Question

MTU and Fragmentation

Asked by: johnpitt

I have a MTU and packet size issue. I have an application running on a Unix box that is regularly sending 1500 byte packets. In order to test my networks ability to handle this size packet, I tried the following “ ping 192.168.x.x – l 1500 –f ” . It will not go, I get the "needs to be fragmented" message. I can see not being able to get a non-fragmented packet through a VPN tunnel without fragmenting but I can’t even get it to go hitting another local device. I have to drop down to 1472. When I capture the 1500 byte ping packet, it shows up as 1528.   1500 – 28 = 1472. Imagine that. I just don’t see where the extra 28 bytes are coming from. If it is a normal part of ICMP then why would most MTU’s default at 1500. I had to change the MTU’s on the machines at one location to get the APP to work right. Outlook would not work from there either until I changed the MTU. It is an oddball location though. Wireless VPN over PPPOE. Lots of overhead. However, I think I could smooth the entire network out if I knew what was going on. Here is the setup. My Unix box is connected to a common switch (HP Procurve 4000m) with fiber. Most all local devices are on that switch. I leave the LAN either on a Cisco 3005 or through a Cisco 3600 router on Lease lines. I could probably reduce the MTU on the UNIX box to 1472 or something but I really need to know what is going on. Reducing the MTU on the UNIX box will be a very last resort.
Thanks in advance for the help.

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Asked On
2004-06-09 at 07:45:41ID21019117
Tags

bytes

,

dropping

,

hp

Topic

Miscellaneous Networking

Participating Experts
2
Points
500
Comments
4

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