Greetings-
I work for a local mom'n'pop Cable Modem ISP provider doing contracted modem
and SOHO networking installs.
Recently, when they shifted to a new Tech Support provider, they "changed"
some settings
in some of their routers and switches.
They use a Cisco 3300 (I think) Router, and Catalyst 1900 switches. The
problem that
one one portion of clients in a part of town is that they cannot send emails
with attachments
and emails that contain more than a "basic" amount of data (ie: they can
send small emails with approx
10 lines or so). They were having this problem for about 4 weeks before
they brought me into the
consultation for a solution. My findings are that somewhere the MTU has
been changed preventing
the users from sending large attachments.
Here's the kicker. I went to a customer's location, plugged into their
network (they were using an
Asante router, Mac Network, MTU cannot be changed in router) with my Dell
Laptop running XP.
I had the same problem using Outlook express, as described. When I plugged
directly into the cable
modem, I had no problem and sent several emails with 300KB attachments. My
guess is that XP automatically
changes MTU as needed. I downloaded a utility for the Mac that changes the
MTU to 1383, and they could
send large emails with attachments. That led me to this conclusion that
their "Tech" guys changed something
relating to packet frame size.
Where should I start looking for this? I've been to training on Cisco
Routers/Switches but that was nearly 2 years
ago and have only used the basic information I've obtained, never had to
configure MTU and network packet
settings.
Thanks for any input!
-IanCT
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