[openib-general] IPoIB broadcast MC group membership

[Roland Dreier]

    Greg> Is this a correct summary of this thread?  * IPoIB uses an
    Greg> InfiniBand multicast group to fake ethernet broadcast * This
    Greg> is optional, I'm not sure what functionality is lost without
    Greg> it

No, IPoIB uses multicast groups to implement IP broadcast and
multicast.  Without multicast, IPoIB can't do ARP (or IP multicast,
etc).  So it's an absolute requirement.

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

Probably right, don't know for sure.

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

No, but an IB multicast group has a speed associated to it.  This is
to allow, say, a 4X port sending multicast packets to use the right
static rate to avoid overrunning a 1X port that is also a member of
the group.

 - R.