When LMC is used who decides which path is used?

[Vangelis Tasoulas <vangelis-S8y2klWGCOFhl2p70BpVqQ@public.gmane.org>]

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Hello,

When LMC is used, the subnet manager may program alternate paths through
the subnet for these various LIDs.
This is done by the routing algorithm, that programs the LFTs of the
switches to forward different LIDs on different ports.

Assume we have an algorithm that is programming alternative paths, can
someone please explain how will different nodes get to choose different
paths when LMC > 0?

Consider the following example:
Node N1 and node N2 wants to communicate with node N3 and node N3 can be
reached either on LID 6 or LID 7 if LMC=1.
DLID 6 is routed through port 1 (p1) on switch 2 (sw2) and port 1 (p1)
on switch 1 (sw1)
DLID 7 is routed through port 2 (p2) on switch 2 (sw2) and port 2 (p2)
on switch 1 (sw1)

                 -----------
                 |sw1      |
                 |    p1 p2|
                 -----------
                 /  /   \  \
          -----------   -----------
          |sw2 p1 p2|   |      sw3|
          |         |   |         |
          -----------   -----------
             /   /         \
            N1  N2         N3
LIDs:       2   4          6
LIDs:       3   5          7


Which entity (SM, user app etc) is responsible for determine and decide
that node N1 is better to send traffic to DLID 6 and node N2 is better
to send traffic to DLID 7 (so that the two communications use different
paths)?

Vangelis


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