Comment 8 for bug 1922891

You can link multiple subnets to an interface in MAAS. The POST /MAAS/api/2.0/nodes/{system_id}/interfaces/{id}/?op=link_subnet can be called on the same interface multiple times. Then to address the original issue, if you look at the API docs for POST /MAAS/api/2.0/nodes/{system_id}/interfaces/{id}/?op=set_default_gateway, an interface with multiple subnets must also supply the link ID.