Digisol DG-FS4526E, Management Manual

Page: 967 / 1162

C
HAPTER
38 | Multicast Filtering Commands
IGMP Snooping
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C
OMMAND
U
SAGE
When a spanning tree topology change occurs, the multicast
membership information learned by the switch may be out of date. For
example, a host linked to one port before the topology change (TC)
may be moved to another port after the change. To ensure that
multicast data is delivered to all receivers, by default, a switch in a
VLAN (with IGMP snooping enabled) that receives a Bridge Protocol
Data Unit (BPDU) with the TC bit set (by the root bridge) will enter into
“multicast flooding mode” for a period of time until the topology has
stabilized and the new locations of all multicast receivers are learned.
If a topology change notification (TCN) is received, and all the uplink
ports are subsequently deleted, a time out mechanism is used to delete
all of the currently learned multicast channels.
When a new uplink port starts up, the switch sends unsolicited reports
for all current learned channels out through the new uplink port.
By default, the switch immediately enters into “multicast flooding
mode” when a spanning tree topology change occurs. In this mode,
multicast traffic will be flooded to all VLAN ports. If many ports have
subscribed to different multicast groups, flooding may cause excessive
loading on the link between the switch and the end host. Flooding may
be disabled to avoid this, causing multicast traffic to be delivered only
to those ports on which multicast group members have been learned.
When the spanning tree topology changes, the root bridge sends a
proxy query to quickly re-learn the host membership/port relations for
multicast channels. The root bridge also sends an unsolicited Multicast
Router Discover (MRD) request to quickly locate the multicast routers in
this VLAN.
The proxy query and unsolicited MRD request are flooded to all VLAN
ports except for the receiving port when the switch receives such
packets.
E
XAMPLE
The following example enables TCN flooding.
Console(config)#ip igmp snooping tcn-flood
Console(config)#