> Apparently you are talking about a behavior of a
> half-router/half-host node, that is, a node that works as a router on
> some subset of its interfaces and works as a host on the other set of
> interfaces.  [...]  The KAME/BSD currently does not support this
> behavior on purpose, due to concerns about possible implementation
> complexity.  I don't know if the concern is a FUD or there is any
> essential issue due to the BSD's networking architecture that makes
> it impossible.

Well, for one thing, in stock BSD there is only one "forwarding"
switch.  If you turn it on, then the machine will forward packets
between any two of its interfaces.  While there are hacks that can
extend this - I've added flags that can be set on an interface to
prevent forwarding packets to or from that interface - the stock design
does not have any way to set up what you describe.

If that's what you're talking about.  It's not entirely clear to me
that it is, though, since I can't see anything wrong with a router
autoconfiguring an interface.  (For example, a machine autoconfigures
interface A and statically configures interface B; it then speaks some
routing protocol to advertise its B-side subnet to those on its A side.
I can see such a thing being useful in, say, a small-static-subnet ISP
environment, where customer-facing routers advertise the small subnets
they're connected to to the rest of the ISP's core network.)

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