chuong7-multicast

Gio trnh kha hc BSCI Chng 7 - Multicast

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CHNG 7: MULTICAST

7.1. Tng quan v multicast

C ba kiu truyn IP traffic trn router v switch:

- Unicast: Cc gi tin c gi t mt a ch ngun n mt a ch ch. Mt router

hoc mt thit b lp 3 s chuyn cc gi tin bng cch tm a ch ch trong bng

nh tuyn. Nu mt thit b l L2, n ch cn da vo a ch MAC.

- Broadcast: Cc gi tin c gi t mt my ngun n mt a ch ch broadcast.

a ch ch c th l a ch tt c cc hosts (255.255.255.255) hoc l mt phn ca

a ch subnet. Mt router hoc mt L3 switch s khng cho php chuyn cc d liu

broadcast ny. Mt thit b L2 s cho php pht tn broadcast traffic ra tt c cc cng

ca n.

- Multicast: Cc gi c gi t mt a ch ngun n mt nhm cc my tnh. a

ch ch tng trng bng cc hosts mun nhn traffic ny. Mc nh, mt router hoc

mt L3 switch s khng chuyn cc gi tin ny tr khi phI cu hnh multicast routing.

Mt thit b L2 switch khng th nhn bit c v tr ca a ch multicast ch. Tt

c cc gi s c pht tn ra tt c cc port ch mc nh.

C hai thi cc c m t y. C ch dng unicast th d liu s i t host n host;

broadcast th traffic s i n tt c cc host trn phn on mng . C ch multicast s nm

gia hai thi cc ny, trong my ngun ch gi nhng gi tin t mt host n cc ngi

dng mun nhn loi traffic . Nhm ny gi l nhm multicast. Cc my nhn multicast

traffic c th nm bt c ni no ch khng ch trn phn on mng cc b.

Cc traffic dng multicast thng l mt chiu (unidirectional). Do c nhiu host nhn

cng mt d liu, nn thng thng cc gi tin khng c php gi ngc v my ngun

trn c ch multicast. Mt host ch s tr traffic ngc v ngun theo c ch unicast. C ch

multicast cng s c truyn theo kiu phi-kt-ni (connectionless). Multicast dng UDP

ch khng dng TCP.

Cc host mun nhn d liu t mt ngun multicast c th tham gia hoc ri khi mt

nhm multicast bt k thi im no. Hn na, mt host s quyn nh c tr thnh thnh

vin ca mt hay nhiu nhm multicast hay khng . Nguyn tc cn quan tm l s hoch


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nh lm th no phn phi cc multicast traffic n cc thnh vin ca nhm m khng

nh hng n cc thnh vin ngoi nhm.

7.1.1. a ch multicast

Cc router v switch phI c phng thc phn bit traffic dng multicast vI dng

unicast hay broadcast. iu ny thc hin thng qua vic gn a ch IP, bng cch dng a

ch lp D t 224.0.0.0 n 239.255.255.255 ch cho multicast. Cc thit b mng c th nhanh

chng lc ra cc a ch multicast bng cch c 4 bit bn tri ca mt a ch. Bn bit ny

ca mt a ch multicast lun lun bng 1110.

Lm th no m mt router v switch kt hp mt a ch multicast ca IP vi mt a ch

MAC. Do khng c c ch tng ng vi c ch ARP, mt dng gi tr c bit dnh ring

cho a ch MAC ca multicast s c dng. Cc a ch ny bt u bng 0100.5e. Phn 28

bit sau ca a ch multicast Ip s c nh x vo 23 bit thp ca a ch MAC bng mt gii

thut n gin.

Hnh trn cho thy c ch nh x a ch. Ch c 23 bit cui ca a ch l c chp t a

ch IP sang a ch MAC.

