I agree that it doesn't sound like an MTU problem. There are often problems with MTU when DSL, VPNs, tunnels, etc. are used, so people might jump to that conclusion. But e-mail messages are often very short and would easily fit into most MTUs even after overhead. To test whether it's an MTU problem, try some oversized pings.
The MTU issue occurs when a full-sized packet arrives at an interface that needs to squeeze it into an MTU along with the overhead. The interface could fragment, but maybe the application or transport layer set the Don't Fragment bit. Quite a few applications do that as part of their MTU discovery process. The problem is made worse if there's an access list that is blocking the ICMP "Fragmentation required but DF bit set" message. Here's a Cisco article on MTU: http://www.cisco.com/warp/public/105/56.html This isn't a criticism of the original poster, who was already doubting the people who told him it was an MTU problem, but it does give me a chance to get on my soapbox about troubleshooting methods. A lot of people troubleshoot using the technique we learned in grade school to match items from Column A with items from Column B. ;-) Column A has network types and Column B has most common problem for network type. It's important to know about common problems, but it's just as important to gather data, research symptoms, and use logic and reasoning. Cisco's troubleshooting method really does work: 1. Define the problem. 2. Gather facts. 3. Consider possibilities. 4. Create an action plan. 5. Implement the action plan. 6. Observe the results. 7. Do problem symptoms stop? If no, go back to 4 or possibly to 2. If yes, problem resolved, document the results. OK, off my soapbox now! :-) _______________________________ Priscilla Oppenheimer www.troubleshootingnetworks.com www.priscilla.com [EMAIL PROTECTED] wrote: > > I found email to be a touchy thing... Especially when dealing > with M$ > 0utlook. Are you sure it's the MTU size that's the problem > with email. > > I know in our situation, I had to add the mail server name & IP > to the host > file of the remote pc. Some times we experience some latency, > but for the > most part it's only been about half a minute. > > Cheers, > mkj > > -----Original Message----- > From: JohnZ [mailto:[EMAIL PROTECTED]] > Sent: Tuesday, October 01, 2002 8:55 PM > To: [EMAIL PROTECTED] > Subject: Confused about MTU size [7:54689] > > > Can some one explain clearly how does MTU size affect windows > applications > where these applications won't work over a network link. I have > a certain > home user that can establish a vpn tunnel through a DSL to > corporate network > and all applications will work except for email. The only > difference is a > cisco router in between the homeuser and corporate network. > Without this > cisco router (with homeuser directly attached to DSL modem) > there are no > problems. Some one mentioned MTU could be the problem, but if > the frames are > larger then MTU don't they get fragmented and re-assembled at > the other end. > How could MTU size fail single application while everything > else works fine. > Thanks for any help. > > Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=54743&t=54689 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
