Comment 13 for bug 1609861

curtin 0.1.0~bzr425-0ubuntu was installed out of xenial-updates and the non-default MTU does indeed get into /e/n/i.

From a deployed node's /e/n/i:
auto ens7f0
iface ens7f0 inet manual
    bond-xmit_hash_policy encap3+4
    bond-mode 802.3ad
    bond-master bond2
    mtu 9202
    bond-lacp_rate fast
    bond-miimon 100

auto ens7f1
iface ens7f1 inet manual
    bond-xmit_hash_policy encap3+4
    bond-mode 802.3ad
    bond-master bond2
    mtu 9202
    bond-lacp_rate fast
    bond-miimon 100

auto bond2
iface bond2 inet6 manual
    address fd06:ffe0:6779::2:0:9/64
    bond-xmit_hash_policy encap3+4
    bond-mode 802.3ad
    bond-slaves none
    mtu 9202
    bond-lacp_rate fast
    hwaddress ether 0c:c4:7a:b7:24:34
    bond-miimon 100

auto br-bond2
iface br-bond2 inet6 static
    address fd06:ffe0:6779::2:0:9/64
    mtu 9202
    hwaddress ether 0c:c4:7a:b7:24:34
    bridge_ports bond2