An example from my configuration. I use GRE tunnels to connect my routers. Sometimes, in addition to IPv4, I also have IPv6 from a specific provider, which I use for transit. This is terminated on the same Ethernet interface. PMTUD doesn’t always work well, and if a given router natively sends IPv6, it’s worth forcing smaller MSSs on it to ensure that these packets properly travel through GRE tunnels as third-party responses, which don’t necessarily return on the same interface due to routing asymmetries. Our route view isn’t always the same as the route view of the server we’re connecting to. My GRE tunnels have an MTU of 1476, and the Ethernet interface itself is 1500. The following trick solves many “packet too big” problems.

Tested on VyOS 1.4.4:

set interfaces ethernet eth0 ipv6 adjust-mss '1360'