Specifications

Understanding Multicast
April 16, 2009 Page 7 of 32
Route Table
EachDVMRP‐enableddevicebuildsaDVMRProutetabletomaintainroutestoallnetworks
involvedinDVMRProuting.Asshowninthefollowingexample,theDVMRProutetable contains
asourcenetwork,hopcount,routeuptime,neighborexpirationtime,associatedinterface,and
associatedIPaddress.
matrix(router-config)# show ip dvmrp route
6.0.0.0/8, [70/2], uptime 00:00:29, expires 00:01:51
via ge.2.1, 1.1.1.1
Inthisexample,network6.0.0.0/8isrunningDVMRPandis2hopsaway,learnedfrominterface
ge.2.1,whichhastheIPaddress1.1.1.1.
Route Reports
DVMRP‐enableddevicessendroutereportpacketstoadjacentDVMRPdevicesevery60seconds.
WhenaDVMRPdevicereceivesone,itcheckstoverifythatthereportisfromaknownneighbor
beforeprocessing.
Thefirsttimeadeviceseesitsownaddressinaneighbor’sprobepacket,itsendsa
unicastcopyof
itsentireroutingtabletotheneighbortoreducestart‐uptime.
Theroutereportpacketcontainsdataaboutallnetworks/routesofwhichthesendingdeviceis
aware.Thisinformationisusedtodeterminethereversepathbacktoaparticularmulticast
source.EveryDVMR Pdevicekeepsa
separatemetricassociatedwitheachroute.Thismetricis
thesumofallinterfacemetricsbetweenthedeviceoriginatingthereportandthesourcenetwork.
DVMRPdevicesacceptroutereportsforaggregatedsourcenetworksinaccordancewithclassless
inter‐domaindevices(CIDR).Thismeansthat,ifapruneorgraft
isreceivedonadownstream
interfaceforwhichthesourcenetworkisaggregated,thenapruneorgraftshouldbesent
upstream(tothemulticastsource).
IfaDVMRPdevicehasalargenumberofDVMRProutes,itwillspreadroutereportsacrossthe
routeupdateinterval(60seconds)toavoid
bottlenecksinprocessingandroutesynchronization
issues.
Forthepurposeofpruning,DVMRPneedstoknowwhichdowns tream routesdependonthe
deviceforreceivingmulticaststreams.Usingpoisonreverse,theupstreamroutermaintainsa
tableofthesourcenetworkandalldownstreamdevicesthataredependentontheupstream
device.
Mroute Table
DVMRP‐enableddevicesusethemroutetabletomaintainasource‐specificforwardingtree.
WhenaDVMRPdeviceisinitialized,itassumestheroleofthedesignatedforwarderforallofits
locallyattachednetworks.Beforeforwardinganypackets,alldevicesuseIGMPtolearnwhich
networkswouldliketoreceive
particularmulticastgroupstreams.Inthecaseofashared
network,thedevicewithalowerinterfacemetric(aconfigurablevalue),orthelowerIPaddress
willbecomethedesignatedforwarder.
ADVMRPdeviceforwardsmulticastpacketsfirstbydeterminingtheupstreaminterface,and
thenbybuildingthedownstreaminterfacelist.
Ifadownstreamrouterhasnohostsforamulticast
stream,itsendsaprunemessagetotheupstreamrouter.Iftheupstreamrouter’soutboundlistis
nowempty,itmaysendaprunemessagetoitsupstreamrouter.
Ifadownstreamdevicehasprunedamulticastgroupthatahost
wouldliketonowreceive,the
downstreamdevicemustsendaDVMRPgraftmessagetoitsupstreamdevice.TheDVMRPgraft
willtraversethesource‐specificmulticastdeliverytreetothedevicethatisreceivingthisstream.