Tuy nhin ch rng c 5 bit ca a ch IP khng c chuyn sang a ch MAC. Kh

nng ny lm cho ny sinh mt vn l c th c 32 a ch MAC khc nhau c th nh x

vo cng mt a ch MAC. Do s nhp nhng ny, mt host multicast c mt vn nh khi

n nhn mt Ethernet frame ca mt a ch multicast. Mt MAC c th tng ng vi 32 a

ch multicast khc nhau. V vy, khi mt host phi nhn v kim tra tt c cc frame c MAC

m n quan tm. Sau host ny phi kim tra phn a ch IP bn trong mi frame nhn

ra phn a ch ca tng nhm multicast.

Mt vi khng gian a ch c dnh ring

- Ton b khng gian a ch multicast:224.0.0.0-239.255.255.255


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- a ch link-local: 224.0.0.0 - 224.0.0.255 c dng bi cc giao thc nh tuyn.

Router s khng chuyn cc gi tin c a ch ny. Cc a ch bao gm a ch tt c

cc host all-hosts 224.0.0.1, tt c cc router 224.0.0.2, tt c cc OSPF routers

224.0.0.5y l a ch cc nhm c nh v cc a ch ny c nh ngha trc.

- Tm a ch dnh cho qun tr (239.0.0.0-239.255.255.255) c dng trong cc vng

multicast ring, ging nh dy a ch dnh ring trong RFC1918. a ch ny khng

c route gia cc domain nn n c th c dng li nhiu ln.

- a ch ton cc (224.0.1.0-238.255.255.255) c dng bi bt c i tng no.

Cc a ch ny c th c route trn Internet, v vy a ch ny phi duy nht.

7.1.2. nh tuyn cho traffic multicast

Cc traffic IP phi c nh tuyn ging nh bt c mt gi tin L3 no. S khc nhau l

im cn phi bit chuyn gi tin v u. Cc gi tin L3 dng unicast ch c mt cng ra

duy nht trn router (ngay c khi c qu trnh load-balancing xy ra), trong khi multicast

traffic c th c chuyn mch ra nhiu cng, tu thuc vo cc my nhn nm u.

Mt vi giao thc nh tuyn multicast hin c. Bi vit ny tp trung vo PIM.

u tin, ta phi cho php nh tuyn multicast bng lnh:

Switch(config)# ip multicast-routing

Cy Multicast

Cc router hoc cc multilayer switch trong mt mng phi xc nh mt tuyn ng

phn phi cc gi tin multicast t my ngun n tng my nhn. Khi , ton b mng ging

nh mt cu trc cy, trong gc ca cy l ngun ca lung traffic . Mi router dc theo

ng i s l mt nhnh r ca cy. Nu mt router bit tt c cc a ch multicast, router

cng phi bit cn phi nhn bn lung multicast ra nhng nhnh no ca cy. Mt vi

router khng c cc my nhn trong cc phn on mng ca n th cc router s khng

chuyn traffic. Cc router khc s c th c cc my nhn multicast traffic.

Cu trc cy ny tng t nh cu trc cy Spanning Tree v n c mt root v cc l.

Cu trc cy ny cng m bo l khng b vng lp sao cho traffic multicast khng b

chuyn ngc v cy.

Reverse Path Forwarding

Cc router thng phi thc hin mt php kim tra trn tt c cc gi multicast m n

nhn. Reverse Path Forwarding (RPF) l mt cng c m bo rng cc gi tin khng b

a ngc tr v cy multicast mt v tr bt k no . Khi mt gi tin c nhn trn mt


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cng ca router, v d cng E0 ca router, a ch ngun ca gi s c kim tra. Sau

router s so snh a ch ngun ny vi mt entry trong bng nh tuyn unicast. Nu ct out-

going interface ca bng nh tuyn cng ng bng cng nhn gi multicast (tc E0 trong v

d ny), gi multicast s c x l v chuyn ra cc nhnh ca cy. Nu cng l khng so

trng, iu ny c ngha l c mt ai a gi vo mt v tr khng mong i, chuyn gi

tin ngc v root. Gi tin lc ny s b loi b. thc hin php kim tra RPF ny, router

chy giao thc PIM phi tm kim a ch ngun trong bng nh tuyn unicast.

