Using the BayStack 450 10/100/1000 Series Switch1-48 302401-D Rev 00IGMP SnoopingBayStack 450 switches can sense IGMP host membership reports from attachedstations and use this information to set up a dedicated path between the requestingstation and a local IP Multicast router. After the pathway is established, theBayStack 450 switch blocks the IP Multicast stream from exiting any other portthat does not connect to another host member, thus conserving bandwidth. Thefollowing describes how BayStack 450 switches provide the same benefit as IPMulticast routers, but in the local area:Internet Group Management Protocol (IGMP) is used by IP Multicast routers tolearn about the existence of host group members on their directly attached subnets(see RFC 2236). The IP Multicast routers get this information by broadcastingIGMP queries and listening for IP hosts reporting their host group memberships.This process is used to set up a client/server relationship between an IP Multicastsource that provides the data streams and the clients that want to receive the data.Figure 1-31 shows how IGMP is used to set up the path between the client andserver. As shown in this example, the IGMP host provides an IP Multicast streamto designated routers which forward the IP Multicast stream on their local networkonly if there is a recipient.The client/server path is set up as follows:1. The designated router sends out a host membership query to the subnet andreceives host membership reports from end stations on the subnet.2. The designated routers then set up a path between the IP Multicast streamsource and the end stations.3. Periodically, the router continues to query end stations on whether to continueparticipation.4. As long as any client continues to participate, all clients, includingnonparticipating end stations on that subnet, receive the IP Multicast stream.IP Multicast can be optimized in a LAN by using IP Multicast filtering switches,such as the BayStack 450 switch.Note: Although the nonparticipating end stations can filter the IP Multicasttraffic, the IP Multicast still exists on the subnet and consumes bandwidth.kombk.book Page 48 Tuesday, June 29, 1999 3:25 PM
