I am thinking of switching our streaming servers to use multicasting instead of registering multiple clients and duplicating the streams manually on the server. It seems to work very well. However, I did notice that on the packet capture, when a server is multicasting the packets show up on every node, even those that have not subscribed to the stream. Is this the way it is supposed to function? Will this cause network problems if I have several of these servers sending out a multicast stream simultaneously(to different addresses obviously)?


  • You need to enable IGMPv2 or v3 on the switch on particular VLAN. – Andrew Smith Aug 30 '12 at 0:22

Multicast, by it's nature, is broadcast based in the sense that the multicast stream is flooded to all switch ports in the same VLAN or broadcast domain. The hosts that have subscribed to and are interested in that stream will then listen to that stream, all other hosts ignoring it. The way to "solve" this "broadcast problem" is to configure IGMP snooping.

  • Will it cause a problem if I leave it as is? – Jonathan Henson Aug 29 '12 at 19:10
  • Only if the volume of multicast traffic creates congestion in the network. Run a packet capture on one or two of the hosts and look for Duplicate Acknowledgements and TCP Retransmissions as these are sure signs of network congestion. – joeqwerty Aug 29 '12 at 19:24
  • Well, there could potentially be hundreds of these guys going at once. also, this is a sub-stream, the main stream is used for normal voip as well. – Jonathan Henson Aug 29 '12 at 19:25
  • That's why it's important to perform packet captures, so you'll know if congestion exists as a result of the multicast streams. – joeqwerty Aug 29 '12 at 19:31
  • I didn't see any such collisions. I am wondering though, would source-specific multicasting fix this? – Jonathan Henson Aug 29 '12 at 20:52

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