những kiến thức cơ bản về bgp

8/3/2019 Nhng kin thc c bn v BGP

1/14

Nhng kin thc c bn v BGPS cn thit ca BGP

BGP l mt giao thc kh phc tp c dng nhiu trn Internet v trong cc cng ty a quc

gia. Mc ch chnh ca BGP l kt ni cc mng rt ln hoc cc Autonomous-System. Cccng ty ln c th dng BGP nh l mt kt ni gia cc mng cc quc gia khc nhau. Mcch ca cc giao thc ngoi nh BGP l khng ch tm ra mt ng i v mt mng no mcn cho php ngi qun tr tm ra cc AS ca cc network. Cc giao thc ni nh EIGRP, RIP, OSPF v ISIS s tm ra network m ngi qun tr cn.

Mt s tnh cht ca BGPBGP l mt giao thc nh tuyn dng path-vector nn vic chn la ng i tt nht thngthng da trn mt tp hp cc thuc tnh c gi l ATTRIBUTE. Do s dng metric khphc tp, BGP c xem l mt giao thc kh phc tp. Nhim v ca BGP l m bo thng

tin lin lc gia cc AS, trao i thng tin nh tuyn gia cc AS, cung cp thng tin v trm kcho mi ch n. BGP s dng giao thc TCP cng 179. Nh cc chng trc cp, ccgiao thc nhm distance vector thng qung b thng tin hin c n cc router lng ging, cnpath vector ch ra chnh xc danh sch ton b ng dn n ch. Ngoi ra cc giao thc nhtuyn hot ng dng path vector gip vic xc nh vng lp trn mng rt tt bng cch xemxt cc con ng m cc router khc gi v xem c chnh bn thn AS trong hay khng, nuc s bit c ngay l lp, v s loi b.BGP h tr cho cc a ch CIDR (Classless Interdomain Routing). BGP cho php dng xc thcv BGP c cc c ch keepalive nh k nhm duy tr quan h gia cc BGP peers. Trong giai on ban u ca ca phin thit lp quan h BGP, ton b cc thng tin routing -update s c gi. Sau , BGP s chuyn sang c ch dng trigger-update. Bt k mt thay

i no trong h thng mng cng s l nguyn nhn gy ra trigger-update.Mt trong cc c im khc bit nht ca BGP l trong cc routing-update ca n. Khi ta xemxt cc BGP update, ta s nhn thy cc routing update ny l kh chnh xc. BGP khng quantm n vic giao tip c y kin thc ca tt c cc subnet bn trong mt cng ty mBGP quan tm n vic chuyn ti y thng tin tm mt AS khc. Cc BGP update thchin qu trnh summarization n mt mc ti a bng cch cho php mt s AS, cho php mts prefix v mt vi thng tin nh tuyn. Tuy nhin, mt phn nh ca BGP update l kh quantrng. BGP m bo rng lp transport truyn cc update v cc c s d liu v ng i c ng b. BGP c th c hin thc bao gm gia cc AS khc nhau hay trong cng 1 AS.Khi dng BGP kt ni cc AS khc nhau, BGP c gi l eBGP. Giao thc ny cng c thc dng mang thng tin gia cc router eBGP trong mt AS. Khi BGP c gi l

iBGP.


2/14

Trong mt AS ta s dng giao thc nh tuyn ni IGP (v d nh RIP, ISIS, EIGRP, OSPF)nhng khi ra ngoi mt AS th phi s dng mt giao thc khc. Vn y chnh l mc chca cc IGP v EGP khng ging nhau. Cc IGP thc hin nh tuyn gi i t ngun n chm khng cn quan tm n chnh sch nh tuyn (policy). Trong khi ra khi phm vi mt ASth chnh sch nh tuyn li l vn quan trng . Xt v d sau:

AS4---AS1---AS2---AS3---(C Company)

|........................................|

|........................................|B Company-----------A Company

Gi s A mun truyn d liu n AS4. A v B l 2 i th ca nhau. B khng mun chuyn dliu cho cng ty A nn A ch c th chuyn d liu thng qua AS3, AS2, AS1 n c AS4,mc d con ng thng qua cng ty B c th l ti u nht. Gi s C thuc AS3 cng mun nAS4 nhng C l i tc ca B nn B sn sng cho qu giang. Nh vy A v C c cng mt ch

n nhng phi i theo nhng cch khc nhau. Cc IGP khng th lm c iu ny v mtnguyn nhn ch yu chnh l cc giao thc u lin quan n metric hay l cost m honton khng quan tm n chnh sch nh tuyn. N ch bit c gng lm sao chuyn c ccgi d liu n ch mt cch hiu qu v ti u nht.Cc thut ng BGP

