Enterasys D-Series - Chapter 13: IGMP Configuration; IGMP Overview

Enterasys D-Series CLI Reference 13-1
13
IGMP Configuration
ThischapterdescribestheIGMPConfigurationsetofcommandsandhowtousethem.
IGMP Overview
About IP Multicast Group Management
TheInternetGroupManagementProtocol(IGMP)runsbetweenhostsandtheirimmediately
neighboringmulticastdevice.Theprotocol’smechanismsallowahosttoinformitslocaldevice
thatitwantstoreceivetransmissionsaddressedtoaspecificmulticastgroup.
Amulticast‐enableddevicecanperiodicallyaskitshostsiftheywantto
receivemulticasttraffic.If
thereismorethanonedeviceontheLANperformingIPmulticasting,oneofthesedevicesis
elected“querier”andassumestheresponsibilityofqueryingtheLANforgroupmembers.
BasedonthegroupmembershipinformationlearnedfromIGMP,adevicecandeterminewhich(if
any)multicast
trafficneedstobeforwardedtoeachofitsports.AtLayer‐3,multicastdevicesuse
thisinformation,alongwithamulticastroutingprotocol,tosupportIPmulticastingacrossanIP
network.
IGMPprovidesthefinalstepinanIPmulticastpacketdeliveryservice,sinceitisonlyconcerned
withforwarding
multicasttrafficfromthelocaldevicetogroupmembersonadirectlyattached
subnetworkorLANsegment.
ThisdevicesupportsIPmulticastgroupmanagementbypassivelysnoopingontheIGMPquery
andIGMPreportpacketstransferredbetweenIPmulticastdevicesandIPmulticasthostgroupsto
learnIPmulticastgrou pmembers.
The
purposeofIPmulticastgroupmanagementistooptimizeaswitchednetwork’sperformance
somulticastpacketswillonlybeforwardedtothoseportscontainingmulticastgrouphostsor
multicastdevicesinsteadoffloodingtoallportsinthesubnet(VLAN).
About Multicasting
Multicastingisusedtosupportreal‐timeapplicationssuchasvideoconferencesorstreaming
audio.Amulticastserverdoesnothavetoestablishaseparateconnectionwitheachclient.It
merelybroadcastsitsservicetothenetwork,andanyhoststhatwanttoreceivethemulticast
registerwiththeirlocalmulticast
switch/router.Althoughthisapproachreducesthenetwork
overheadrequiredbyamulticastserver,thebroadcasttrafficmustbecarefullyprunedatevery
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IGMP Overview 13-1
Configuring IGMP at Layer 2 13-2