Working on renewing my knowledge of multiple spanning tree, in the lab I have three switches in a triangle topology. I have configured mst instance 23 and in this instance vlans 100-400 on Switch2 and Switch3, here I have also configured Switch2 as the mst root bridge for instance 0 and 23.
When I issued the show spanning-tree mst command, in the output I can see this odd reference to PVST:
From the above screenshot g0/0 is connected to Switch3 which is also running mst and g0/3 is connected to Switch1 which is not running mst.
After some searching on the internet I found the blog linked below which points out that the above output is simply indicating that g0/3 is a mst boundary port meaning that the switch which is connected to this port (Switch1) is not running mst.
Since this is the case Switch2 will generate and send a PVST bpdu on g0/3. Note that I have configured mst root primary on Switch2 for both instance 0 and 23.
I also found an article on cisco.com which explains in detail how mst switches inter operate with non mst switches: