngng x l ngay khi c mt match c thc hin, ging nh mt

ACL.

- Trong mt route map, mi pht biu c nh s th t v c th c son tho

ring l.

- S th t c dng ch ra th t trong cc iu kin c kim tra. Nh vy

nu hai pht biu trong route map c tn l BESTTEST, mt pht biu c ch s l 5,

mt pht biu c ch s l 15 th pht biu c ch s l 5 s c kim tra trc. Nu

khng c mt pht biu match trong pht biu 5 th pht biu th 15 s c kim tra.

- Route map c th dng cc IP access-list chun hoc m rng thit lp cc chnh

sch nh tuyn.

- Cc access-list m rng c th c dng ch ra tiu ch match da trn phn a

ch ngun v a ch ch, ng dng, kiu giao thc, kiu dch v ToS v u tin.

- Lnh match trong cc cu hnh route map c dng nh ngha iu kin phi

kim tra.

- Lnh set trong cu hnh route map c dng nh ngha hnh ng theo sau mt

pht biu match.

- Mt route map c th ch cc php AND v OR. Ging nh mt access-list, c mt

pht biu ngm nh DENY cui mt route map. Hnh ng theo sau ca pht biu

deny ny ty thuc route map c dng nh th no. hiu iu ny mt cch

chnh xc, bn cn hiu chnh xc route map hot ng nh th no.

Danh sch sau y s gii thch logic ca hot ng route-map:

- Pht biu ca route map dng cho PBR c th c nh du nh l permit hoc deny.

- Ch nu pht biu c nh du nh permit v packet b match, lnh set mi c p

dng.


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- Cc pht biu trong route-map s tng ng vi cc dng ca mt access-list. Ch ra

mt iu kin match trong route map th cng tng t nh ch ra ngun v ch trong

access list.

- Cc pht biu trong route map c so snh vi ng i ca gi xem c mt

match no hay khng. Cc pht biu ny s c ln lt kim tra t trn xung

di.

- Mt pht biu match c th cha nhiu iu kin. t nht mt iu kin trong pht biu

match phi l ng. y l php logic OR.

- M route-map c th cha nhiu pht biu match. Tt c cc pht biu match trong

route map phi c xem xt l ng cho pht biu ca route map l match. iu

kin ny gi l php logic AND.

5.2. nh tuyn theo chnh sch (Policy-Based Routing PBR)

Cc route map c dng trong cu hnh PBR cho php chn la nhiu tiu chun nh a

ch IP, cc ng dng, giao thc, kch thc gi tin. Khi c chn la, cc lnh ca PBR s

hin thc chnh sch cho cc route c chn la.

Cc routes c route theo policy v cc tuyn ng tnh c rt nhiu im chung. Tuy

nhin nh tuyn tnh s chuyn cc gi da trn a ch ch, trong khi policy s chuyn gi

theo a ch ngun. Nu cc access-list c dng vi route-map, cc thng s trong mt

access-list m rng c th c dng route traffic da trn nhng tiu chun nh a ch

ch, chiu di, giao thc IP, u tin hoc port number. Kh nng ny cho php iu chnh

tt hn na cc tiu chun m qua gi tr next-hop l c quyt nh.

Cc lut p dng nh ngha PBR l nh sau:

- Traffic c th c iu chuyn da trn a ch ngun v a ch ch.

- PBR ch nh hng n qu trnh nh tuyn trong router m n c cu hnh.

- PBR khng c nh hng n a ch ch ca gi tin nhng n c th nh hng n

vic chn la ng i bng cch thay i gi tr next-hop.

- PBR khng cho php traffic gi ti mt AS khc thay i ng i c chn la

bi AS .

- PBR ch c th nh hng ch n vic traffic s n router lng ging no.

- Khi PBR kim tra a ch source, n c cu hnh trn inbound interface.

- Nu khng c match, gi tin s khng b policy route v s c route bnh thng

n ch.


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- Cch s dng route map trong PBR th hi khc so vi cc cch dng route-map khc.

Khi c s dng cho PBR, nu mt gi khng match vi cc tiu ch c ch ra

trong route-map th gi tin s khng b loi b. Thay vo , gi tin s c gi n

qu trnh nh tuyn v c route bnh thng da trn a ch ch. Nu bn mun

loi b packet, bn nn dng lnh set route packet n null interface trong dng

cui cng ca route-map.

- PBR cng cho php mt c ch nh du cc packet vi cc gi tr ToS khc nhau.

c im ny c th c dng kt hp vi cc k thut hng I sao cho mt vi loi

traffic nhn c cc dch v u tin hn.

