Tracking Group Member Locations - 3Com 4007 Implementation Manual

212 C 11: IP M F IGMP
HAPTER ULTICAST ILTERING WITH
Routers and switches both construct filters on ports that do not require
group traffic to be forwarded. On each device, one group's traffic may be
forwarded to one set of ports and another group's traffic may be
forwarded to a different set of ports.

Tracking Group Member Locations

The ability to detect the location of group members is a product of two
IGMP functions — querying and snooping — working together to
construct individual delivery trees for each IP multicast group.
In each subnetwork or broadcast domain (VLAN), all switches and routers
perform snooping (for their own connections), but only one of the
devices needs to perform the querying. If there are multiple devices
(routers and switches) in the subnetwork or broadcast domain that
support querying, the one with the lowest IP address is elected as the
querier.
The querier periodically sends a query message to all hosts on the
subnetwork or broadcast domain and requests that they reply with the IP
multicast groups for which they want to receive traffic.
A host responds to a query by sending an IGMP report. The querier as
well as IGMP-capable devices between hosts and the querier snoop on
the report's content to track which hosts (and their associated ports)
belong to which groups.
If, after a certain period of time, a router or switch does not receive
responses from any host on a given interface or port for a particular IP
multicast group, the router or switch prunes the interface or port from
the delivery tree to conserve network bandwidth.
If a switch that has downstream group members, an upstream router and
upstream sources is elected as the querier, the switch must forward host
reports to the upstream router to maintain the flow of IP multicast traffic
to the switch.