A principal switch selection can be triggered by a Build Fabric or Reconfigure Fabric event. Since you didn't notice any interruptions it's probably not the latter one, because that's a disruptive event. However, something happened that caused this. While in itself not a problem, you might want to figure out what the cause exactly was.
The 'show fcdomain statistics' and 'show fcdomain internal event-history vsan 1' will give you some insight into what exactly happened.
DM_VSF_E10_DOMAIN_ID_LIST_UPDATED means that a new domain was added to VSAN1. This could also mean that a trunk config has changed which involves VSAN1 or a complete link has been flapped. This is just the output for VSAN1 of course.
You now know when the event occured: Wed Oct 20 14:41:33 2010. This means you can correlate with the 'show accounting log' what happened at that time, but you'd have to do it on all switches involved.
As you can see in the statistics, this has been non-disruptive.
If you really want a full root-cause analysis it's best to open a case and include a full 'show tech-support detail' output.
It depends what kind of time you are willing to invest in something that has had virtually no impact. The fact there is nothing in the accounting log means that most probable this isn't caused by human interaction.
Is the `show fcdomain internal event-history` complete? It appears truncated
If you require root cause, a full analysis can be done in a Service Request because I do not recommend attaching a full show tech-support in a public forum such as this.
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