1Aggregation


3/14

L qu trnh tm tt cc route (summarization)

2

Attribute

Tng t nh metric. Cc bin ny s m t cc c im ca ng i ti mt a ch ch no

. Khi c nh ngha, cc c im ny c th cdng ra quyt nh v nn i theong i no.

3Autonomous System

nh ngha mng ca mt t chc. Trong mt AS, cc router s c cng giao thc nh tuyn.Nu ta kt ni ra Internet, ch s AS ny phi l duy nht v c cung cp bi cc y banInternet.

4

Exterior Gateway Protocol (EGP)

Thut ng chung cho mt giao thc c chy gia cc AS khc nhau. Cng c mt giao thcc tn l EGP l tin thn ca BGP

5EBGP

Gi thng tin nh tuyn gia cc AS khc nhau

6Interior Gateway Protocol (IGP)

y l cc giao thc nh tuyn chy bn trong mt AS. Trong qu kh, thut ng gatewaythng c dng nh ngha mt router.

7IBGP

Giao thc ny c dng bn trong mt AS. Cc router khng yu cu phi l lng ging canhau v phng din kt ni vt l v thng ngoi ra ca mt AS. IBGP c dng gia ccrouter chy BGP trong cng mt AS.

8

Originator-ID

y l thuc tnh ca BGP. N l mt thuc tnh ty chn. Thuc tnh ny s cha gi trrouterID ca router pht sinh ra ngi . Mc ch ca thuc tnh ny l ngn ngarouting loop. Nu mt router nhn c mt update t chnh n, router s b qua update .


4/14

9

policy-based routing

C ch ny cho php ngi qun tr lp trnh giao thc nh tuyn bng cch nh ngha traffic s

c route nh th no. y l mt dng ca nh tuyn tnh. PBR c lp vi cc giao thcnh tuyn v dng route-map to ra cc qu trnh ring l p t cc quyt nh nhtuyn.

10

prefix list

Prefix list c dng nh mt thay th cho distribute-list kim sot BGP hc hoc qung bcc cp nht nh th no. Prefix-list th nhanh hn, uyn chuyn hn v t tn ti nguyn ca hthng hn.

11

Route-reflectory l router c cu hnh chuyn cc routes t cc router iBGP khc. Khi cu hnh route -reflector, cc iBGP khng cn phi fully-mesh na. Mt mng fully-mesh th khng c kh nngm rng.

12

Route-Reflector Client

Mt client l mt router c mt TCP session vi mt router khc hot ng nh mt route-reflector-server. Client khng cn thit phi thit lp peer vi cc client khc.

13

Route_reflector ClustorMt cluster l mt nhm bao gm mt route-reflector v clients. C th c nhiu hn mt route-reflector server trong mt cluster.

14

transit autonomous system

L AS c dng mang cc BGP traffic qua cc AS khc.

Khi no th dng BGP?

Khi mng ca mt cng ty kt ni n nhiu ISP hoc cc AS khc v ang dng cc kt niny. Nhiu cng ty dng cc kt ni khc nhau nhm mc ch d phng. Chi ph c th gim


5/14

thiu nu tt c cc kt ni u c dng. Trong trng hp ny, PBR c th cn thit trinkhai trn tng kt ni. BGP cn c dng khi chnh sch nh tuyn ca nh cung cp dch vv ca cng ty khc nhau. Hoc traffic trong cng ty cn phi c phn bit vi traffic ca ISP.Mng ca hai t chc khng th xut hin nh mt AS. Mt trng hp khc phi dng BGP lkhi mng ca ta l mt ISP. Nu l mt ISP, h thng mng ny phi cho php cc traffic khc i

qua AS ca mnh. Lc ny n hot ng nh mt transit domain.

Khi no th khng dng BGP?

