RuggedSwitch™ User Guide
Problem Four
• Multicast streams of some groups are not forwarded properly.
Some segments without subscribers receive the traffic while some
segments with subscribers don’t.
Ensure there are you do have a situation where differing 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’s a proper switch behaviour. When the switch detects a change in the
network topology through RSTP it acts to avoid loss of multicast traffic. It
immediately starts issuing its own IGMP queries to quickly obtain group
membership information. It also starts forwarding all multicast traffic to all ports
that are not Edge Ports (because they may potentially link to routers). This may
result in some undesired flooding of multicast traffic, however, it guarantees that
all devices interested in the traffic will keep receiving it with no break. The
flooding will stop when the “false” router ports are aged out (about 2 switch query
intervals). Note that the same behaviour will be observed when the switch resets
or when IGMP Snooping is being enabled for the VLAN (in the latter case
flooding will only be observed within the VLAN being configured).
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