Comment 6 for bug 1585822

When MAAS commissions a machine, it aims to automatically configure its network topology: subnet, vlan and fabric. MAAS uses a heuristic to "discover" the right vlan and fabric. The heuristic assumes that the subnets are unique, and that machines that appear on the same subnet can communicate (i.e. they are also on the same vlan and fabric). This is a convenience mechanism, that prevents manual network configuration after commissioning, which allows immediate deployment after a machine has been commissioned.

We recognise that this heuristic, while convenient for basic setups, is wrong as it requires that subnets are globally unique (which MAAS data model consequently enforces). We do not want to regress in the convenience of the deployment after commissioning, and need to find another model of initial discovery of network topology that doesn't solely depend on the IP address of a NIC.

The choice of the network model has been made a long time ago and its consequences are affecting many functions of MAAS. Simply lifting the uniqueness constraint will lead to a different behaviour during commissioning, which would then require manual correction of vlan and fabric in cases where a non-unique subnet is present, as MAAS currently lacks more sophisticated methods of discovering network topology. Would such a change be an acceptable step forward?