22. Multicast Filtering
ROX™ v2.2 User Guide 221 RuggedBackbone™ RX5000
Ensure that you do not have a situation where different multicast groups have multicast IP addresses
that map to the same multicast MAC address. The switch forwarding operation is MAC address-based
and will not work properly for several groups mapping to the same MAC address.
Problem Five
Computers on my switch issue join requests but don’t receive multicast streams from a router.
Is your multicast router running IGMP version 2? It must run IGMP version 2 in order for IGMP Snooping
to operate properly.
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 a few minutes),
however, it guarantees that all devices interested in the traffic will keep receiving it without a break.
Note that the same behavior will be observed when the switch resets or when IGMP Snooping is being
disabled for the VLAN.
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