FYI,
I implemented it with the "whole other ifnet" approach for my vrrp6
implementation. It has been committed to KAME tree.

http://www.kame.net/dev/cvsweb2.cgi/kame/kame/sys/net/if_vrrp.c

>     Yes, like ip aliases.  I'm not so sure the "whole other ifnet"
> approach will work for what I have in mind; there are a bunch of
> ambiguity cases that arise.  It doesn't strike me as terribly
> dis-similar to using two physical ethernet cards attached to the same
> link layer.
>
>     The big issues seem to revolve around routing; how and why do I
> pick one interface over the other?
>
>     This would be a big problem for ??rp implementations; typically,
> the system has a management ip address/burned in mac, and advertises
> services attached to a service ip address on the same network, but
> with a virtual link layer name associated with that service ip
> address.  The ??rp implementation only transmits a select few frames
> using the virtual address; namely arp replies and ??rp announcements
> for the sake of programming learning bridge mac tables.
>
>
>     Disregarding that issue, I see no reason why a seperate ifnet
> isn't tractible.  It's probably even doable to have both; the ifnet
> based version seems to have similar requisites to having them within
> one.  I'll certainly keep the approach in mind.
--
Hideki ONO