7.2. IGMP

Lm th no mt router bit c cc my cn nghe multicast traffic? nhn multicast

traffic t mt ngun, c ngun v cc my nhn u tin phi gia nhp (join) vo mt nhm

multicast. Nhm ny c xc nh thng qua a ch multicast. Mt host c th tham gia vo

mt nhm multicast bng cch gi cc yu cu n router gn nht. Tc v ny c thc

hin thng qua giao thc IGMP. IGMPv1 c nh ngha trong RFC1112 v bn ci tin ca

n, IGMPv2 c nh ngha trong RFC2236. Khi c vi host mun tham gia vo nhm, giao

thc PIM s thng bo cho nhau gia cc router v hnh thnh nn cy multicast gia cc

routers.

7.2.1. IGMPv1

tham gia vo mt nhm multicast, mt host s gi mt thng ip ng k tham gia

vo nhm n router cc b ca n. Thng ip ny c tn l Membership Report IGMP.

Thng ip ny s thng bo cho router v a ch multicast m host mun tham gia vo. a

ch multicast 224.0.0.1 all-hosts c dng nh a ch ch. Trong thng ip ny c cha

a ch nhm multicast. C mi 60 giy, mt router trn mi phn on mng s gi truy vn

n tt c cc host kim tra xem cc host ny c cn quan tm nhn multicast traffic na

khng? Router ny gi l IGMPv1 Querier v chc nng ca n l mi cc host tham gia vo

nhm. Nu mt host mun tham gia vo mt nhm, hoc n mun tip tc nhn traffic t mt

nhm m n tham gia, n phi tr li li bng thng ip membership-report.

Cc host c th tham gia vo cc nhm multicast bt k thi im no. Tuy nhin

IGMPv1 khng c c ch cho php mt host ri khi mt nhm nu host khng cn

quan tm n ni dung ca nhm multicast . Thay vo , router s kt lun l mt cng

giao tip ca b khng cn thuc v mt nhm multicast no nu router khng nhn c

membership-report trong ba chu k truy vn lin tip. iu ny c ngha l, ch mc


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nh, cc multicast traffic vn gi vo mt phn on mng trong ba chu k truy vn lin tip

sau khi tt c cc thnh vin ca nhm khng cn lng nghe multicast traffic na.

Ngoi ra, router khng c gi mt danh sch y cc host thnh vin cho tng nhm

multicast. Thay vo , n cn phi lu nhng nhm multicast no l ang tn ti trn nhng

cng no ca n.

7.2.2. IGMPv2

Phin bn IGMPv2 gii thiu vi s khc bit so vi phin bn u tin. Cc gi tin truy

vn by gi c gi l General Queries. Cc gi ny c th gi ti a ch all-hosts hoc ti

tng nhm c th. Mt ci tin khc na l cc host c php ri khi nhm. Khi mt host

quyt nh ri khi mt nhm n tham gia, n s gi thng ip LeaveGroup n a ch

all-router 224.0.0.2. Tt c cc router trn mt phn on mng ni b s lu thng ip ny

v router truy vn s tip tc qu trnh. Router s tr li thng ip trn bng thng ip truy

cp gi theo nhm. Thng ip ny s hi rng c cn host no mun nhn traffic cho nhm

na khng? Bt c host no cng phi tr li li bng thng ip membership report. Nu

khc i, router s kt lun mt cch an ton l khng cn thit chuyn traffic cho nhm

trn phn on mng .

