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GRE dont-fragment - inherit from where?

Posted: Tue Sep 10, 2019 5:02 am
by Paternot
Ok, I understand the concept of "don't fragment". But on a GRE interface we have two options:

1) "No". Meaning "ok, fragment if needed"
2) "inherit". Meaning "inherit the value from..." From where? That's what I want to know.

Where does the value come from, if I choose "inherit"?

Re: GRE dont-fragment - inherit from where?

Posted: Tue Sep 10, 2019 9:58 am
by docmarius
I'm pretty sure it means the parent device' setting if there is one, otherwise yes.

Re: GRE dont-fragment - inherit from where?

Posted: Tue Sep 10, 2019 1:12 pm
by Paternot
Which parent device? The GRE tunnel just exists, it isn't attached to some interface. Would it be the interface through the connection is made?

Re: GRE dont-fragment - inherit from where?  [SOLVED]

Posted: Tue Sep 10, 2019 2:12 pm
by andriys
I always assumed it inherits it from the packet being encapsulated. Not sure this assumption is correct, though.

Re: GRE dont-fragment - inherit from where?

Posted: Thu Sep 12, 2019 12:46 am
by 16again
afaik, inherent indeed means "use DF flag" of original packet. Note a GRE interface is unaware of its parent interface, all you specify is an IP local/remote addresses for the tunnel

Re: GRE dont-fragment - inherit from where?

Posted: Thu Sep 12, 2019 2:30 am
by Paternot
Well, it makes sense, to use the flag from the encapsulated packet.

Re: GRE dont-fragment - inherit from where?

Posted: Sat Sep 14, 2019 12:59 pm
by docmarius
Sorry, that was a bad wording... I meant the interface by which the connection was made through...

Re: GRE dont-fragment - inherit from where?

Posted: Sat Sep 14, 2019 2:09 pm
by pe1chl
But I think andriys and others are right, it means to copy don't fragment from the encapsulated packet to the outer GRE packet.
(same for DSCP)