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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 9180. 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. --Jamshid