Create "MTU discovery tool"
This is discovery tool will propose you the best values for MTU and TCP-MSS for a specific interface.
For example nmap
vyos@r2-roll# sudo nmap -e eth0 --script path-mtu -F 1.1.1.1 Starting Nmap 7.70 ( https://nmap.org ) at 2020-09-10 15:32 UTC Nmap scan report for one.one.one.one (1.1.1.1) Host is up (0.0063s latency). Not shown: 97 filtered ports PORT STATE SERVICE 53/tcp open domain 80/tcp open http 443/tcp open https Host script results: |_path-mtu: PMTU == 1500
Change mtu
set interfaces ethernet eth0 mtu 1300
Nmap
vyos@r2-roll# sudo nmap -e eth0 --script path-mtu -F 1.1.1.1 Starting Nmap 7.70 ( https://nmap.org ) at 2020-09-10 15:34 UTC Nmap scan report for one.one.one.one (1.1.1.1) Host is up (0.0063s latency). Not shown: 97 filtered ports PORT STATE SERVICE 53/tcp open domain 80/tcp open http 443/tcp open https Host script results: |_path-mtu: PMTU == 1300
MTU size - 40 bytes = tcp-mss