7.3. Giao thc PIM

Protocol Independent Multicast (PIM) l mt giao thc nh tuyn c th c dng

chuyn cc multicast traffic. PIM hot ng c lp vI cc giao thc nh tuyn IP v vy

PIM s dng bng nh tuyn IIP v khng gi cc bng multicast routing. Cn ch l bng

unicast routing cng khng ph thuc vo cc giao thc nh tuyn v nhiu giao thc nh

tuyn c th ng gp vo cng mt bng nh tuyn. PIM c th hot ng hai ch :

- PIM Dense Mode

- PIM Sparse Mode

- PIM Sparse Dense Mode (do Cisco a ra)

- PIM Dense Mode

Cc PIM router c th c cu hnh theo kiu Dense Mode (cn gi l PIM-DM) nu cc

host tham gia vo multicast group nm khp ni trn tt c cc subnet. a ch multicast

ngun tr thnh gc ca cy v cy multicast c xy dng t ngun n ch. C ch ny

cn c gi bng k hiu (S,G) trong ng i t ngun n cc thnh vin trong nhm

l duy nht v c xc nh.


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Cy multicast c xy dng bng cch cho php pht tn cc traffic t ngun n tt c

cc router trong mng. Cy s pht trin t trn xung di. Trong mt thI gian ngn, cc

traffic khng cn thit s c lu chuyn ging nh trong broadcast traffic. Tuy nhin khi

cc router nhn c traffic cho mt nhm, router s quyt nh n c cc my nhn mun

nhn d liu hay khng? Nu l mun, router s duy tr tnh trng im lng v dng traffic

tip tc. Nu khng c host no ng k cho nhm multicast (thng qua IGMP), router s

gi thng ip Prune n cc router lng ging ca n (theo hng v gc ca cy. Nhnh ca

cy sau s b loi b (prune) sao cho cc traffic khng cn thit s khng c pht tn v

hng .

Cy multicast s c xy dng theo mt ln sng ca cc yu cu tham gia vo nhm.

Sau cc switch khng c cc host tham gia s b xa ra khi cy. Cy kt qu sau cng

c hin th hnh k tip.


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PIM-DM s nhn bit cc thit b lng ging bng cch trao i cc gi hello. Thng tin

lng ging ny c dng trc xy dng cy n tt c cc lng ging. Sau , cc nhnh

ca cy s ln lt c loi b. Nu mt dng multicast bt u, cy s c xy dng, cy

s ch tn ti khi cc thnh vin tch cc cn tn ti. Nu mt host mi ng k tham gia

nhm, nhnh ca phn on mng s c nh thm vo cy.

cu hnh PIM Dense Mode trn mt cng, dng lnh:

Switch(config-if)# ip pim dense-mode

7.3.1. PIM Sparse Mode

PIM Sparse Mode (PIM-SM) dng mt gii php khc. Cy multicast khng m rng n

router cho n khi no mt host tham gia vo mt nhm. Cy multicast c xy dng

bng cc thnh vin cc node l v m rng ngc v root. Cy c xy dng t di ln.

SM cng hot ng da trn tng cu trc shared-tree, trong gc ca cy khng nht

thit l ngun ca multicast. Thay vo , root l router PIM-SM thng c t trung tm

ca mng. Router lm gc ny gi l Rendezvous Point (RP). Cy t im RP n cc thnh

vin tht ra l mt cy con ca cy t ngun n cc thnh vin. Nu mt router bt k u

trong mng c th ng k vi RP, cu trc cy ny s hon tt. Ch spare-mode cn c

gi l Shared tree. Cc dng multicast c m t nh (*,G) bi v cy lun cho php bt c

ngun no gi n mt nhm.


