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15–2 MULTILINK ML2400 ETHERNET COMMUNICATIONS SWITCH – INSTRUCTION MANUAL
IGMP CHAPTER 15: IGMP
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
agent.
The Internet Group Management 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.