There are two possible causes for a path migration:
Software command.
Software commands the two QPs or two EECs to perform a path migration. Some of the circumstances that might cause software to trigger a path migration are:
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Load-balancing.After examining the various performance counters (see “Performance Management” on page 1019) throughout the fabric, software might trigger a path migration to ease traffic congestion in the fabric.
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Excessive soft failures.
Software may determine (by examining various performance counters) that a specific link or a switch or router is experiencing frequent soft (i.e., correctable) errors that indicate a possible imminent hard failure.
Repetitive hardware failure.
A repetitive hardware failure causes the two QPs or two EECs to automatically migrate to the alternate path.