More on MTU discovery...

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
from one.

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...

  i would be one of those two sites, and i responded tonight. the mtu problem
you found (and matt found it because he worked for this particular customer
this weekend) was on the customer end of one of our links. i couldn't quite
figure out what you wanted me to do about it. i'll just say for the record
that it seems to be a problem between the customer's vendors' router and our
router :slight_smile: the customer is aware of the problem and is following up with their
vendor. i was a little taken back by your note which basically sounded like:

"you've got an obvious problem, you need to fix it quick as everyone depends
on this resource"

that may not be what you meant, but it sounded that way.