Mt h thng mng n gin l mt h thng mng d dng qun l v bo tr. y l l dochnh trnh dng BGP trong mt h thng mng. V vy, nu h thng mng c cc c imsau, nn dng nhng cch thc khc, chng hn nh static hoc default-routing.

- Mng ca ISP v mng ca cng ty c chung mt chnh sch nh tuyn

- Mc d cng ty ca bn c nhiu kt ni n ISP, cc kt ni ny l d phng v v vy khngcn mt k hoch kch hot nhiu hn mt kt ni n Internet.

- Ti nguyn mng l c gii hn, chng hn nh b nh v CPU ca router.

- Bng thng gia cc AS l thp v cc ph tn cho nh tuyn s nh hng n qu trnhchuyn d liu.

Mt s thuc tnh quan trng ca BGPBGP nh tuyn traffic bng cch s dng cc thuc tnh. Vic s dng cc thuc tnh m chn vic s dng cc bin trong qu trnh chn la ng i trong BGP. Cc thuc tnh ca BGPkhng ch l danh sch cc bin m qua route c chn la. Mt vi thng tin c mangtrong cc thng ip cp nht l quan trng hn cc thng tin khc. Mt s thng tin khc l rtquan trng cho hot ng ca BGP, v vy cc thng tin ny phI c mang n tt c ccrouter BGP trong mng. Qu trnh chnla ng i da trn nhng thuc tnh v cc gi tr can. Cc thuc tnh c chia thnh hai nhm: nhm nI ting (well-known) v nhm ty chn(optional). C hai nhm ny s c tip tc chia ra lm cc nhm con.

NhmM t

Well-known mandatory

Cc thuc tnh ny l bt buc v c cng nhn bi tt c cc router BGP

Well-known Discretionary


6/14

Khng yu cu cc thuc tnh ny tn ti trong cc cp nht nhng nu chng tn ti, tt c ccrouter s cng nhn v s c hnh ng tng ng da trn thng tin c cha bn trong thuctnh ny.

Optional Transitive

Router c th khng cng nhn cc thuc tnh ny nhng nu router nhn c thuc tnh ny,n s nh du v gi y cp nht ny n router k tip. Cc thuc tnh s khng thay i khi i qua router nu thuc tnh ny khng c cng nhn bi router.

Optional NontransitiveCc thuc tnh ny b loi b nu cp nht mang thuc tnh ny i vo router m router khnghiu hoc khng cng nhn thuc tnh. Cc thuc tnh ny s khng truyn ti cc BGP.

1. Thuc tnh AS_PATH: y l thuc tnh quan trng trong vic xc nh ng i ti u,mang 2 chc nng. Th nht, AS_PATH cng ngn cng c router u tin hn. (v d t AS3n AS4 c 2 con ng khc nhau: AS3 - B - AS4, hoc AS3 - AS2 - AS1AS4 th router su tin con ng ngn hn l qua B). Chc nng th hai l ngn nga vng lp. Bi v thuctnh AS_PATH rt quan trng cho qu trnh hot ng ca BGP nn tt c cc nh cung cp phih tr n v l mt thuc tnh mang tnh bt buc, ngha l tt c cc b nh tuyn phi chuynthuc tnh ny n tt c cc lng ging trong mi thng ip cp nht ca BGP.

2. Thuc tnh ngun gc (Origin-attribute): l mt thuc tnh xc nh ngun gc ca cc routingthng tin cpnht nh tuyn. Nu l i th ch rng routing thng tin cp nht nh tuyn cbit thng qua mt IGP (RIP, OSPF). Nu gi tr e th routing thng tin cp nht nh tuync bit thng qua mt EGP. Nu gi tr ? (incomplete) th ch rng khng bit ngun gcca routing thng tin cp nht nh tuyn. (iu ny c th xut hin nu routing thng tin cpnht nh tuyn c bit thng qua IGP ri sau li c thng bo qua EGP.). Khi n hcng bng lnh network th n thuc tnh ny l: i. Khi n hc ng qua cc lnh nhredistribute v.v... th n thuc tnh l: ?.Cn sau cc mng ny c qung b ti ccEGP router lng ging th thuc tnh ny c i thnh e v router mi hc c cc tuynng ny nh EGP. Thuc tnh origin khng thay i qua giao thc EBGP m vn gi nguyn.Thuc tnh origin l e khi tuyn c hc t giao thc EGP (y l giao thc nh tuyn gia ccAS, c s dng nhiu trn Internet khi BGP cha ra i nhng hin nay hu nh khng cnc s dng)

