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BayStack 350 10/100/1000 Series Switches
309979-A Rev 00
1-37
IGMP Snooping
BayStack 350 switches can sense IGMP host membership reports from attached
stations and use this information to set up a dedicated path between the requesting
station and a local IP Multicast router. After the pathway is established, the
BayStack 350 switch blocks the IP Multicast stream from exiting any other port
that does not connect to another host member, thus conserving bandwidth. The
following discussion describes how BayStack 350 switches provide the same
benefit as IP Multicast routers, but in the local area.
Internet Group Management Protocol (IGMP) is used by IP Multicast routers to
learn about the existence of host group members on their directly attached subnets
(see RFC 2236). The IP Multicast routers get this information by broadcasting
IGMP 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 Multicast
source that provides the data streams and the clients that want to receive the data.
Figure 1-26
shows how IGMP is used to set up the path between the client and
server. As shown in this example, the IGMP host provides an IP Multicast stream
to designated routers which forward the IP Multicast stream on their local network
only 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 and
receives host membership reports from end stations on the subnet.
2.
The designated routers then set up a path between the IP Multicast stream
source and the end stations.
3.
Periodically, the router continues to query end stations on whether to continue
participation.
4.
As long as any client continues to participate, all clients, including
nonparticipating 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 350 switch.
Note:
Although the nonparticipating end stations can filter the IP Multicast
traffic, the IP Multicast still exists on the subnet and consumes bandwidth.