MTU size question


While troubleshooting FortiClient VPN frequent disconnects for a friend, I ran across MTU size calculation suggestions using ping test.
Followed a tutorial similar to following posted at: Netgear
My impression from reading various OpenWrt comments is to leave OpenWrt default of 1500 alone.
However ran the ping test (mentioned at link above) multiple times and the figure I obtained was 1420 (1392+28)!?
Any number above 1392 resulted in Packets need to be fragmented... message.

  1. Just wish to confirm that the problem is with the ISP (Internet Service Provider = cable internet) and not OpenWrt.
  2. Any ideas as to what is happening with the ISP resulting in such numbers for the MTU?
  3. Confirm procedure to change MTU with current snapshot releases with LuCI is: Network > Devices > br-lan > General device options > MTU


EDIT: Corrected grammar

What does the test at say? Instead of you pinging some address on the Internet, that site attempts to ping you. If the site doesn't agree with your results, then there's something else going on.

You need to allow your router to reply to ICMP ECHO REQUEST for the test to work.

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OpenWrt uses MSS clamping to adjust the effective maximal packet size for TCP and seems to silently expect UDP connections to use path MTU discovery....
In the VPN case something obviously does not work as expected... is the VPN using TCP or UDP?