GE ML1600 Switch User Manual

zero. On the other hand, a transient group is dynamically assigned an address when the
group is created, at the request of a host. A transient group ceases to exist, and its address
becomes eligible for reassignment, when its membership drops to zero.
The creation of transient groups and the maintenance of group membership is the
responsibility of “multicast agents”, entities that reside in internet gateways or other
special-purpose hosts. There is at least one multicast agent directly attached to every IP
network or sub-network that supports IP multicasting. A host requests the creation of new
groups, and joins or leaves existing groups by exchanging messages with a neighboring
The Internet Group MGMNT Protocol (IGMP) is an internal protocol of the Internet Protocol
(IP) suite. IP manages multicast traffic by using switches, multicast routers, and hosts that
support IGMP (in the MultiLink implementation of IGMP, a multicast router is not necessary
as long as a switch is configured to support IGMP with the querier feature enabled). A set of
hosts, routers, and/or switches that send or receive multicast data streams to or from the
same source(s) is termed a multicast group, and all devices in the group use the same
multicast group address. The multicast group running version 2 of IGMP uses three
fundamental types of messages to communicate:
Query: A message sent from the querier (multicast router or switch) asking for a
response from each host belonging to the multicast group. If a multicast router
supporting IGMP is not present, then the switch must assume this function in order
to elicit group membership information from the hosts on the network (if you need
to disable the querier feature, you can do so using the IGMP configuration MIB).
Report: A message sent by a host to the querier to indicate that the host wants to
be or is a member of a given group indicated in the report message.
Leave Group: A message sent by a host to the querier to indicate that the host has
ceased to be a member of a specific multicast group. Thus, IGMP identifies
members of a multicast group (within a subnet) and allows IGMP-configured hosts
(and routers) to join or leave multicast groups.
When IGMP is enabled on the MultiLink family of switches, it examines the IGMP packets it
receives to:
Learn which ports are linked to IGMP hosts and multicast routers/queriers
belonging to any multicast group.
Become a querier if a multicast router/querier is not discovered on the network.
Once the switch learns the port location of the hosts belonging to any particular multicast
group, it can direct group traffic to only those ports, resulting in bandwidth savings on
ports where group members do not reside. The following example illustrates this operation.