Based on the number of replies, it sounds like there is quite a lot of
interest in this subject. I didn't want to call out any sites in
particular, because I didn't want to pick on any providers. I sent
mail to three people with problems this morning at around the same
time I sent the nanog mail, and have so far only heard a response back
The one who responded turned out to be exactly what Matt suggested --
mismatched ATM MTUs on Cisco routers. Apparently the Cisco default is
something like 4470 even though ATM default MTU is supposed to be
The other two sites haven't responded. The IP hop in question in both
cases was, I believe, a cisco router (based on the prompt I got when I
telnetted to them). However, since I know that in most cases Ciscos
seem to do the right thing, I suspect that these sites have other
problems down at layer two. Lots of people out there make bridging
products who have never heard of RFC1191...
If I hear back from the providers for the other two sites, I'll post
the answers to the list.