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I am setting up a small LAN(20ish hosts) with a many devices which output multicast traffic. I am able to select their destination IP address and port number. There are 1-10 computers on the LAN which will subscribe to 0-to-all multicast streams. Some of the hosts will be unicasting back to some of the devices as well.

One option is to use the same destination IP address for each multicast device and then separate streams by port numbers. This may be easiest for users since they only need to remember 1 IP address and a handful of port numbers. A pitfall of this though is that if a host subscribes to any one of the multicast streams, all the multicast streams go to their ethernet port. This was verified with Wireshark. Also, IGMPv3 join requests don't seem to contain port number information (at least the way we're doing them).

Another option is to use different IP addresses and the same port number. I think this may upset the switch(es?) in the system so I don't want to do this.

The last option is to use different IP address and different port numbers. This may be the best option, but it means users would have to remember many IP addresses and port numbers, and would take up more of the IP pool (not that that's an issue for our small LAN).

Can any of your help me choose between these options based on your experience or best practices?

Thanks,

fooOnYou
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1 Answers1

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The notion of ports happens at L4 (i.e. UDP) and is of no interest to either IGMP or the routers and switches in your environment. As such, if you run all of your multicast traffic on a single group but differentiate streams by port then all hosts joining that group will receive all of the traffic associated with that group (plus or minus source lists if you are using IGMPv3 and SSM). I doubt this is what you're trying to achieve.

So - for purposes of network planning the numbers matter, the ports don't. That said, it would be wise to familiarize yourself with the mapping of multicast group addresses to MAC addresses as there is actually some overlap that needs to be borne in mind. Check out this link from Cisco for how this works as well as an excellent treatment of the topic of multicast overall.

Honestly the best option overall would be to use some kind of lookup / mapping system so that your users don't have to remember addresses or port numbers.

rnxrx
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  • Thank you for that link. It lead me to another by Cisco which goes into other details: http://www.cisco.com/c/dam/en/us/support/docs/ip/ip-multicast/ipmlt_wp.pdf We are planning on using the 239.255.* range so I think we'll avoid MAC address collisions. I was unaware of that issue, so thank you for pointing that out. – fooOnYou Dec 16 '16 at 17:32