Comment #8 made me think about why this works in our functional test, so I went to investigate that. In our functional test we use Bionic images, and sure enough the netplan.yaml [9] written there does _NOT_ include the MTU! The OpenStack Metadata source remains the same [10].
9: ubuntu@banana-1:~$ lsb_release -a No LSB modules are available. Distributor ID: Ubuntu Description: Ubuntu 18.04.5 LTS Release: 18.04 Codename: bionic ubuntu@banana-1:~$ cat /etc/netplan/50-cloud-init.yaml # This file is generated from information provided by the datasource. Changes # to it will not persist across an instance reboot. To disable cloud-init's # network configuration capabilities, write a file # /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg with the following: # network: {config: disabled} network: ethernets: ens2: dhcp4: true match: macaddress: fa:16:3e:a3:34:78 set-name: ens2 version: 2 10: $ curl http://169.254.169.254/openstack/2018-08-27/network_data.json {"links": [{"id": "tapc352887e-0f", "vif_id": "c352887e-0fff-481b-af47-7df9f7c2ff05", "type": "ovs", "mtu": 8950, "ethernet_mac_address": "fa:16:3e:a3:34:78"}], "networks": [{"id": "network0", "type": "ipv4_dhcp", "link": "tapc352887e-0f", "network_id": "f8123ceb-e29d-4f4a-b200-6fb3bf3984ba"}], "services": []} -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1899487 Title: cloud-init hard codes MTU configuration at initial deploy time To manage notifications about this bug go to: https://bugs.launchpad.net/cloud-init/+bug/1899487/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs