Specifications

Understanding Multicast
April 16, 2009 Page 8 of 32
Asshowninthefollowingexample,theMroutetabledisplaystheincominginterfaceIPaddress,
themulticastgroupaddress,theuptimeofthestream,incominginterfaceportnumber,andthe
outgoinginterfaceportnumber.
matrix(router-config)# show ip mroute
Multicast Routing Table
(6.6.6.6, 235.1.1.1), uptime: 00:00:38
Incoming interface: ge.2.1
Outgoing interface list:
ge.2.7
(6.6.6.6, 235.1.1.2), uptime: 00:00:37
Incoming interface: ge.2.1
Outgoing interface list:
ge.2.7
Inthisexample,thedeviceisreceivingmulticaststreamsforgroups235.1.1.1and235.1.1.2fromIP
address6.6.6.6onportge.2.1.Thisdeviceisforwardingmulticaststreamstothe235.1.1.1and
235.1.1.2groupsonportge.2.7aspartoftheoutgoinginterfacelist.
Prune Messages
IfadevicereceivesadatagramthathasnoIGMPgroupmemberspresent,andallthedowns tre am
networksareleafnetworks,thedevicesendsaprunepacketupstreamtothesourcetree.
Whensendingapruneupstream,thedevice:
1. Decidesiftheupstreamneighboriscapableofreceivingprunes.
•Ifitis
not,thenthesendingdeviceproceedsnofurther.
•Ifitis,thenthesendingdeviceproceedsasfollows.
2. Stopsanypendinggraftsawaitingacknowledgments.
3. Determinestheprunelifetime.
Thisvalueshouldbetheminimumofthedefaultprunelifetime(randomizedtoprevent
synchronization)andtheremainingprunelifetimesofthedownstreamneighbors.
4. Forms
andtransmitsthepackettotheupstreamneighborforthesource.
Toensurethepruneisaccepted,theDVMRP‐enableddevicesetsanegativecachepruneentryfor
threeseconds.Ifthetraffichasnotstoppedafterthreeseconds,thedevicesendsanotherprune
anddoublesthecacheentry.Thismethod
iscalledexponentialback‐off.Themoreprunesthatare
dropped,thelongertheback‐offbecomes.
Aftertheprunelifetimeexpires(twohours),theprunetransmissionprocessisrepeated.
Whenreceivingaprune,theupstreamdevice:
1. Decidesifthesendingneighborisknown.
•Iftheneighborisunknown,itdiscardsthe
receivedprune.
•Iftheneighborisknown,thereceivingdeviceproceedsasfollows.
2. Ensurestheprunemessagecontainsatleastthe correctamountofdata.
3. Copiesthesourceaddress,groupaddress,andprunetime‐outvalue,and,ifitisavailablein
thepacket,thenetmaskvaluetodeterminetherouteto
whichthepruneapplies.