Next-Hop attribute:


7/14

Trong eBGP, gi tr next-hop l a ch IP ca router t bn ngoi ca vng t tr AS qung bvo bn trong AS. a ch IP c ch ra bi lnh neighbor. Tuy nhin trong mi trngmultiaccess, nu mt route n t mt router, s l khng khn ngoan nu mt router qung ba ch ca n nh l a ch ngun v iu ny s dn n vic cc packets b gi n vi routertrn mng multiacess trc khi c th tm ra c ngun gc xut pht tht s. V vy, c mt

lut rng, a ch ca router xut pht gi tin BGP s vn gi nh a ch ngun.

Khi BGP khai bo mt mng, router t nh r bn thn router nh l Next Hop. Tuy nhin,nu router gi thng ip update cho mt router lng ging khc trong cng mt AS th n litip tc dng router ngoi AS lm Next hop. Do ta phi cu hnh cho cc router cn li trongAS bit c chnh router bin cng AS vi n l router Next Hop ch khng phi l routerkhc AS.

Bc 1: Router A gi cp nht v mng 192.33.33.0

Bc 2:Router D nhn update v truyn. Tuy nhin nu routerD thay i a ch source -addressthnh a ch a ch ca chnh n, s c mt s nhm ln xy ra v trong mi trng multiaccess,mi router s cng b n l next-hop. trnh tnh hung ny, a ch ngun ca update khngthay i.

Bc 3: Router B bit routerA l next-hop ca mng 192.33.33.0

The Next-Hop Attribute and a Nonbroadcast Multiaccess Network

Cng ging nh trong mi trng multiaccess, mt mng NBMS cng tun theo cng mt quilut.Tuy nhin s c tim n mt vn v cc router khc s khng c kh nng kt ni trctip vi router ngun nu m my NBMA c cu hnh hub-and-spoke.


8/14

Thuc tnh Nexthopself trong BGP

150.10.0.0 (AS 100)

RouterA--------(150.10.3.1)---IBGP--------RouterB---||(170.10.20.1)

|

| (170.10.20.2)RouterC

|_ 170.10.0.0 (AS 300)

Thuc tnh BGP nexthop l mt a ch next hop IP m n s c dng i n mt s mng

ch. i vi EBGP, next hop lun lun l a ch IP ca router lng ging c khai botrong cu lnh neighbor. Theo v d trn, RouterC s qung co network 170.10.0.0 cho RouterAvi mt next hop l 170.10.20.2 v ngc li RouterA s qung co li 150.10.0.0 cho RouterCvi mt next hop l 170.10.20.1. i vi IBGP, c mt nguyn tc l: the next hop advertisedby EBGP should be carried into IBGP. Chnh v quy lut ny, nn Router A s qung co li170.10.0.0 cho cc IBGP peer ca n, trong trng hp ny l RouterB vi mt next hop l170.10.20.2. V th, lc ny RouterB s c next hop i n network 170.10.0.0 s l170.10.20.2 ch KHNG phi l 150.10.30.1Lc ny th bn phi m bo rng RouterB c th reach 170.10.20.2 thng qua IGP, nu khngRouterB s drop nhng packet c dest. l 170.10.0.0 v lc ny a ch next hop address tr nninaccessible. n y th bn hy hnh dung nu nh trong mi trng NBMA, v c thm mt

RouterD (network 180.20.0.0) c thm vo trong AS 300, m RouterA khng c PVC kt nitrc tip n RouterD th lc ny nexthop ca RouterD qung co cho RouterA m RouterA skhng reach c. Lc ny routing s fail. khc phc tnh trng ny th NEXT -HOPE-SELFc s dng.

neighbor {ip-address| peer-group-name1 } next-hop-self

Cu lnh ny s cho php p t BGP dng a ch mnh khai bo thnh next hop thay v BGP


9/14

t chn next hop.

RouterC#

router bgp 300

neighbor 170.10.20.1 remote-as 100

neighbor 170.10.20.1 next-hop-self

RouterC s qang co 180.20.0.0 vi NextHop = 170.10.20.2. Next -hop-self ch c li trong mitrng nonmeshed.

