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RS400 205 ROS™ v3.5
Problem Six
I connect or disconnect some switch ports and multicast goes everywhere. Is IGMP
broken?
No, it may be a proper switch behavior. When the switch detects a change in the network
topology through RSTP it acts to avoid loss of multicast traffic – if configured to do so, it starts
forwarding all multicast traffic to all ports that are not RSTP Edge ports (because they may
potentially link to routers). This may result in some undesired flooding of multicast traffic (which
will stop after few minutes), however, it guarantees that all devices interested in the traffic will
keep receiving it with no break. Note that the same behavior will be observed when the switch
resets or when IGMP Snooping is being enabled for the VLAN.