2. When a node affiliates with multiple routers, upper-layer hints of
forward progress MUST NOT be used for reachability confirmation.
Instead, the node MUST actively probe neighboring routers using
unicast Neighbor Solicitation messages as in the second paragraph
of RFC2461
I don't understand how this is a solution. Requiring this probing will
seriously limit the scalability of ISATAP. Given the architecture for
ISATAP, NS/NA messages will need to be sent unicast between ISATAP hosts
and routers. What this means is that each ISATAP host will be sending
an NS message every 30 seconds or so (depending on the host
configuration, but 30 seconds is the default timer for ND). The ISATAP
router would need to reply to all of these messages, since there is no
way for hosts to passively listen for ND messages sent to
ipv6_all_nodes. This would create a packet storm of ND messages on a
semi-large network deploying ISATAP. Requiring (or even making this
mechanism a SHOULD) is a really bad idea. At best it should be a MAY
with strong wording suggesting the scalability issues I mentioned above.