I've done nothing overnight but now I get 3.7 megs on ipv6 on the same test page (consistently,
though of course it's now a different time of day)
ipv6 mtu 1452
on my router's tunnel0 (and on the HE config page).
(the ip mtu on my pppoa dialer interface is
ip mtu 1478 and I use ip tcp adjust-mss 1438
using ping6 -s (value) www.youtube.com
it works up to value = 1404
it's possible that my immediate problem has gone away for no particular reason
the beginning of "show ipv6 traffic" looks like this:
lounge#show ipv6 traffic
Rcvd: 762495 total, 9918 local destination
0 source-routed, 155 truncated
139 format errors, 50 hop count exceeded
0 bad header, 0 unknown option, 0 bad source
0 unknown protocol, 0 not a router
0 fragments, 0 total reassembled
0 reassembly timeouts, 0 reassembly failures
Sent: 11896 generated, 402539 forwarded
0 fragmented into 0 fragments, 0 failed
0 encapsulation failed, 0 no route, 486 too big
1286 RPF drops, 0 RPF suppressed drops
Mcast: 65 received, 849 sent
it seems plausible that the errors are mostly me trying to send overly big packets.
I am using IOS 15.1(4)M4 (advanced ip services) on a cisco 877.
I am happy enough with 3.7 megs of IPv6 but if there are errors in my config I'm all ears.