I have been playing around a bit with max MTU path discovery, and I came across something strange.
My setup is following:
I have OpenVPN tunnel established beetwen R1 and R2 (MikroTiks), R1 access internet through PPPoE, R2 have ISP with pure ethernet.
Then i have my setup following
-PC1 goes from R1 through OpenVPN tunnel to R2, and there on the internet. (OpenVPN is established, through internet connection of R1-PPPoE), when i try to run max MTU discover on PC1, it is 1500 (it goes thorugh OpenVPN that is established through PPPoE).
-PC2 goes on internet directly from R1 through PPPoE, and when i try to run max MTU discovery it is 1480.
Now my question is how is it possible that max mtu discovered through PPPoE and than additionally through OpenVPN is higher, than directly through PPPoE? Shouldnt OpenVPN add its own overhead of lets say 69 bytes so if we calculate max MTU on PPPoE is 1480 - 69, that should be 1411? Can someone please explain me what I do not understand correctly.