NEC INTELLIGENT L2 SWITCH N8406-022A Application Manual

Also see for N8406-022A: Reference guide

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NEC INTELLIGENT L2 SWITCH N8406-022A Application Manual Manual pdf 64 page image

IGMP Snooping 64IGMP SnoopingIntroductionIGMP Snooping allows the switch to forward multicast traffic only to those ports that request it. IGMP Snoopingprevents multicast traffic from being flooded to all data ports. The switch learns which server hosts are interested inreceiving multicast traffic, and forwards it only to ports connected to those servers.The following topics are discussed in this chapter:• Overview• FastLeave• IGMP Filtering• Static Multicast Router• IGMP Snooping Configuration exampleOverviewInternet Group Management Protocol (IGMP) is used by IP Multicast routers to learn about the existence of hostgroup members on their directly attached subnet (see RFC 2236). The IP Multicast routers get this information bybroadcasting IGMP Query Reports and listening for IP hosts reporting their host group memberships. This processis used to set up a client/server relationship between an IP Multicast source that provides the data streams and theclients that want to receive the data.IGMP Snooping conserves bandwidth. With IGMP Snooping, the switch learns which ports are interested inreceiving multicast data, and forwards multicast data only to those ports. In this way, other ports are not burdenedwith unwanted multicast traffic.The switch currently supports snooping for IGMP version 1 and version 2.The switch can sense IGMP Membership Reports from attached host servers and act as a proxy to set up adedicated path between the requesting host and a local IP Multicast router. After the pathway is established, theswitch blocks the IP Multicast stream from flowing through any port that does not connect to a host member, thusconserving bandwidth.The client-server path is set up as follows:• An IP Multicast Router (Mrouter) sends Membership Queries to the switch, which forwards them to all ports ina given VLAN.• Hosts that want to receive the multicast data stream send Membership Reports to the switch, which sends aproxy Membership Report to the Mrouter.• The switch sets up a path between the Mrouter and the host, and blocks all other ports from receiving themulticast.• Periodically, the Mrouter sends Membership Queries to ensure that the host wants to continue receiving themulticast. If the host fails to respond with a Membership Report, the Mrouter stops sending the multicast tothat path.• The host can send a Leave report to the switch, which sends a proxy Leave report to the Mrouter. Themulticast path is terminated immediately.FastLeaveWhen the switch with IGMP Snooping enabled receives an IGMPv2 leave message, it sends a Group-SpecificQuery to determine if any other devices in the same group (and on the same port) are still interested in thespecified multicast group traffic. The switch removes the affiliated port from that particular group, if the followingconditions apply:• If the switch does not receive an IGMP Membership Report message within the query-response-interval• If no multicast routers have been learned on that port.With Fastleave enabled on the VLAN, a port can be removed immediately from the port list of the group entry whenthe IGMP Leave message is received, unless a multicast router was learned on the port.Enable FastLeave only on VLANs that have only one host connected to each physical port.