Frag 8 bytes earlier: case 1 VPN

[osgpcqosg]

Was looking to understand MTU size in this ticket https://supportforums.cisco.com/thread/2067814
Now I understand how it's work but when i try to test it, found this strange behavior.

The MTU tunel is 1446, I'm trying to do a ping from a linux box to another one.

  linux box client -- cisco 877 (vpn end punt) ---- internet ---- cisco 878 (nat static) -- linux vpn server -- linux box

1446 (MTU) -20 (ip header) -8 (icmp header) = 1418

So I should be able to send a ping a 1418 bytes of data without frag, but in fact the frag start with 1411 bytes of data ping.

• ping -c 1 -s 1410 192.168
  

•     Tot ok

• ping -c 1 -s 1411 192.168

•     The tcpdump are allright , but have in the tunel frag.

Do somebody have the same problem ?
Any solutions ?

Best regards.

[osgpcqosg]

In fact in the same problem I have with a simple ping thought nat: https://supportforums.cisco.com/thread/2066638

[osgpcqosg]

Seems to be a problem between us and our provider.

  upload MTU 1500

  download MTU lower than 1500.

When i correct this I will post a comment to explain the result.

Really sorry for the noise.