v6 troubles ("cannot forward")

[der Mouse <mouse%Rodents-Montreal.ORG@localhost>]

The scenario: my house network has v6 space, but its uplink is through
a non-v6-capable router.  So I tunnel.  The tunnel endpoint has
historically been a SPARCstation IPX running my mutant 1.4T.  Today I
replaced it, hardwarily, with something a little more recent (a P4)
running 3.1...and v6 broke.

sk0 is configured - by /etc/rc.d/network - as "inet6 2610:98:0:ffff::1
prefixlen 127", and defaultroute6 is set to 2610:98:0:ffff:: - same as
on the SPARCstation (well, mutatis mutandis - le0 and /etc/netstart).

But when it comes up, v6 doesn't work.  I can't ping6
2610:98:0:ffff::1, not even directly from 2610:98:0:ffff::.  But if I
manually run

route delete -inet6 default
ifconfig sk0 inet6 delete 2610:98:0:ffff::1
ifconfig sk0 inet6 2610:98:0:ffff::1 prefixlen 127
route add -inet6 default 2610:98:0:ffff::

then everything starts working just fine - even if I paste all four
commands at once into the shell, thereby executing them with very
little delay between them, an observation that seems to me to imply
it's not something like waiting a few seconds for DAD.

The setup is actually somewhat more complicated than the above, because
of the tunnel, but when it's broken I can't even ping6
2610:98:0:ffff::1 directly from 2610:98:0:ffff::, which inclines me
towards exonerating the rest of the setup.  I can of course give
details of the tunnel if anyone thinks they're likely to be relevant.
For that matter, I can provide more details about pretty much any part
of this setup....

Ideas, anyone?  I really would much rather fix this than kludge it (if
I'm content with a kludge, I could just have rc.local delay 60 seconds
and run the above, or some such).

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