Hi Francois and everybody else
Please have a look at the attached Visio and see if you could plese help me with an aswer
The configuration shoud not lead to loops, or at least, not lead to permanent loop.
The simplest is keep assuming that the CST region is a group of STP unaware bridges (some kind of hubs). PVST is able to compute a loop free topology taking a hub into account. Now, there is a problem with STP and hub that can come into play here.
Suppose you have the following simple topology:
Bridge A 2--- HUB--.
Bridge B 2|-- HUB--.
Bridge A & B are redundantly connected via a link A1-B1 and via another link A2-B2. The link A2-B2 goes through 2 hubs. Suppose port B1 is blocking, everything is fine.
Now, the link between the two hub breaks.
Bridge A 2--- HUB--.
Bridge B 2--- HUB--.
Port B1 has to move to forwading. However, bridge B could not detect the failure of the path through the hubs. It has to wait for max-age before it can consider putting port B1 as a root port, replacing the previous root port B2. Performance are not that good, but so far so good. B1 eventually goes to forwarding and connectivity is re-established between A and B.
Now suppose that the link between the two hubs comes back up. Again, bridge A and B have no clue that this link came up (they don't see a link change, as the link coming up is behind the hubs). The problem is that when the link between the hub become operational, ports A1 A2 B1 and B2 are forwarding. There is a short bridging loop until B2 receives a BPDU from A2 and blocks B1 again.
This kind of problem is exactly what may happen in your network. Your PVST+ region is redundantly connected to the CST region. If the CST region splits, the PVST+ region will not see a redundant path any more through the CST region. It will open its ports. Now, if the CST region heals (and that's something that can happen automatically because the CST region is actually running STP!), then you will have a short bridging loop between the CST region and the PVST+ region, at least until the PVST+ region sees its BPDUs going through the CST region.