Hi Daniel,
Assuming that adaptive routing is not enabled, available paths will be statically balanced, between each pair of Local IDs (LIDs). If only one LID exists on each end of the 2 cables, only one cable will be used. If a server has 2 ports (2 LIDs), there will be two sets of paths to that server from every other LID in the subnet, and the routing algorithms will try to statically allocate half of the paths to each cable. But the existence of multiple available paths doesn’t mean they’ll be utilized. Load balancing among two ports on the same server is a function of the Upper Layer Protocol (IPoIB, SRP, etc.) IPoIB doesn’t do load balancing; MPI stacks do..
See some examples here as well regarding unused links.
Ophir.