Local-Preference attribute: y l thuc tnh khng bt buc (tc l khng cn phi xut hintrong thng ip Update), ch mang ngha cc b trong phm vi AS m khng bao gi gi rakhi AS . N c truyn n cc local BGP neighbors. Thuc tnh Local-Preference chophp chng ta chn con ng mong mun nht ra khi mt AS.

Weight attribute: y cng l thuc tnh khng bt buc, cc nh cung cp khng cn phi h trcho n. Thc t thuc tnh ch c cung cp cho mt b nh tuyn n v khng gi trong btk thng ip Update no. Thuc tnh Weight c dng iu khin hng truyn cc localpacket n router khi router c hai route khc nhau ra khi AS(multihomed).

Thuc tnh MULTI_EXIT_DISC (MED)

y l thuc tnh c AS dng tham chiu cho vic nn chn router no i n cng mtch trong mt AS. Trong BGP cng c mt ATTRIBUTE c gi l metric, l MED(Multiexit Discrimination). Tuy nhin MED ny c ngha hi khc so vi metric trong ccIGP. BGP dng MED (metric) hng dn cho cc router lng ging la chn ng i ttnht. MED(multiple-exit-discriminator) l 1 attribute ca BGP, n gii thiu v nhng routerlng ging bn ngoi v 1 mng c chn vo 1 AS c nhiu exit point, ci no c MED thphn s c chn (do c l n ging vi metric), khng ging nh Local Preference, MEDc trao i gia cc AS nhng n s khng ra khi AS nu n vo (khi 1 thng tin cpnht nh tuyn vo 1 AS vi gi tr MEd xc nh th n s c s dng thc hin quytnh trong ni b AS .Nu 1 route xut pht t chnh AS th MED s c tnh theo metricca IBGP. MED l mt attribute yu (trong th t route-selection ca BGP), nn control cn cn phi care c cc thuc tnh c mc u tin cao hn (weight-localpreference-originate-ASpath-origin code). Hn na, originated AS khng th iu khin qu trnh t MED trn ccoriginating AS (neighbor AS), hoc ngc li, nn him khi c dng trong thc t. Ngoi ra,vi nhiu Customer vi ccyu cu chnh sch chn outgoing traffic khc nhau, MED khnghiu qu. Ngi ta hay s dng community attribute gii quyt vn chn incomming trafficthay cho as-path prepending hoc MED.

Communtity attribute:

y l mt thuc tnh khng bt buc. Thuc tnh ny c dng cung cp chnh sch chomt nhm cc router i qua cc AS. Do cc router ny c cng mt tnh cht v c chung


10/14

chnh sch. Nu mt router nhn c mt thng ip Thng tin cp nht nh tuyn vi thuctnh Community c thit lp,n s x l thng ip ny mt cch hp l. Nu n khng hiuthng ip ny th s gi thuc tnh cho router lng ging router x l. (Tuy nhin, nu nhiu th s khng bao gi gi thuc tnh cho lng ging router tr khi c ch nh lm thmiu g ).

Tng quan hot ng ca BGP

BGP l mt giao thc hng kt ni. Khi thit lp mt quan h neighbor, mt phin lm vicTCP s c thit lp v duy tr. BGP s gi ra cc thng ip keepalive kim tra ngtruyn v duy tr phin lm vic TCP ny. Cc thng ip keepalive ny ch l nhng header cgi tr 19-bytes. Sau cc router s gi cc cp nht nh k ch khi c thay i xy ra. Sau khi iu chnh bng BGP, qu trnh BGP s truyn n tt c cc lng ging cc thay i.

C 4 kiu thng ip c dng trong BGP:

Thng ip Open: c dng thit lp kt ni vi cc router BGP khc.

Thng ip keepalive: c gi nh k gia cc BGP peers duy tr kt nI v kim trang i. Cc thng ip keepaliveny c gi theo c ch khng tin cy. Nu khong thigian nh k c gn v bng 0, s khng c thng ip keepalive no c gi.

Thng ip cp nht: cha cc ng i v cc a ch ch v cc thuc tnh ca ng i. Cccp nht bao gm cc route khng cn tn ti. Thng tin cha trong cc cp nht bao gm ccthuc tnh v ng dn, bao gm thuc tnh origin, AS_PATH, neighbor.

