How Hosts Join And Leave Multicast Groups - Dell PowerConnect J-EX4200-24T Software Manual

How Hosts Join and Leave Multicast Groups

IGMP Snooping Support for IGMPv3
Hosts can join multicast groups in either of two ways:
By sending an unsolicited IGMP join message to a multicast router that specifies the
IP multicast group that the host is attempting to join.
By sending an IGMP join message in response to a general query from a multicast
router.
A multicast router continues to forward multicast traffic to a VLAN provided that at least
one host on that VLAN responds to the periodic general IGMP queries. For a host to remain
a member of a multicast group, therefore, it must continue to respond to the periodic
general IGMP queries.
To leave a multicast group, a host can either not respond to the periodic general IGMP
queries, which results in a "silent leave" (the only leave option for hosts connected to
switches running IGMPv1), or send a group-specific IGMPv2 leave message.
NOTE: A host does not leave a group if its link goes down—for example, if a
user disconnects from the port. The host remains a member of the group
until group membership times out and a silent leave occurs. This means that
if another user connects to the port before the silent leave occurs, the host
resumes receiving the group multicast traffic until the silent leave, even
though it never sent an IGMP join message.
IGMPv3 allows IGMP snooping to filter multicast streams based on the source address
of the multicast stream. Junos OS for J-EX Series switches supports IGMPv3 packets
that are in INCLUDE or EXCLUDE mode.
When a host sends an IGMPv3 INCLUDE report through a switch interface to indicate
that it wants to receive a multicast stream from a source address, the switch adds the
source address to the source list. In INCLUDE mode, the switch requests that packets be
sent to the specified multicast address only from those IP source addresses listed in the
source-list parameter. However, because J-EX Series switches do not support forwarding
on a per-source basis, the switch merges all IGMPv3 reports for a VLAN to create a
route with the appropriate next hop. This next hop contains all the interfaces on the VLAN
that are interested in group
When IGMP snooping for IGMPv3 is used with an RVI, the same
the snooping information in the RVI's output interface list (
When a host sends an IGMPv3 EXCLUDE report, the host indicates that it wants to join
a multicast group and receive packets for that group except from those IP source
addresses in the source-list parameter. However, because J-EX Series switches do not
support forwarding on a per-source basis, the switch ignores the source information and
creates a route. A host can also send an EXCLUDE report in which the source-list
(*,G,V)
parameter is empty, which is known as an EXCLUDE NULL report. An EXCLUDE NULL
Chapter 75: Understanding IGMP Snooping and Multicast
G .
(*,G,V)
route is added to
(* ,G,V)
).
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