D-Link DWS-4026 [208/741] Configuring igmp snooping

ConfiguringIGMPSnooping

D-Link UnifiedWiredandWirelessAccessSystem

November2011 Page208

D‐LinkUWSUserManual

ConfiguringIGMPSnooping

InternetGroupManagementProtocol(IGMP)Snooping is afeaturethatallowsaswitchtoforwardmulticast

trafficintelligentlyontheswitch.MulticastIPtrafficistrafficthatisdestinedtoahostgroup.Hostgr oupsare

identifiedbyclassDIPaddre sses,whichrangefrom224.0.0.0to239.255.255.255.BasedontheIG

MPquery

andreportmessages,theswitchforwardstrafficonlytotheportsthatrequestthemulticasttraffic.This

preventstheswitchfrombroadcastingthetraffictoallportsandpossiblyaffectingnetworkperformance.

AtraditionalEthernetnetworkmaybeseparatedintodifferentnetworksegmentstoprev entplacingtoomany

deviceson

tothesamesharedmedia.Bridgesandswitchesconnectthesesegments.Whenapacketwitha

broadcastormulticastdestinationaddressisreceived,theswitchwill forwardacopyintoeachoftheremaining

networksegmentsinaccordancewiththeIEEEMACBridgestandard.Eventually,thepacketismadeaccessible

toallnode

sconnectedtothenetwork.

Thisapproachworkswellforbroadcastpacketsthatareintendedtobeseenorprocessedbyallconnected

nodes.Inthecaseofmulticastpackets,however,thisapproachcouldleadtolessefficientuseofnetwork

bandwidth,particularlywhenthepacketisintendedforonlyasmallnu

mberofnodes.Packetswillbeflooded

intonetworksegmentswherenonodehasanyinterestinreceivingthepacket.Whilenodeswillrarelyincur

anyprocessingoverheadtofilterpacketsaddressedtoun‐requestedgroupaddresses,theyareunableto

transmitnewpacketsontothesharedmediafo

rtheperiodoftimethatthemulticastpack etisflooded.The

problemofwastingbandwidthisevenworsewhentheLANsegmentisnotshared,forexampleinFullDuplex

links.

AllowingswitchestosnoopIGMPpacketsisacreativeefforttosolvethisproblem.Theswitchusesthe

informationintheIGMPpa

cketsastheyarebeingforwardedthroughoutthenetworktodeterminewhich

segmentsshouldreceivepacketsdirectedtothegroupaddress.