Thng ip Notification: c dng thng bo cho cc router khc v nhng nguyn nhn gyra kt ni b ngt.

CIDR v Route Aggregation

BGP cn phi truyn mt s thng tin gia cc AS nhng c l khng phi tt c cc thng tin lcn thit. Nu h thng mng c thit k h tr qu trnh summarization, tng s ti nguynmng s c gim i bi v c rt t b nh, bng thng v CPU c dng gi bng nhtuyn. Tht ra, bng cch tm tt cc lp a ch IANA, mng Internet s tr thnh mt mt mitrng nhanh hn v tin cy hn. Cch dng CIDR l mt trong nhng gii php c dng


11/14

nhiu trong nhng nm gn y. Cc a ch s c hp nht li thnh mt s a ch dngsummary. Thay v mt a ch c mt gi tr netmask nhn ra phn a ch network, n s cmt gi tr prefix mask. Gi tr ny n gin l mt con s ch ra s bit dnh cho phnnetwork. Qu trnh summarization s gim thiu s prefix. Nu mt mng c cp tm a chlp C, n c th c qung b nh mt network.

BGP s truyn cc prefix v cc prefix mask. Mt router c th truyn route c tm tt, ccroute ring l cha c summarize hoc truyn c hai.

C ch hot ng ca IBGP

IBGP khng thay i thuc tnh AS-PATH ca mt tuyn do chng loop, s dng qui lutsplit-horizon: mt BGP peer s khng gi cc tuyn c hc t IBGP n mt BGP peer khc.


12/14

Nh vy, c th forward d liu cho cc AS khc, cc router ca transit AS phi thc hin:- Gii php th nht: phn phi li cc tuyn BGP vo IGP tha mn qui lut ng b, nhngphng php ny khng nn s dng trong cc mng core ca ISP do s tuyn trn Internet rtln, cc IGP c thit k khng phi c th qun l s tuyn qu ln nh vy, ngoi ra, nudng phng php ny th khi c s thay i v 1 tuyn trn Internet s lm cc IGP phi thc

hin gii thut tm ng -> chim nhiuCPU v b nh gy mt n nh.- Gii php th hai: cc BGP peer c full-mesh m bo cc router u c thng tin nhtuyn forward packet. Khi ny c th tt chc nng ng b ca IBGP. Nhng full -mesh dnn vic kh qun l, tn bng thng cp nht nh tuyn, tn CPU,mem. C 2 cch giiquyt vn ny. S dng Route Reflector: thay i qui lut split-horizon ca IBGP. Cc RR cth gi cc tuyn IBGP n cc BGP peer khc. chng lp vng, cc RR thm vo cc thuctnh: cluster-id v originator. S dng Confederation: thay i cch x l AS-PATH ca IBGP:Chia mt AS thnh nhiu member-AS. BGP gia cc BGP peer thuc cc member-AS khcnhau s l EBGP (tc l s thm member-AS vo AS-PATH chng loop) nhng cc thuctnh khc nh Local Preference, MED, Next-hop s khng thay i. u im ca gii php ny

l cc member-AS c th c qun l c lp, tng kh nng chnh sch nh tuyn routing.Cc member-AS c th s dng cc IGP khc nhau. Nhng qun l mt Confederation th c ths rt phc tp do n ch thch hp cho cc ISP cc ln (cc ISP kt ni trc tip vo ccim trung chuyn quc t nh NAP,...) hoc cc cng ty a quc gia cn c s linh ng v clp gia cc thnh phn ca h thng. la chn tuyn cho cc Incoming traffic, c th sdng cc cch sau. S dng thuc tnh MED: cch ny t s dng do MED l mt thuc tnh yuv non-transitive. S dng community: phng php ny i hi phi c upstream ISP h tr,ch nh hng n ISP trc tip, trong nhiu trng hp khng lm vic tt. S dng AS-Prepend: phng php ny c s dng thng dng hn do c th lm vic tt trong tt c ccm hnh. Nhng mt iu cn phi nh, quyt nh tuyn cho Incoming traffic rt kh thc hinchnh xc do cc AS khc c th thay i cc attribute.

set metric, bn c th dng lnh default-metric

Router(config-router)#default-metric number

hoc c th dng route-map:

router bgp 100

neighbor 1.1.1.1 route-map freddy out

!

ip as-path access-list 1 permit ^690_ip as-path access-list 2 permit .*

!

route-map freddy permit 10match as-path 1

set metric 127

!


