I have been recently playing with MSTP more closely, trying to get better under its hood, and one issue surprised me: when a MSTP-enabled switch detects a RSTP neighbor on an access port, it speaks to it using RSTP as expected. The same behavior can be observed if the neighbor is another MSTP switch that belongs to a different region - the two switches fall back to using RSTP on the link.
However, if the link interconecting the two switches is a trunk then the MSTP reverts to PVST+ operation, not to RPVST+ as I originally expected. Is there any reason for that?
Thank you very much for all your insight!