linadm@maas-controller:~$ sudo maas root vlan read 0 0
Success.
Machine-readable output follows:
{
    "name": "untagged", 
    "vid": 0, 
    "mtu": 1500, 
    "fabric": "fabric-0", 
    "id": 0, 
    "resource_uri": "/MAAS/api/1.0/vlans/0/"
}
linadm@maas-controller:~$ sudo maas root vlan update 0 0 mtu=9000
{"__all__": ["Cannot modify the default VLAN for a fabric."]}