Comment 1 for bug 1543195

It affects me too.

$ maas 1st version read
Success.
Machine-readable output follows:
{
    "subversion": "trusty1",
    "version": "1.9.1+bzr4543-0ubuntu1",
    "capabilities": [
        "networks-management",
        "static-ipaddresses",
        "ipv6-deployment-ubuntu",
        "devices-management",
        "storage-deployment-ubuntu",
        "network-deployment-ubuntu"
    ]
}

$ maas 1st vlan update 0 103 mtu=9000
Success.
Machine-readable output follows:
{
    "name": "",
    "vid": 103,
    "mtu": 9000,
    "fabric": "fabric-0",
    "id": 5003,
    "resource_uri": "/MAAS/api/1.0/vlans/5003/"
}

$ maas 1st vlan update 0 0 mtu=9000
{"__all__": ["Cannot modify the default VLAN for a fabric."]}