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Emulab FAQ: Troubleshooting: Why isn't my multicast traffic being correctly shaped?

If you have setup shaped links in your experiment, but your multicast traffic is not being properly shaped, then most likely your multicast traffic is using the control network.

This is because the default route for nodes is out the (unshaped) control network and unless you have setup an explicit route for your multicast addresses, they will go via the default route.

In FreeBSD and Linux you typically use setsockopt to explicitly bind multicast sockets to interfaces (IP_MULTICAST_IF). If the application you are using doesn't allow you to specify the interface via a command line or config file option, you can also just add a system wide route for the multicast address range in question. For FreeBSD it would look something like:

route add -net 239.0.0.0/8 -iface em2

See also this FAQ entry and the control net section of the tutorial.