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Khi mt host tham gia vo mt nhm multicast dng IGMP, router cc b s chuyn cc

thng ip Membership report v gc ca cy multicast. Mi router dc theo ng i s

thm nhnh vo cy dng chung shared-tree. Qu trnh loi b nhanh ch thc hin khi mt

thnh vin ca nhm b xa ra khi mt nhm. Qu trnh ny c hin th hnh di y:

Ch l qu trnh ny ch bao gm 1 bc. Cc router khng tham gia vo nhm s khng

b loi b v n khng bao gi l mt thnh phn ca cy.

cu hnh PIM Sparse Mode trn mt cng, dng lnh sau:

Switch(config-if)# ip pim sparse-mode

7.3.2. PIM Sparse-Dense Mode

PIM c kh nng h tr c hai ch Dense v Sparse Mode bi v c hai tn ti trn

nhng nhm multicast khc nhau trn mt mng. Cisco cho php ch lai sparse-dense

mode cho php mt PIM router dng ch dense hay ch sparse ty thuc vo tng

nhm. Nu mt nhm c RP c nh ngha, Sparse-mode s c dng, nu khng c,

dense-mode s c dng. cu hnh mt router chy sparse-dense mode trn mt cng

giao tip, dng lnh:

Switch(config-if)# ip pim sparse-dense-mode


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7.3.3. PIM Version 1

i vi cc router chy PIMv1, cc router RP c th c cu hnh bng tay hoc theo c

ch t ng. ch nh mt router l RP, dng lnh sau:

Switch(config)# ip pim rp-address ip-address [ access-list-number]

[override]

Ta c th gii hn phm vi cc nhm multicast c h tr bi RP bng cch dng mt

access-list. T kha override cho php RP c u tin hn bt c mt RP no c bu chn

bi qu trnh t ng. RP phi c nh ngha trn tt c cc vng mng chy PIM, k c

trn router RP. Cisco cng cung cp mt phng thc t ng thng bo v PIM-SM cho

mt nhm. Phng thc ny gi l Auto-RP. Qu trnh ny c thc hin bng cch ch ra

mt router nm trung tm v cc router kt ni vo n gi l mapping agent. Cc mapping

agent s hc thng tin ca tt c cc ng c vin cho RP. Cc router mun lm ng c vin

phI gI ra mt thng ip Cisco-RP-Announce v a ch multicast 224.0.1.39. nh

ngha router nh mt mapping agent, dng lnh:

Switch(config)# ip pim send-rp-discovery scope ttl

Router mapping agent s gi cc thng tin nh x t RP-nhm n tt c cc PIM router

khc dng thng ip Cisco-RP-Discovery v a ch 224.0.1.40. Gi tr Time-to-live c

thit lp trong nhng thng ip ny hn ch tm vc ca thng p ny. Thng s ny s ch

ra l thng tin s cn hp l trong bao nhiu hop na. Ngi qun tr phi nh ngha tng

minh cc ng c vin cho RP. Khi mt router c xem nh l RP, n s bt u gi cc

thng ip n router mapping agent. Hy cu hnh router nh RP bng lnh:

Switch(config)# ip pim send-rp-announce type mod/num scope ttl group-list

access-listnumber

Cc cng giao tip s qung b a ch router RP. Phm vi ca thng p qung b ny s

b gii hn bi thng s TTL. Router cng s qung b chnh n nh l mt candidate RP cho

nhm c nh ngha trong access-list.

7.4. Switching Multicast Traffic

Cc router hoc cc MLS switch c th xy dng cc cy multicast v chuyn cc gi tin

i mt cch hiu qu. Tuy nhin lp 2, mt switch ch kim tra phn header ca frame

Ethernet tm a ch ngun v a ch ch. Cc switch ny khng th hot ng theo yu

cu ging nh router. Thng tin tt nht m mt switch bit c l a ch multicast ch v

khi frame cn phi c pht tn ra tt c cc cng ca vlan. C hai phng thc c


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pht trin cho php cc switch chuyn cc gi tin multicast mt cch thng minh: dng

IGMP snooping v dng CGMP. Mt phng thc i hi phn cng mnh, phng thc kia

th hc hi thng tin t cc router lng ging.

