[2.1] setting larger MTU on vlan does not set MTU on base device

[Frank Rosquin]

When setting MTU on VLAN interfaces, MAAS leaves the MTU on the base interface at 1500.

As we don't use untagged traffic, I never touched the default vlan. This caused my VLANs to never come up, with no feedback from MAAS as to why.

generated /etc/network/interfaces (edited for IP's and take out interfaced that are not relevant)

auto lo
iface lo inet loopback
    dns-nameservers 8.8.8.8
    dns-search maas

auto enp2s0f0
iface enp2s0f0 inet static
    dns-nameservers 172.20.20.1
    address 172.20.20.10/24
    mtu 1500

auto enp3s0f0
iface enp3s0f0 inet manual
    mtu 1500

auto enp3s0f0.4011
iface enp3s0f0.4011 inet static
    address 172.20.21.10/24
    vlan-raw-device enp3s0f0
    mtu 9000
    vlan_id 4011

auto enp3s0f0.4012
iface enp3s0f0.4012 inet static
    address 172.20.22.10/24
    vlan-raw-device enp3s0f0
    mtu 9000
    vlan_id 4012

source /etc/network/interfaces.d/*.cfg

[Blake Rouse (blake-rouse)]

http://bazaar.launchpad.net/~maas-committers/maas/trunk/view/head:/src/maasserver/models/interface.py#L585

Is how the effective MTU is set on an interface for the preseed generation and for the `effective_mtu` interface value over the API. This code only takes into account the VLAN attached to the interface and not the the children interface VLAN's. It needs to look at the children and ensure that the parent is the same size. Without the parent being the same size then the children will never work.