[openib-general] IPoIB multicast group creation

[Hal Rosenstock]

On Wed, 2006-03-01 at 23:10, Fabian Tillier wrote:
> On 3/1/06, Sean Hefty <sean.hefty at intel.com> wrote:
> > Can someone explain when ipoib creates a multicast group?
> 
> Here's how the Linux IPoIB handles multicast:
> 
> For the broadcast group, it depends on the SM having pre-created the
> broadcast group.  It only does a join (i.e. component mask doesn't
> provide all the settings to create the group) for the broadcast group.
> 
> Once in the broadcast group, it uses it as a template for all other
> groups, which allows it to issue a join with all paramters necessary
> to create the group if it doesn't exist already.

That's what the IPoIB I-D says.

> I believe the only field that changes is the MGID.
> 
> Since all IPoIB instances in the fabric use the same broadcast group,
> their settings for all MC groups are consistent, no matter which
> instance creates particular groups.

This is on a per partition basis (one broadcast group per partition
assuming IPoIB is running on that partition).

-- Hal

> In Windows, only the broadcast group join is different.  In this case,
> the code handles the case where the broadcast group is not
> pre-created, and the first instance to start will create it.  In that
> case, it picks some values for the broadcast group, mostly hardcoded
> except for the qkey which it generates dynamically.  Once in the
> broadcast group, it uses it as a template for all other MC groups.
> 
> - Fab
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