- Thay v nh tuyn bng cch dng a ch ch, PBR cho php bn xc nh v ci t

chnh sch nh tuyn bng cch cho php hoc deny mt gi tin da trn cc thng s

nh a ch ca mt thit b u cui, ng dng ang chy, giao thc ang dng, kch

thc ca gi tin.

5.2.1. Cc li ch ca PBR

- Chn la nh cung cp dch v da trn ngun gc traffic: Cc nh cung cp dch

v thng dng PBR ra cc quyt nh nh tuyn da trn a ch ngun. Cc

traffic ca cc khch hng khc nhau s c route n cc kt ni Internet khc nhau.

- Cht lng dch v: Bng cch thit lp cc kiu cht lng dch v (ToS: Type Of

Service) trong gi IP, cc t chc c th cung cp dch v QoS. Theo cch ny, cc

traffic c th khc nhau v cc c ch hng i c th c ci t u tin traffic

da trn gi tr QoS trn lp core v backbone ca mng.

- Tit kim chi ph: Cc traffic c to ra bi mt hot ng c bit c th c p

t cho mt mc bng thng cao hn trong mt khong thi gian ngn.

- Chia s ti: PBR cho php hin thc cc chnh sch phn phi traffic gia cc

ng i khc nhau da vo loi traffic. Tnh nng ny khng ging vi c tnh chia

s ti ng trong cc giao thc nh tuyn.

5.2.2. Cc im bt li ca PBR

- Nn dng thm mt ng i d phng trong trng hp router k tip b down. Nu

khng c mt gii php d phng, PBR dng bng nh tuyn.

- Phi dng thm nhiu chu k CPU v tt c cc a ch source u phi c kim tra.

- Phi cu hnh thm.

- S tn ti kh nng cc loi traffic khc b hy.

5.2.3. Hot ng ca route-map v ca Policy based Routing (PBR)


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PBR p dng vo cc gi tin i vo hoc cc gi tin c to ra bi router. Khi mt gi tin

nhn c trn mt interface c cu hnh PBR, gi tin s tri qua mt qui trnh nh sau:

- Nu c mt match xy ra v hnh ng l cho php, gi tin s c route theo hnh

ng c ch ra trong lnh set.

- Nu c mt match xy ra v hnh ng l t chi gi tin th packet s khng c

route theo chnh sch nhng s c nh tuyn nh bnh thng.

- Nu khng c match no v nu khng c thm on cu hnh no ch ra phi lm g

trong tnh hung ny, mc nh gi tin s b deny, ngha l s chuyn v nh tuyn

bnh thng. Qu trnh nh tuyn bnh thng s c th b loi b nu ta dng thm

mt pht biu set nh tuyn packet v null0. Ni cch khc, y l cch loi b

gi tin.

5.2.4. Cu hnh Route Maps cho Policy-Based Routing

Cch cu hnh route map nh sau:

Router(config)#route-map map-tag [{permit | deny} sequence-number]

5.2.5. Cu hnh Fast Switching vi Policy-Based Routing

Tc i qua mt h thng mng b nh hng bi kh nng ca cc thit b mng trong

x l traffic. C ch fast switching cho PBR b tt ch mc nh. cu hnh fast-switch

cho PBR, thc hin cc bc sau:

- Bc 1: Cu hnh PBR trc khi bn c th cu hnh fast-switch.

- Bc 2: Khi PBR c cu hnh, bt fast-switching vi lnh.

Router(config-if)#ip route-cache policy

C ch fast-switch PBR h tr tt c cc dng ca lnh match v phn ln cc lnh set

ngoi tr vic lnh set ip default khng c h tr.

Lnh set interface c h tr ch trn nhng kt ni point-to-point tr phi c mt entry

c cache dng cng mt interface trong lnh set interface trong route map. Route cache

l mt phn ca b nh c dnh cho cc quyt nh nh tuyn. Trong process switching,

bng nh tuyn s c tham kho tm ng i n ch. Trong fast switching, h iu

hnh khng kim tra bng nh tuyn bi v c thng tin c cache li. Thay vo , nu

mt gi l match, h iu hnh s nhanh chng chuyn gi tin ra interface ngay lp tc.

ROUTE OPTIMIZATIONRedistributionTng quanRouting Metrics v RedistributionChn la ng i gia cc giao thc nh tuynTrnh vng lp khi redistributionTrnh vn nh tuyn khng ti u khi thc hin redistributionTrnh cc vn vi hi t mng khi redistributionCu hnh RedistributionCu hnh Default MetricRoute Maps

nh tuyn theo chnh sch (Policy-Based Routing PBR)Cc li ch ca PBRCc im bt li ca PBRHot ng ca route-map v ca Policy based Routing (PBR)Cu hnh Route Maps cho Policy-Based RoutingCu hnh Fast Switching vi Policy-Based Routing

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