Comment 6 for bug 1936842

Hmm, I'm not sure where the difference comes from. With Juju 2.9.16 I still see mtu=1442 on VM NIC (expected) and mtu=1450 (bigger than underlying NIC) on fan-252 bridge.

ubuntu@juju-913ba4-k8s-on-openstack-0:~$ brctl show
bridge name bridge id STP enabled interfaces
fan-252 8000.0653e0778a0a no ftun0
                                                        veth966fdd48
lxdbr0 8000.00163e2848ef no
ubuntu@juju-913ba4-k8s-on-openstack-0:~$ ip l
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: ens3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1442 qdisc fq_codel state UP mode DEFAULT group default qlen 1000
    link/ether fa:16:3e:27:18:ce brd ff:ff:ff:ff:ff:ff
3: fan-252: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1450 qdisc noqueue state UP mode DEFAULT group default qlen 1000
    link/ether 06:53:e0:77:8a:0a brd ff:ff:ff:ff:ff:ff
4: ftun0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1392 qdisc noqueue master fan-252 state UNKNOWN mode DEFAULT group default qlen 1000
    link/ether 82:8a:c4:02:b5:77 brd ff:ff:ff:ff:ff:ff
5: lxdbr0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN mode DEFAULT group default qlen 1000
    link/ether 00:16:3e:28:48:ef brd ff:ff:ff:ff:ff:ff
7: veth966fdd48@if6: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1450 qdisc noqueue master fan-252 state UP mode DEFAULT group default qlen 1000
    link/ether 06:53:e0:77:8a:0a brd ff:ff:ff:ff:ff:ff link-netnsid 0