Enterasys SecureStack C2 - Chapter 11: IGMP Configuration; IGMP Overview

SecureStack C2 Configuration Guide 11-1
11
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the
Internet.
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group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).
InadditiontopassivelymonitoringIGMPqueryandreportmessages, theSecureStack
C2canalso
activelysendIGMPquerymessagestolearnlocationsofmulticastdevicesandmemberhostsin
multicastgroupswithineachVLAN.
However,notethatIGM P neitheraltersnorroutesanyIPmulticastpackets.SinceIGMPisnot
concernedwiththedeliveryofIPmulticastpacketsacrosssubnetworks,anexternal
IPmulticast
deviceisneededifIPmulticastpacketshavetoberoutedacrossdifferentsubnetworks.
For information about... Refer to page...
IGMP Overview 11-1
Configuring IGMP at Layer 2 11-2
Configuring IGMP on Routing Interfaces 11-9