WAN MTU Confusion

  • I have a bonded ADSL2+ modem in bridge mode connected to my pfSense using PPPoE.

    Last year when I initially setup the PPPoE connection on pfSense I left the MTU blank so that it would use defaults. If I remember right, going to Status > Interfaces the WAN was reporting an MTU of 1500.

    After doing some reading I found out DSL MTU should be 1492 so I went back to the pfSense WAN interface and entered an MTU of 1492. I verified the WAN MTU was set to 1492 in  Status > Interfaces. Up until recently I didn't look any more into it.

    While trying to find information on another subject I read an article explaining how to find the proper MTU so I thought I would give it a try.

    First I went back into pfSense Status > Interfaces and found my WAN MTU now reporting as 1484. I found this odd as the MTU is set to 1492 in the WAN interface settings and I had verified in the past that the interface was indeed reporting that it was set to 1492.

    Next, using the command line on my PC I determined my optimal MTU by entering "ping www.tp-link.com -f -l 1472". 1472 was as high as I could go. Anything higher would return a result of "Packet needs to be fragmented but DF set."

    According to the article I read I would now take 1472 and add 28(for IP and ICMP headers) to it to find my correct MTU which in my case would be 1472 + 28 = 1500.

    I then went into the WAN interface and changed the MTU to 1500. Once the new MTU was applied I rebooted pfSense and checked Status > Interfaces. Once again I found it reporting an MTU of 1484.

    After this I cleared the WAN MTU setting, saved and rebooted to see what would happen. Going to Status > Interfaces the WAN is still reporting an MTU of 1484.

    So I am confused about a few things here and I am hoping some one can shed some light on this for me.

    1.) How could my MTU auto-magically change from 1492 to 1484 on its own especially considering it was set to 1492?

    2.) If according to what I have read, DSL MTU should be 1492 why does my ping test indicate my MTU setting should be 1500?

    3.) If the WAN interfaces MTU field is left blank or I set it to 1500 why is my MTU still defaulting to 1484?

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