North American Network Operators Group|
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Re: 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... 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. -brett > If I hear back from the providers for the other two sites, I'll post > the answers to the list. > > --Jamshid