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> > 
> > I thought NUD as specified in the ND spec could be quite sufficiently 
> > used on all types of links to verify two-way reachability.  Am I missing 
> > something?
> I don't think NUD is sufficient on multi-access links to verify that
> multicast routing updates are reaching all neighboring routers.  NUD
> serves to detect the unreachability only of those destinations or next-
> hop routers to which one is sending unicast packets.  So yes, we could
> depend on NUD on p-to-p links and tunnels, if we specified that RIPng
> must use unicast destination addresses over those types of links; however,
> we'd still have to employ a different strategy on multi-access links, and
> presumably that same strategy would also work in the degenerate case
> of a p-to-p link without requiring it to be treated as a special case.

I'm not sure that a router sending multicast routing updates should ever care
if his updates are reaching all neighboring routers (and if he learned that they
don't, what he is going to do about that?).  But receivers of routing
updates should be able to verify that they can reach the advertising router
if they chose to send traffic through it.  NUD is quite sufficient for
that.  And,  if a router relying on the reception of routing updates does
not get them or can't use them for lack of reachability to the advertiser,
someone will scream rather sooner than later. 

> Steve