More on MTU discovery...
Brett D. Watson
bwatson at mci.net
Tue Feb 13 03:15:36 UTC 1996
> 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 :) 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.
> If I hear back from the providers for the other two sites, I'll post
> the answers to the list.
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