I've been playing around with routed trying to get it to pick the best
route based on IRDP information received over the network.

The first thing I noticed is that when I delete the default route (so
that routed will give me a new one), multicast routing no longer works.
This makes routed unhappy, since it tries to find routers via sending
a router solicitation to 224.0.0.2 (all-routers.mcast.net), and if it
can't send a multicast packet, it marks the interface as "SICK" and
we now have to wait until a router advertisement appears on the segment.

After some surfing, I found a couple of posts that suggest creating a
route for multicast packets to the loopback interface with:

    route add -net 224.0.0.0 127.0.0.1

But that effectively throws the packets away - at least on the NetBSD 1.4
and 1.5 where I tried it.

I suppose I could always add a route before I start routed that points
all multicast traffic to the local subnet, but what if I have two
physical interfaces?  Wouldn't you want a broadcast to "all-routers" to
go to all physical interfaces?