Re: using same IP subnet on multiple interfaces (was: dual HCAs with upstream kernel)

[Jason Gunthorpe <jgunthorpe-ePGOBjL8dl3ta4EC/59zMFaTQe2KTcn/@public.gmane.org>]

On Sun, Aug 15, 2010 at 10:50:17AM +0300, Or Gerlitz wrote:

> the echo requests go on the wire, the replies not, probably (...)
> internally, Patrick?

What all these settings do is let a socket that is bound to a device
resolve the local host's address through ARP. The socket that is bound
to a device will then use its device for sending, but other sockets
not bound to devices will do route lookups and use the lo device.

Do:
ip route get 192.168.20.100 dev ib0
ip route get 192.168.20.1 src 192.168.20.100

To see the difference in each side.

To really effect a full external loopback you need to have both sides
bound to their respective devices. Note that binding to a device and
binding to a source IP are not the same thing in Linux.

In the RDMA CM case the listening side doesn't do any IP
routing operations at all so a device bind isn't necessary.

Jason
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