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Multimedia Traffic Control with IP Multicast (IGMP)

How IGMP Operates

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 Hewlett-Pack-

ard’s 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 through the CLI, using the IGMP configuration MIB. See

“Configuring the Querier Function” on page 2-11.)

■ Report (Join): A message sent by a host to the querier to indicate that

the host wants to b e o r is a m em be r of a g iven 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.

N o te on IGMP When an IGMPv3 Join is received by the switch, it accepts the host request

version 3

and begins to forward the IGMP traffic. This means that ports which have not

support

joined the group and are not connected to routers or the IGMP Querier will

not receive the group's multicast traffic.

The switch does not support the IGMPv3 “Exclude Source” or “Include

Source” options in the Join Reports. Rather, the group is simply joined from

all sources.

The switch does not support becoming a version 3 Querier. It will become a

version 2 Querier in the absence of any other Querier on the network.

An IP multicast packet includes the multicast group (address) to which the

packet belongs. When an IGMP client connected to a switch port needs to

receive multicast traffic from a specific group, it joins the group by sending

an IGMP report (join request) to the network. (The multicast group specified

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ProCurve 2900 Betriebsanweisung Seite 36