Qtech QSW-3900-2x10GE [135/245] Processing of multicast protocol messages
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Qtech QSW-3900-2x10GE [135/245] Processing of multicast protocol messages

Qtech QSW-3900-48-SFP-DC [135/245] Processing of multicast protocol messages
QTECH Software Configuration Manual
10-134
address will suppress their own reports upon hearing this report, and this will prevent the
switch from knowing whether any hosts attached to that port are still active members of the
reported multicast group.
10.4.3.3 When receiving a leave group message
When an IGMPv1 host leaves a multicast group, the host does not send an IGMP leave group message, so
the switch cannot know immediately that the host has left the multicast group. However, as the host stops sending
IGMP reports as soon as it leaves a multicast group, the switch deletes the forwarding entry for the member port
corresponding to the host from the forwarding table when its aging timer expires.
When an IGMPv2 or IGMPv3 host leaves a multicast group, the host sends an IGMP leave group message
to the multicast router.
When the switch hears a group-specific IGMP leave group message on a member port, it first checks
whether a forwarding table entry for that group exists, and, if one exists, whether its outgoing port list contains that
port.
l If the forwarding table entry does not exist or if its outgoing port list does not contain the
port, the switch discards the IGMP leave group message instead of forwarding it to any port.
l If the forwarding table entry exists and its outgoing port list contains the port, the switch
forwards the leave group message to all router ports in the VLAN. Because the switch does
not know whether any other hosts attached to the port are still listening to that group address,
the switch does not immediately removes the port from the outgoing port list of the
forwarding table entry for that group; instead, it resets the member port aging timer for the
port.
Upon receiving the IGMP leave group message from a host, the IGMP querier resolves from the message the
address of the multicast group that the host just left and sends an IGMP group-specific query to that multicast group
through the port that received the leave group message. Upon hearing the IGMP group-specific query, the switch
forwards it through all its router ports in the VLAN and all member ports for that multicast group, and performs the
following :
l If any IGMP report in response to the group-specific query is heard on a member port
before its aging timer expires, this means that some host attached to the port is receiving or
expecting to receive multicast data for that multicast group. The switch resets the aging
timer of the member port.
l If no IGMP report in response to the group-specific query is heard on a member port before
its aging timer expires, this means that no hosts attached to the port are still listening to that
group address : the switch removes the port from the outgoing port list of the forwarding
table entry for that multicast group when the aging timer expires.
10.4.4 Processing of Multicast Protocol Messages
With Layer 3 multicast routing enabled, an IGMP Snooping switch processes multicast protocol messages
differently under different conditions, specifically as follows :
1) If only IGMP is enabled, or both IGMP and PIM are enabled on the switch, the switch handles multicast
protocol messages in the normal way.
2) In only PIM is enabled on the switch :
l The switch broadcasts IGMP messages as unknown messages in the VLAN.
l Upon receiving a PIM hello message, the switch will maintain the corresponding router
port.
3) When IGMP is disabled on the switch, or when IGMP forwarding entries are cleared (by using the reset
igmp group command) :
l If PIM is disabled, the switch clears all its Layer 2 multicast entries and router ports.
l If PIM is enabled, the switch clears only its Layer 2 multicast entries without deleting its
router ports.
4) When PIM is disabled on the switch :

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