[ofa-general] IPOIB subnet on IB fabric with routers

[Rolf Manderscheid]

In order for an ipoib subnet to work on an IB fabric consisting of
multiple IB subnets, the MGIDs used for multicast can't have
link-local scope otherwise packets will get dropped by the first IB
router.  Currently, the scope is hard-coded to link-local in the
ip*_ib_mc_map() routines.  A reasonable first step is to modify said
routines to use the scope from the link level broadcast address of the
ipoib device in question, and I have a proposed patch which I will
post right away.

The second step is to make the scope in the link level broadcast
address configurable, but I'm not sure what the best way to accomplish
that would be.  The ip command supports changing the link level broadcast
address, but it uses an ioctl which limits the hardware address to 14
bytes (six bytes short) ... so it fails.  Even if that approach would
work it would be a little error prone, we only care about four bits out
of a 20 byte value.

My current favourite approach is to add a "broadcast_scope" attribute
under /sys/class/net/ibX/ that could be configured before bringing up
the link.  Does anybody have a better solution?

    Rolf