It took me a bit to figure out how to set an interface MTU using the new netplan that ships with Ubuntu 18.04. Netplan works by taking a YAML file and generating systemd-networkd configuration scripts. I found another post that indicated the reason MTU settings wasn’t working had to do with the generated “Match” clause for the inteface name was against the original (pre-udev-renaming) name. The post indicated they switched to a macaddress match instead, this also worked for me:
network: version: 2 renderer: networkd ethernets: # Configure MTU bigmtu1: match: macaddress: 00:e0:8d:7e:1f:37 mtu: "9000"
00:00 [13:24:19 hyperion:~]$ more /run/systemd/network/10-netplan-bigmtu1.link [Match] MACAddress=00:e0:8d:7e:1f:37 [Link] WakeOnLan=off MTUBytes=9000