13/14

route-map freddy permit 20

match as-path 2

trong v d trn, cc route as 690 s c set metric l 127.

Tm tt gii thut tm ng i tt nht ca BGP.

BGP s gn ng i hp l u tin nh l ng i tt nht hin c. Sau BGP s so snhng i tt nht ny vI ng i k tip trong danh sch cho n khi no hon tt tt c ccng trong danh sch cc ng i hp l. DI y l danh sch cc lut c dng xcnh ng i tt nht.

* u tin ng i c trng s WEIGHT cao nht. WEIGHT l mt thng s do Cisco a ra.Thng s ny ch c nh hng trong phm vi ni b ca router c cu hnh dng thng sny.

* u tin ng i c LOCAL_PREF cao nht. Nhng ng i khng c thuc tnhLOCAL_PREF s c gi tr mc nh l 100.

* u tin nhng ng i c ngun gc t lnh network hoc lnh aggregate hoc thng qua qutrnh redistribution t mt IGP. Cc ng i c ngun gc t lnh network hoc lnhredistribution c u tin hn nhng ng i c ngun gc t lnh aggregate.

* u tin nhng ng i c thuc tnh AS_PATH ngn nht. Bc ny s b b qua nu binbgp bestpath as-path c cu hnh. Mt tp hp AS_SET s c m t 1, bt chp c baonhiu AS bn trong tp hp. Cc bin AS_CONFED_SEQUENCE v AS_CONFED_SET skhng bao gm trong AS_PATH.

* u tin cc ng i c ngun gc thp nht. IGP th thp hp EGP v EGP th thp hnINCOMPLETE.

* u tin nhng ng i c gi tr MED nh nht. S so snh ny ch c thc hin nu ASu tin l ging nhau cho c hai ng i. Bt c mt AS con no ca confederation s b bqua. Ni cch khc, MED c so snh ch nu gi tr AS u tin trong AS_SEQUENCE lging nhau. Nu lnh bgp always-compare-med c cu hnh, MED c so snh cho tt ccc ng i c th. Nu dng chn la ny, tt c cc router trong AS phI c cu hnh vIchn la bgp always-compare-med, nu khng routing loop s xy ra. Nu thng s bgp bestpathmed-confed c cu hnh, MED s c so snh trn tt c cc ng i m ch cha binAS_CONFED_SEQUENCE ( l nhng ng i xut pht t local confederation). Nhngng i c nhn t mt router lng ging c gi tr MED l 4,294,967,295 s c gi tr MEDthay I n 4,294,967,294 trc khi c chn vo bng BGP. Nhng ng i c nhn mkhng c MED c cu hnh s c gn gi tr MED bng 0.


14/14

* u tin nhng ng i eBGP so vI iBGP. Nu ng i tt nht c chnla, qua bc 9.

* u tin nhng ng i c IGP metric thp nht n BGP next-hop.

* Kim tra xem c nhiu ng i d phng cn phi ci t vo bng routing hay khng. Nuc hai ng i l t bn ngoi, u tin ng i c nhn trc (ng i c nht). Bc nys b b qua nu

-Lnh bgp best path compare-routerid c dng

-RouterID l ging nhau cho nhiu ng i v cc routes c nhn t cng 1 router.

-Hin ti khng c mt ng i tt nht.

* u tin nhng ng i n t BGP router c routerID nh nht. Gi tr routerID l a ch IP

cao nht trn router. Gi tr ny cng c th c gn bng lnh bgp router-id. Nu mt ng icha 1 route-reflector, thuc tnh Originator ID ca Route-Reflector s thay th cho routerID.

* Nu RouterID l ging nhau cho nhiu ng i, u tin ng i c s cluster l t nht. iuny ch ng trong mi trng BGP route-reflector. N cho php nhng thnh vin trong Route-Reflector thit lp quan h peer vI cc thnh vin trong cc RR khc.

* u tin nhng ng i n t nhng lng ging c a ch thp nht. a ch ny l a chc dng trong lnh neighbor.

những kiến thức cơ bản về bgp

Documents