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 th