7.4.1. IGMP Snooping

Trong ch hot ng bnh thng, mt host mun tham gia vo mt nhm multicast

phi lin lc vi mt router gateway router a host vo nhm multicast. IGMP

snooping cho php mt switch lng nghe cc thng ip IGMP membership report ny sao

cho n c th tm ra host no ang yu cu nhm no. tham gia vo mt nhm, mt host

phi gi cc thng ip multicast membership report v chnh a ch multicast ca nhm .

Mt switch L2 phi lng nghe n tt c cc multicast frame tm ra thng tin IGMP. y r

rng l mt gnh nng cho CPU ca switch. Mt thit b L3 switch th c li th r rng hn,

n c th tch ra thng tin L3 trong mt frame. Kiu switch ny phi lng nghe mi gi

IGMP. Khi mt thng p membership report c lng nghe, switch s thm a ch MAC

ca nhm multicast vo bng CAM ca n cng vi port ngun ni m mt gi IGMP c

nhn. Tc v ny s lin kt a ch nhm vi cc host yu cu tham gia nhm. Khi cc

host khc cng yu cu tham gia nhm, cc switchport tng ng s c thm vo bng

CAM. Khi c mt frame cn n mt a ch multicast, n c th c nhn bn ra chnh xc

cc cng ca cc host nhn.

Vi IGMP snooping, c hai trng hp c bit ca thnh vin nhm trong bng CAM:

- Tt c cc a ch IGMP l nhn bit bi switch (hc ng) cng s c lu trong

bng CAM. Cc frame multicast phi c nhn bn v pha cc router cc traffic

ny c th c route nu cn thit.

- CPU ca switch cng l mt thnh vin ca nhm multicast v n c th xem cc gi

IGMP n v i. Ch c traffic ca IGMP l c x l. CPU s khng kim tra cc

frame multicast khc. IGMP snooping c cho php trn tt c cc cng ca switch

v cc interface vlan. Cc switch 2950,3550,4500 v 6500 l c h tr IGMP

snooping. cho php hoc tt IGMP, dng lnh:

Switch(config)# [no] ip igmp snooping

7.4.2. CGMP

Khi mt L2 switch khng th thc hin tc v IGMP snooping, mt router multicast lng

ging s tr gip. Cisco pht trin ra mt giao thc l Cisco Group Membership Protocol

(CGMP) nhm mc ch ny.


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Mt router hoc mt multilater switch c cu hnh cho nh tuyn multicast c th c

cu hnh cho CGMP. Khi cc host gi cc thng ip membership report tham gia v ri

khi mt nhm, Router CGMP s trung chuyn cc thng ip ny n cc switch quan tm.

Cc thng ip CGMP dng cc a ch multicast ni ting l 0100.0cdd.dddd. Theo nh

ngha, a ch nhm multicast ny s c pht tn nh l mt trng hp c bit sao cho

cc thng ip CGMP c th c truyn trn cc non-CGMP switch. Cc thng ip CGMP

bao gm a ch MAC ca host cng vi a ch MAC ca nhm multicast n mun tham gia

hay ri b. Khi mt layer 2 switch nhn thng ip CGMP ny, tc v n gin ca n l

thm vo a ch nhm multicast v cc host ca nhm vo bng CAM. Router s tr thnh

thit b tr thnh cho mt switch trong vic lng nghe cc thng ip CGMP.

ch mc nh, CGMP l tt trn tt c cc cng ca multicast router. cho php,

dng lnh:

Switch(config-if)# ip cgmp

Ch c router multicast phI c cu hnh cho CGMP. tt c cc IOS-based L2 switch

u c CGMP cho php ch mc nh, v vy switch s t ng x l CGMP t routers.

MULTICASTTng quan v multicasta ch multicastnh tuyn cho traffic multicast

IGMPIGMPv1IGMPv2

Giao thc PIMPIM Sparse ModePIM Sparse-Dense ModePIM Version 1

Switching Multicast TrafficIGMP SnoopingCGMP

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