the openstack-backports-archive-keyring package, which doesn't work anymore
    if using Buster.

    controller nodes.

    of using apt-get update / apt-get install --allow-unauthenticated (which
    method doesn't work anymore in Buster).

    backport repositories (as apt exit 1 without it starting in Buster).

    has mtu != 0, allowing to set (for example) mtu = 9000.