[openib-general] IPoIB broadcast MC group membership

[Greg Lindahl]

Is this a correct summary of this thread?

* IPoIB uses an InfiniBand multicast group to fake ethernet broadcast
  * This is optional, I'm not sure what functionality is lost without it

* MVAPICH uses a multicast group for some MPI collectives
  * This can be turned off by setting env var DISABLE_HARDWARE_MCST

* An IB multicast group has to use ports of the same speed
  * This one was a surprise to me

Ergo, when you mix 1X, 4X SDR, and 4X DDR hosts, it behaves
differently from a homogeneous network.

-- greg