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Chapter 7 – Configuring Multicast Filtering
Note that VLAN 3 does not have an external multicast router. The switch will
operating in its “routerless” mode and issue general membership queries as if it is
the router.
Processing Joins
If host C1 desires to subscribe to the multicast streams for both P1 and P2, it will
generate two joins. The join from C1 on VLAN 2 will cause the switch to
immediately initiate its own join to multicast router 1 (and to issue its own join as
a response to queries).
The join from C1 for VLAN 3 will cause the switch to immediately begin
forwarding multicast traffic from P2 to C2.
Processing Leaves
When host C1 decides to leave a multicast group it will issue a leave request to the
switch. If the switch is in active mode it will specific poll the port to determine if
C1 is the last member of the group on that port. If C1 is the last (or only)
member, the switch will issue a leave to the router and the group will immediately
be pruned from the port.
If the switch is configured to run in passive mode, it will age out the membership
of C1 in the group before pruning.
Should host C1 leave the multicast group without issuing a leave group message
and then fail to respond to a general membership query, the switch will stop
forwarding traffic after two queries.
Multicast Sources
When a multicast source starts multicasting, the traffic stream will be immediately
blocked on segments from which joins have not been received.
Should a multicast source stop issuing multicast traffic, the switch will age out
knowledge of the multicast source after about two query intervals.
IGMP And RSTP
An RSTP change of topology can render the routes selected to carry multicast
traffic as incorrect. This results in lost multicast traffic.
If RSTP detects change in the network topology, IGMP will take some actions to
avoid loss of multicast connectivity:
• The switch will immediately issue IGMP queries (if in IGMP active mode)
to obtain potential new group membership information.
• The switch will immediately begin to flood all multicast traffic through all
the ports that are not configured as RSTP Edge Ports for about 2 switch
query intervals.
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