Jonn,
As interesting as it may sound, I was not able to find a definitive answer. Let me at least summarize what I was able to find out.
The RFC 4861, Section 6.3.6 discusses the selection of the default router. However, it limits itself only to say that preferably, only those routers which are known to be reachable or probably reachable shall be preferred to routers whose reachability is questionable. If no routers are currently known as reachable, the available routers shall be tried in round-robin fashion. It says, furthermore, that if multiple equivalent routers are available and reachable, some hints at the implementation of load sharing can be found in RFC 4311. The RFC 4311 is surprisingly quite vague, too, and simply states that a host should support some arbitrary kind of load sharing mechanism to make use of several gateways. From this it follows that there is considerable freedom in the implementation.
What can be interesting, however, is the Router Preference Option - an optional extension to the Router Advertisement message described in RFC 4191. According to this RFC, a router can declare its relative priority to other routers on the segment in three levels - high, medium (default) and low. Hosts implementing this extension may then decide which router should be chosen. Cisco supports this extension using the command ipv6 nd router-preference on the interface level. I suppose that both Windows and Linux hosts support this extension - I've seen the support in the Linux kernel, and the RFC itself was authorized by people from Microsoft.
If anybody has more information please share it with us!
Best regards,
Peter
