>There is no requirement for the link-local part of an IPv6 address to
>actually be the MAC address of *that* interface - a machine with multiple
>interfaces can use the same link local part on all connected links
>etc. just as long as they're unique.

	couterexample: if you put multiple interface into single ethernet
	broadcast domain, your plan will not work.  for ppp interface I feel
	it much easier to borrow interface ID from other interfaces
	(as there's no possibility of having two interface into the same
	ppp domain).

itojun